The syntax is mostly derived from C and C++. Unlike C++, Java has no global functions or variables, but has data members which are also regarded as global variables. All code belongs to classes and all values are objects. The only exception is the primitive data types, which are not considered to be objects for performance reasons (though can be automatically converted to objects and vice versa via autoboxing). Some features like operator overloading or unsigned integerdata types are omitted to simplify the language and avoid possible programming mistakes.
The Java syntax has been gradually extended in the course of numerous major JDKreleases, and now supports abilities such as generic programming and anonymous functions (function literals, called lambda expressions in Java). Since 2017, a new JDK version is released twice a year, with each release improving the language incrementally.
Integer literals are of int type by default unless long type is specified by appending L or l suffix to the literal, e.g. 367L. Since Java SE 7, it is possible to include underscores between the digits of a number to increase readability; for example, a number 145608987 can be written as 145_608_987.
Variables
Variables are identifiers associated with values. They are declared by writing the variable's type and name, and are optionally initialized in the same statement by assigning a value.
intcount;// Declaring an uninitialized variable called 'count', of type 'int'count=35;//Initializing the variableintcount=35;// Declaring and initializing the variable at the same time
Multiple variables of the same type can be declared and initialized in one statement using comma as a delimiter.
inta,b;// Declaring multiple variables of the same typeinta=2,b=3;// Declaring and initializing multiple variables of the same type
Since Java 10, it has become possible to infer types for the variables automatically by using var.
// stream will have the FileOutputStream type as inferred from its initializervarstream=newFileOutputStream("file.txt");// An equivalent declaration with an explicit typeFileOutputStreamstream=newFileOutputStream("file.txt");
Code blocks
The separators { and } signify a code block and a new scope. Class members and the body of a method are examples of what can live inside these braces in various contexts.
Inside of method bodies, braces may be used to create new scopes, as follows:
voiddoSomething(){inta;{intb;a=1;}a=2;b=3;// Illegal because the variable b is declared in an inner scope..}
Comments
Java has three kinds of comments: traditional comments, end-of-line comments and documentation comments.
Traditional comments, also known as block comments, start with /* and end with */, they may span across multiple lines. This type of comment was derived from C and C++.
/* This is a multi-line comment.It may occupy more than one line. */
End-of-line comments start with // and extend to the end of the current line. This comment type is also present in C++ and in modern C.
// This is an end-of-line comment
Documentation comments in the source files are processed by the Javadoc tool to generate documentation. This type of comment is identical to traditional comments, except it starts with /** and follows conventions defined by the Javadoc tool. Technically, these comments are a special kind of traditional comment and they are not specifically defined in the language specification.
/** * This is a documentation comment. * * @author John Doe */
Universal types
Classes in the package java.lang are implicitly imported into every program, as long as no explicitly-imported types have the same names. Important ones include:
java.lang.System
java.lang.System is one of the most fundamental classes in Java. It is a utility class for interacting with the system, containing standard input, output, and error streams, system properties and environment variables, time, and more.
java.lang.Object
java.lang.Object is Java's top type. It is implicitly the superclass of all classes that do not declare any parent class (thus all classes in Java inherent from Object). All values can be converted to this type, although for primitive values this involves autoboxing.
java.lang.Record
All records implicitly extend java.lang.Record. Thus, a record, while treated as a class, cannot extend any other class.
java.lang.Enum
All enumerated types (enums) in Java implicitly extend java.lang.Enum. Thus, an enum, while treated as a class, cannot extend any other class.
java.lang.Class<T>
java.lang.Class<T> is a class that represents a class or interface in the application at runtime. It cannot be constructed via direct instantiation, but is rather created by the JVM when a class is derived from the bytes of a .class file. A class literal can be obtained through .class on such a class. For instance, String.class returns Class<String>. An unknown class being modeled can be represented as Class<?> (where ? denotes a wildcard).
java.lang.String
java.lang.String is Java's basic string type. It is immutable. It does not implement Iterable<Character>, so it cannot be iterated over in a for-each loop, but can be converted to char[]. Some methods treat each UTF-16 code unit as a char, but methods to convert to an int[] that is effectively UTF-32 are also available. String implements CharSequence, so chars in the String can be accessed by the method charAt().
java.lang.Throwable
java.lang.Throwable is the supertype of everything that can be thrown or caught with Java's throw and catch statements. Its direct known subclasses are java.lang.Error (for serious unrecoverable errors) and java.lang.Exception (for exceptions that may naturally occur in the execution of a program).
java.lang.Error
java.lang.Error is the supertype of all error classes and extends Throwable. It is used to indicate conditions that a reasonable application should not catch.
java.lang.Exception
java.lang.Exception is the supertype of all exception classes and extends Throwable. It is used to indicate conditions that a reasonable application may have reason to catch.
java.lang.Math
java.lang.Math is a utility class containing mathematical functions and mathematical constants (such as Math.sin(), Math.pow(), and Math.PI).
java.lang.IO
java.lang.IO is a class introduced in Java 25 (previously residing in java.io as java.io.IO). It allows simpler access to the standard input and output streams over System.in and System.out.
Primitives
Each primitive type has an associated wrapper class (see primitive types).
Program structure
Java applications consist of collections of classes. Classes exist in packages but can also be nested inside other classes.
Every Java application must have an entry point. This is true of both graphical interface applications and console applications. The entry point is the main method. There can be more than one class with a main method, but the main class is always defined externally (for example, in a manifest file). The main method along with the main class must be declared public. The method must be static and is passed command-line arguments as an array of strings. Unlike C++ or C#, it never returns a value and must return void. However, a return code can be specified to the operating system by calling System.exit().
publicstaticvoidmain(String[]args){// ...}
Packages
Packages are a part of a class name and they are used to group and/or distinguish named entities from other ones. Another purpose of packages is to govern code access together with access modifiers. For example, java.io.InputStream is a fully qualified class name for the class InputStream which is located in the package java.io.
A package is essentially a namespace. Packages do not have hierarchies, even though the periods may suggest so. A package can be controlled whether it is accessible externally or internally of a project using modules.
A package is declared at the start of the file with the package declaration:
packagecom.myapp.mylibrary;publicclassMyClass{}
Classes with the public modifier must be placed in the files with the same name and java extension and put into nested folders corresponding to the package name. The above class com.myapp.mylibrary.MyClass will have the following path: com/myapp/mylibrary/MyClass.java.
Modules are used to group packages and tightly control what packages belong to the public API. Contrary to Jar files, modules explicitly declare which modules they depend on, and what packages they export.[1] Explicit dependency declarations improve the integrity of the code, by making it easier to reason about large applications and the dependencies between software components.
The module declaration is placed in a file named module-info.java at the root of the module’s source-file hierarchy. The JDK will verify dependencies and interactions between modules both at compile-time and runtime.
For example, the following module declaration declares that the module com.foo.bar depends on another com.foo.baz module, and exports the following packages: com.foo.bar.alpha and com.foo.bar.beta:
The public members of com.foo.bar.alpha and com.foo.bar.beta packages will be accessible by dependent modules. Private members are inaccessible even through a means such as reflection. Note that in Java versions 9 through 16, whether such 'illegal access' is de facto permitted depends on a command line setting.[2]
The JDK itself has been modularized in Java 9.[3] For example, the majority of the Java standard library is exported by the module java.base.
Import declaration
An import statement is used to resolve a type belonging to another package (namespace). It can be seen as similar to using in C++.
Type import declaration
A type import declaration allows a named type to be referred to by a simple name rather than the full name that includes the package. Import declarations can be single type import declarations or import-on-demand declarations. Import declarations must be placed at the top of a code file after the package declaration.
packagecom.myapp;importjava.util.Random;// Single type declarationpublicclassImportsTest{publicstaticvoidmain(String[]args){/* The following line is equivalent to * java.util.Random random = new java.util.Random(); * It would have been incorrect without the import. */Randomrandom=newRandom();}}
Import-on-demand declarations are mentioned in the code. A "glob import" imports all the types of the package. A "static import" imports members of the package.
importjava.util.*;/* This form of importing classes makes all classes in package java.util available by name, could be used instead of the import declaration in the previous example. */importjava.*;/* This statement is legal, but does nothing, since there are no classes directly in package java. All of them are in packages within package java. This will not import all available classes. */
This type of declaration has been available since J2SE 5.0. Static import declarations allow access to static members defined in another class, interface, annotation, or enum; without specifying the class name:
import staticjava.lang.System.out;// 'out' is a static field in java.lang.SystempublicclassHelloWorld{publicstaticvoidmain(String[]args){/* The following line is equivalent to System.out.println("Hi World!"); and would have been incorrect without the import declaration. */out.println("Hello World!");}}
Import-on-demand declarations allow to import all the fields of the type:
import staticjava.lang.System.*;/* This form of declaration makes all fields in the java.lang.System class available by name, and may be used instead of the import declaration in the previous example. */
Enum constants may also be used with static import. For example, this enum is in the package called screen:
publicenumColorName{RED,BLUE,GREEN};
It is possible to use static import declarations in another class to retrieve the enum constants:
importscreen.ColorName;import staticscreen.ColorName.*;publicclassDots{/* The following line is equivalent to 'ColorName foo = ColorName.RED', and it would have been incorrect without the static import. */ColorNamefoo=RED;voidshift(){/* The following line is equivalent to if (foo == ColorName.RED) foo = ColorName.BLUE; */if(foo==RED)foo=BLUE;}}
Module import declaration
Since Java 25, modules can be used in import statements to automatically import all the packages exported by the module.[4] This is done using import module. For example, importmodulejava.sql; is equivalent to
importjava.sql.*;importjavax.sql.*;// Remaining indirect exports from java.logging, java.transaction.xa, and java.xml
Similarly, importmodulejava.base;, similarly, imports all 54 packages belonging to java.base.
importmodulejava.base;/** * importing module java.base allows us to avoid manually importing most classes * the following classes (outside of java.lang) are used: * java.text.MessageFormat * java.util.Date * java.util.List * java.util.concurrent.ThreadLocalRandom */publicclassExample{publicstaticvoidmain(String[]args){List<String>colours=List.of("Red","Orange","Yellow","Green","Blue","Indigo","Violet");IO.println(MessageFormat.format("My favourite colour is {0} and today is {1,date,long}",colours.get(ThreadLocalRandom.current().nextInt(colours.size())),newDate()));}}
Operators
Operators in Java are similar to those in C++. However, there is no delete operator due to garbage collection mechanisms in Java, and there are no operations on pointers since Java does not support them. Another difference is that Java has an unsigned right shift operator (>>>), while C's right shift operator's signedness is type-dependent. Operators in Java cannot be overloaded. The only overloaded operator is operator+ for string concatenation.
Also, a ?: operator can be used in place of simple if statement, for example
inta=1;intb=2;intminVal=(a<b)?a:b;
switch statement
Switch statements in Java can use byte, short, char, and int (not long) primitive data types or their corresponding wrapper types. Starting with J2SE 5.0, it is possible to use enum types. Starting with Java SE 7, it is possible to use Strings.[6] Other reference types cannot be used in switch statements.
Possible values are listed using case labels. These labels in Java may contain only constants (including enum constants and string constants). Execution will start after the label corresponding to the expression inside the brackets. An optional default label may be present to declare that the code following it will be executed if none of the case labels correspond to the expression.
Code for each label ends with the break keyword. It is possible to omit it causing the execution to proceed to the next label, however, a warning will usually be reported during compilation.
switch(ch){case'A':doSomething();// Triggered if ch == 'A'break;case'B':case'C':doSomethingElse();// Triggered if ch == 'B' or ch == 'C'break;default:doSomethingDifferent();// Triggered in any other casebreak;}
switch expressions
Since Java 14 it has become possible to use switch expressions, which use the new arrow syntax:
enumResult{GREAT,FINE,// more enum values}Resultresult=switch(ch){case'A'->Result.GREAT;case'B','C'->Result.FINE;default->thrownewException();};
Alternatively, there is a possibility to express the same with the yield statement, although it is recommended to prefer the arrow syntax because it avoids the problem of accidental fall throughs.
Iteration statements are statements that are repeatedly executed when a given condition is evaluated as true. Since J2SE 5.0, Java has four forms of such statements. The condition must have type boolean or java.lang.Boolean.
For example, the following code is valid in C but results in a compilation error in Java.
In the while loop, the test is done before each iteration.
while(i<10){doSomething();}
do ... while loop
In the do ... while loop, the test is done after each iteration. Consequently, the code is always executed at least once.
// doSomething() is called at least oncedo{doSomething();}while(i<10);
for loop
for loops in Java include an initializer, a condition and a counter expression. It is possible to include several expressions of the same kind using comma as delimiter (except in the condition). However, unlike C, the comma is just a delimiter and not an operator.
for(inti=0;i<10;i++){doSomething();}// A more complex loop using two variablesfor(inti=0,j=9;i<10;i++,j-=3){doSomething();}
Like C, all three expressions are optional. The following loop never terminates:
Foreach loops have been available since J2SE 5.0. This type of loop uses built-in iterators over arrays and collections to return each item in the given collection. Every element is returned and reachable in the context of the code block. When the block is executed, the next item is returned until there are no items remaining. This for loop from Java was later added to C++11. Unlike C#, this kind of loop does not involve a special keyword, but instead uses a different notation style.
for(inti:intArray){doSomething(i);}
Jump statements
Labels
Labels are given points in code used by break and continue statements. The Java goto keyword cannot be used to jump to specific points in code.
start:someMethod();
break statement
The break statement breaks out of the closest loop or switch statement. Execution continues in the statement after the terminated statement, if any.
for(inti=0;i<10;i++){while(true){break;}// Will break to this point}
It is possible to break out of the outer loop using labels:
outer:for(inti=0;i<10;i++){while(true){breakouter;}}// Will break to this point
continue statement
The continue statement discontinues the current iteration of the current control statement and begins the next iteration. The following while loop in the code below reads characters by calling getChar(), skipping the statements in the body of the loop if the characters are spaces:
intch;while(ch==getChar()){if(ch==' '){continue;// Skips the rest of the while-loop}// Rest of the while-loop, will not be reached if ch == ' 'doSomething();}
Labels can be specified in continue statements and break statements:
outer:for(Stringstr:stringsArr){char[]strChars=str.toCharArray();for(charch:strChars){if(ch==' '){/* Continues the outer cycle and the next string is retrieved from stringsArr */continueouter;}doSomething(ch);}}
return statement
The return statement is used to end method execution and to return a value. A value returned by the method is written after the return keyword. If the method returns anything but void, it must use the return statement to return some value.
voiddoSomething(booleanstreamClosed){// If streamClosed is true, execution is stoppedif(streamClosed){return;}readFromStream();}intcalculateSum(inta,intb){intresult=a+b;returnresult;}
return statement ends execution immediately, except for one case: if the statement is encountered within a try block and it is complemented by a finally, control is passed to the finally block.
voiddoSomething(booleanstreamClosed){try{if(streamClosed){return;}readFromStream();}finally{/* Will be called last even if readFromStream() was not called */freeResources();}}
Exception handling statements
try-catch-finally statements
Exceptions are managed within try ... catch blocks.
try{// Statements that may throw exceptionsmethodThrowingExceptions();}catch(Exceptionex){// Exception caught and handled herereportException(ex);}finally{// Statements always executed after the try/catch blocksfreeResources();}
The statements within the try block are executed, and if any of them throws an exception, execution of the block is discontinued and the exception is handled by the catch block. There may be multiple catch blocks, in which case the first block with an exception variable whose type matches the type of the thrown exception is executed.
Java SE 7 also introduced multi-catch clauses besides uni-catch clauses. This type of catch clauses allows Java to handle different types of exceptions in a single block provided they are not subclasses of each other.
try{methodThrowingExceptions();}catch(IOException|IllegalArgumentExceptionex){//Both IOException and IllegalArgumentException will be caught and handled herereportException(ex);}
If no catch block matches the type of the thrown exception, the execution of the outer block (or method) containing the try ... catch statement is discontinued, and the exception is passed up and outside the containing block (or method). The exception is propagated upwards through the call stack until a matching catch block is found within one of the currently active methods. If the exception propagates all the way up to the top-most main method without a matching catch block being found, a textual description of the exception is written to the standard output stream.
The statements within the finally block are always executed after the try and catch blocks, whether or not an exception was thrown and even if a return statement was reached. Such blocks are useful for providing cleanup code that is guaranteed to always be executed.
The catch and finally blocks are optional, but at least one or the other must be present following the try block.
try-with-resources statements
try-with-resources statements are a special type of try-catch-finally statements introduced as an implementation of the dispose pattern in Java SE 7. In a try-with-resources statement the try keyword is followed by initialization of one or more resources that are released automatically when the try block execution is finished. Resources must implement java.lang.AutoCloseable. try-with-resources statements are not required to have a catch or finally block unlike normal try-catch-finally statements.
The throw statement is used to throw an exception and end the execution of the block or method. The thrown exception instance is written after the throw statement.
voidmethodThrowingExceptions(Objectobj){if(obj==null){// Throws exception of NullPointerException typethrownewNullPointerException();}// Will not be called, if object is nulldoSomethingWithObject(obj);}
To make a code block synchronized, it is preceded by the synchronized keyword followed by the lock object inside the brackets. When the executing thread reaches the synchronized block, it acquires a mutual exclusion lock, executes the block, then releases the lock. No threads may enter this block until the lock is released. Any non-null reference type may be used as the lock.
/* Acquires lock on someObject. It must be ofa reference type and must be non-null */synchronized(someObject){// Synchronized statements}
assert statement
assert statements have been available since J2SE 1.4. These types of statements are used to make assertions in the source code, which can be turned on and off during execution for specific classes or packages. To declare an assertion the assert keyword is used followed by a conditional expression. If it evaluates to false when the statement is executed, an exception is thrown. This statement can include a colon followed by another expression, which will act as the exception's detail message.
// If n equals 0, AssertionError is thrownassertn!=0;/* If n equals 0, AssertionError will be thrownwith the message after the colon */assertn!=0:"n was equal to zero";
Primitive types
Primitive types in Java include integer types, floating-point numbers, UTF-16 code units and a Boolean type. Unlike C++ and C#, there are no unsigned types in Java except char type, which is used to represent UTF-16 code units. The lack of unsigned types is offset by introducing unsigned right shift operation (>>>), which is not present in C++, methods such as .toUnsignedInt(). Nevertheless, criticisms have been leveled about the lack of compatibility with C and C++ this causes.[7]
−9,223,372,036,854,775,808 through +9,223,372,036,854,775,807
64-bit (8-byte)
0
byte
java.lang.Byte
integer
-128 through 127
8-bit (1-byte)
0
float
java.lang.Float
floating point number
±1.401298E−45 through ±3.402823E+38
32-bit (4-byte)
0.0
double
java.lang.Double
floating point number
±4.94065645841246E−324 through ±1.79769313486232E+308
64-bit (8-byte)
0.0
boolean
java.lang.Boolean
Boolean
true or false
8-bit (1-byte)
false
char
java.lang.Character
single Unicode character
'\u0000' through '\uFFFF'
16-bit (2-byte)
'\u0000'
void
java.lang.Void
N/A
N/A
N/A
N/A
null has no type, neither primitive nor class. Any object type may store null.
The class Void is used to hold a reference to the Class object representing the keyword void. It cannot be instantiated as void cannot be the type of any object. For example, CompletableFuture<Void> signifies that a CompletableFuture performs a task that does not return a value.
The class java.lang.Number represents all numeric values which can be converted to byte, double, float, int, long, and short.
char does not necessarily correspond to a single character. It may represent a part of a surrogate pair, in which case Unicode code point is represented by a sequence of two char values.
Boxing and unboxing
This language feature was introduced in J2SE 5.0. Boxing is the operation of converting a value of a primitive type into a value of a corresponding reference type, which serves as a wrapper for this particular primitive type. Unboxing is the reverse operation of converting a value of a reference type (previously boxed) into a value of a corresponding primitive type. Neither operation requires an explicit conversion.
Example:
intfoo=42;// Primitive typeIntegerbar=foo;/* foo is boxed to bar, bar is of Integer type, which serves as a wrapper for int */intfoo2=bar;// Unboxed back to primitive type
Reference types
Reference types include class types, interface types, and array types. When the constructor is called, an object is created on the heap and a reference is assigned to the variable. When a variable of an object gets out of scope, the reference is broken and when there are no references left, the object gets marked as garbage. The garbage collector then collects and destroys it some time afterwards.
A reference variable is null when it does not reference any object.
Arrays
Arrays in Java are created at runtime, just like class instances. Array length is defined at creation and cannot be changed.
Due to the nature of the multi-dimensional arrays, sub-arrays can vary in length, so multi-dimensional arrays are not bound to be rectangular unlike C:
int[][]numbers=newint[2][];// Initialization of the first dimension onlynumbers[0]=newint[3];numbers[1]=newint[2];
Classes
Classes are fundamentals of an object-oriented language such as Java. They contain members that store and manipulate data. Classes are divided into top-level and nested. Nested classes are classes placed inside another class that may access the private members of the enclosing class. Nested classes include member classes (which may be defined with the static modifier for simple nesting or without it for inner classes), local classes and anonymous classes.
classFoo{voidbar(){// Local class within a methodclassFoobar{// ...}}}
Anonymous class
classFoo{voidbar(){// Creation of a new anonymous class extending ObjectnewObject(){};}}
Instantiation
Non-static members of a class define the types of the instance variables and methods, which are related to the objects created from that class. To create these objects, the class must be instantiated by using the new operator and calling the class constructor.
Foofoo=newFoo();
Accessing members
Members of both instances and static classes are accessed with the . (dot) operator.
Accessing an instance member Instance members can be accessed through the name of a variable.
Stringfoo="Hello";Stringbar=foo.toUpperCase();
Accessing a static class member Static members are accessed by using the name of the class or any other type. This does not require the creation of a class instance. Static members are declared using the static modifier.
publicclassFoo{publicstaticvoiddoSomething(){// ...}}// Calling the static methodFoo.doSomething();
Modifiers
Modifiers are keywords used to modify declarations of types and type members. Most notably there is a sub-group containing the access modifiers.
abstract - Specifies that a class only serves as a base class and cannot be instantiated.
static - Used only for member classes, specifies that the member class does not belong to a specific instance of the containing class.
final - Classes marked as final cannot be extended from and cannot have any subclasses.
strictfp - Specifies that all floating-point operations must be carried out conforming to IEEE 754 and forbids using enhanced precision to store intermediate results.
By default, all methods in all classes are concrete, unless the abstract keyword is used. An abstract class may include abstract methods, which have no implementation. By default, all methods in all interfaces are abstract, unless the default keyword is used. The default keyword can be used to specify a concrete method in an interface.
//By default, all methods in all classes are concrete, unless the abstract keyword is used.publicabstractclassDemo{// An abstract class may include abstract methods, which have no implementation.publicabstractintsum(intx,inty);// An abstract class may also include concrete methods.publicintproduct(intx,inty){returnx*y;}}//By default, all methods in all interfaces are abstract, unless the default keyword is used.interfaceDemoInterface{intgetLength();//The abstract keyword can be used here, though is completely useless//The default keyword can be used in this context to specify a concrete method in an interfacedefaultintproduct(intx,inty){returnx*y;}}
A final class cannot be subclassed. As doing this can confer security and efficiency benefits, many of the Java standard library classes are final, such as java.lang.System and java.lang.String.
The access modifiers, or inheritance modifiers, set the accessibility of classes, methods, and other members. Members marked as public can be reached from anywhere. If a class or its member does not have any modifiers, default access is assumed.
The following table shows whether code within a class has access to the class or method depending on the accessing class location and the modifier for the accessed class or class member:
Modifier
Same class or nested class
Other class inside the same package
Extended Class inside another package
Non-extended inside another package
private
yes
no
no
no
default (package private)
yes
yes
no
no
protected
yes
yes
yes
no
public
yes
yes
yes
yes
This image describes the class member scope within classes and packages.
Constructors and initializers
A constructor is a special method called when an object is initialized. Its purpose is to initialize the members of the object. The main differences between constructors and ordinary methods are that constructors are called only when an instance of the class is created and never return anything. Constructors are declared as common methods, but they are named after the class and no return type is specified:
classFoo{Stringstr;// Constructor with no argumentsFoo(){}// Constructor with one argumentFoo(Stringstr){this.str=str;}}
Initializers are blocks of code that are executed when a class or an instance of a class is created. There are two kinds of initializers, static initializers and instance initializers.
Static initializers initialize static fields when the class is created. They are declared using the static keyword:
classFoo{static{// Initialization}}
A class is created only once. Therefore, static initializers are not called more than once. On the contrary, instance initializers are automatically called before the call to a constructor every time an instance of the class is created. Unlike constructors instance initializers cannot take any arguments and generally they cannot throw any checked exceptions (except in several special cases). Instance initializers are declared in a block without any keywords:
classFoo{{// Initialization}}
Since Java has a garbage collection mechanism, there are no destructors. However, every object has a finalize() method called prior to garbage collection, which can be overridden to implement finalization.
Methods
All the statements in Java must reside within methods. Methods are similar to functions except they belong to classes. A method has a return value, a name and usually some parameters initialized when it is called with some arguments. Similar to C++, methods returning nothing have return type declared as void. Unlike in C++, methods in Java are not allowed to have default argument values and methods are usually overloaded instead.
classFoo{intbar(inta,intb){return(a*2)+b;}/* Overloaded method with the same name but different set of arguments */intbar(inta){returna*2;}}
A method is called using . notation on an object, or in the case of a static method, also on the name of a class.
Foofoo=newFoo();intresult=foo.bar(7,2);// Non-static method is called on foointfinalResult=Math.abs(result);// Static method call
The throws keyword indicates that a method throws an exception. All checked exceptions must be listed in a comma-separated list.
importjava.io.IOException;importjava.util.zip.DataFormatException;// Indicates that IOException and DataFormatException may be thrownvoidoperateOnFile(Filef)throwsIOException,DataFormatException{// ...}
Modifiers
abstract - Abstract methods can be present only in abstract classes, such methods have no body and must be overridden in a subclass unless it is abstract itself.
static - Makes the method static and accessible without creation of a class instance. However static methods cannot access non-static members in the same class.
final - Declares that the method cannot be overridden in a subclass.
native - Indicates that this method is implemented through JNI in platform-dependent code. Actual implementation happens outside Java code, and such methods have no body.
strictfp - Declares strict conformance to IEEE 754 in carrying out floating-point operations. Now obsolete.
synchronized - Declares that a thread executing this method must acquire monitor. For synchronized methods the monitor is the class instance or java.lang.Class if the method is static.
Access modifiers - Identical to those used with classes.
A final method cannot be overridden or hidden by subclasses.[8] This is used to prevent unexpected behavior from a subclass altering a method that may be crucial to the function or consistency of the class.[9]
A common misconception is that declaring a method as final improves efficiency by allowing the compiler to directly insert the method wherever it is called (see inline expansion). Because the method is loaded at runtime, compilers are unable to do this. Only the runtime environment and JIT compiler know exactly which classes have been loaded, and so only they are able to make decisions about when to inline, whether or not the method is final.[10]
Machine code compilers that generate directly executable, platform-specific machine code, are an exception. When using static linking, the compiler can safely assume that methods and variables computable at compile-time may be inlined.
This language feature was introduced in J2SE 5.0. The last argument of the method may be declared as a variable arity parameter, in which case the method becomes a variable arity method (as opposed to fixed arity methods) or simply varargs method. This allows one to pass a variable number of values, of the declared type, to the method as parameters - including no parameters. These values will be available inside the method as an array.
Fields, or class variables, can be declared inside the class body to store data.
classFoo{doublebar;}
Fields can be initialized directly when declared.
classFoo{doublebar=2.3;}
Modifiers
static - Makes the field a static member.
final - Allows the field to be initialized only once in a constructor or inside initialization block or during its declaration, whichever is earlier.
transient - Indicates that this field will not be stored during serialization.
volatile - If a field is declared volatile, it is ensured that all threads see a consistent value for the variable.
Inheritance
Classes in Java can only inherit from one class. A class can be derived from any class that is not marked as final. Inheritance is declared using the extends keyword. A class can reference itself using the this keyword and its direct superclass using the super keyword.
classFoo{}classFoobarextendsFoo{}
If a class does not specify its superclass, it implicitly inherits from java.lang.Object class. Thus all classes in Java are subclasses of Object class.
If the superclass does not have a constructor without parameters the subclass must specify in its constructors what constructor of the superclass to use. For example:
classFoo{publicFoo(intn){// Do something with n}}classFoobarextendsFoo{privateintnumber;// Superclass does not have constructor without parameters// so we have to specify what constructor of our superclass to use and howpublicFoobar(intnumber){super(number);this.number=number;}}
Overriding methods
Unlike C++, all non-final methods in Java are virtual and can be overridden by the inheriting classes.
An Abstract Class is a class that is incomplete, or is to be considered incomplete, so cannot be instantiated.
A class C has abstract methods if any of the following is true:
C explicitly contains a declaration of an abstract method.
Any of C's superclasses has an abstract method and C neither declares nor inherits a method that implements it.
A direct superinterface of C declares or inherits a method (which is therefore necessarily abstract) and C neither declares nor inherits a method that implements it.
A subclass of an abstract class that is not itself abstract may be instantiated, resulting in the execution of a constructor for the abstract class and, therefore, the execution of the field initializers for instance variables of that class.
This language feature was introduced in J2SE 5.0. Technically enumerations are a kind of class containing enum constants in its body. Each enum constant defines an instance of the enum type. Enumeration classes cannot be instantiated anywhere except in the enumeration class itself.
enumSeason{WINTER,SPRING,SUMMER,AUTUMN}
Enum constants are allowed to have constructors, which are called when the class is loaded:
Interfaces are types which contain no fields and usually define a number of methods without an actual implementation. They are useful to define a contract with any number of different implementations. Every interface is implicitly abstract. Interface methods are allowed to have a subset of access modifiers depending on the language version, strictfp, which has the same effect as for classes, and also static since Java SE 8.
An interface is implemented by a class using the implements keyword. It is allowed to implement more than one interface, in which case they are written after implements keyword in a comma-separated list. A class implementing an interface must override all its methods, otherwise it must be declared as abstract.
interfaceRequestListener{intrequestReceived();}classActionHandlerimplementsActionListener,RequestListener{publicvoidactionSelected(intaction){// ...}publicintrequestReceived(){// ...}}// Calling method defined by interface// ActionHandler can be represented as RequestListener...RequestListenerlistener=newActionHandler();// ...and thus is known to implement requestReceived() methodlistener.requestReceived();
Functional interfaces and lambda expressions
These features were introduced with the release of Java SE 8. An interface automatically becomes a functional interface if it defines only one method. In this case an implementation can be represented as a lambda expression instead of implementing it in a new class, thus greatly simplifying writing code in the functional style. Functional interfaces can optionally be annotated with the @FunctionalInterface annotation, which will tell the compiler to check whether the interface actually conforms to a definition of a functional interface.
// A functional interface@FunctionalInterfaceinterfaceCalculation{intcalculate(intsomeNumber,intsomeOtherNumber);}// A method which accepts this interface as a parameterintrunCalculation(Calculationcalculation){returncalculation.calculate(1,2);}// Using a lambda to call the methodrunCalculation((number,otherNumber)->number+otherNumber);// Equivalent code which uses an anonymous class insteadrunCalculation(newCalculation(){@Overridepublicintcalculate(intsomeNumber,intsomeOtherNumber){returnsomeNumber+someOtherNumber;}})
Lambda's parameters types do not have to be fully specified and can be inferred from the interface it implements. Lambda's body can be written without a body block and a return statement if it is only an expression. Also, for those interfaces which only have a single parameter in the method, round brackets can be omitted.[11]
// Same call as above, but with fully specified types and a body blockrunCalculation((intnumber,intotherNumber)->{returnnumber+otherNumber;});// A functional interface with a method which has only a single parameterinterfaceStringExtender{StringextendString(Stringinput);}// Initializing a variable of this type by using a lambdaStringExtenderextender=input->input+" Extended";
Method references
Java allows method references using the operator :: (it is not related to the C++ namespace qualifying operator ::). It is not necessary to use lambdas when there already is a named method compatible with the interface. This method can be passed instead of a lambda using a method reference. There are several types of method references:
Reference type
Example
Equivalent lambda
Static
Integer::sum
(number, otherNumber) -> number + otherNumber
Bound
"LongString"::substring
index -> "LongString".substring(index)
Unbound
String::isEmpty
string -> string.isEmpty()
Class constructor
ArrayList<String>::new
capacity -> new ArrayList<String>(capacity)
Array constructor
String[]::new
size -> new String[size]
The code above which calls runCalculation could be replaced with the following using the method references:
runCalculation(Integer::sum);
Inheritance
Interfaces can inherit from other interfaces just like classes. Unlike classes it is allowed to inherit from multiple interfaces. However, it is possible that several interfaces have a field with the same name, in which case it becomes a single ambiguous member, which cannot be accessed.
/* Class implementing this interface must implement methods of bothActionListener and RequestListener */interfaceEventListenerextendsActionListener,RequestListener{}
Default methods
Java SE 8 introduced default methods to interfaces which allows developers to add new methods to existing interfaces without breaking compatibility with the classes already implementing the interface. Unlike regular interface methods, default methods have a body which will get called in the case if the implementing class does not override it.
interfaceStringManipulator{StringextendString(Stringinput);// A method which is optional to implementdefaultStringshortenString(Stringinput){returninput.substring(1);}}// This is a valid class despite not implementing all the methodsclassPartialStringManipulatorimplementsStringManipulator{@OverridepublicStringextendString(Stringinput){returnString.format("%s Extended",input);}}
Private methods were added in the Java 9 release. An interface can have a method with a body marked as private, in which case it will not be visible to inheriting classes. It can be called from default methods for the purposes of code reuse.
Annotations in Java are a way to embed metadata into code. This language feature was introduced in J2SE 5.0.
Annotation types
Java has a set of predefined annotation types, but it is allowed to define new ones. An annotation type declaration is a special type of an interface declaration. They are declared in the same way as the interfaces, except the interface keyword is preceded by the @ sign. All annotations are implicitly extended from java.lang.annotation.Annotation and cannot be extended from anything else.
@interfaceBlockingOperations{}
Annotations may have the same declarations in the body as the common interfaces, in addition they are allowed to include enums and annotations. The main difference is that abstract method declarations must not have any parameters or throw any exceptions. Also they may have a default value, which is declared using the default keyword after the method name:
Annotations may be used in any kind of declaration, whether it is package, class (including enums), interface (including annotations), field, method, parameter, constructor, or local variable. Also they can be used with enum constants. Annotations are declared using the @ sign preceding annotation type name, after which element-value pairs are written inside brackets. All elements with no default value must be assigned a value.
Besides the generic form, there are two other forms to declare an annotation, which are shorthands. Marker annotation is a short form, it is used when no values are assigned to elements:
@Unused// Shorthand for @Unused()voidtravelToJupiter(){}
The other short form is called single element annotation. It is used with annotations types containing only one element or in the case when multiple elements are present, but only one elements lacks a default value. In single element annotation form the element name is omitted and only value is written instead:
/* Equivalent for @BlockingOperations(fileSystemOperations = true).networkOperations has a default value anddoes not have to be assigned a value */@BlockingOperations(true)voidopenOutputStream(){}
Generics, or parameterized types, or parametric polymorphism, is one of the major features introduced in J2SE 5.0. Before generics were introduced, it was required to declare all the types explicitly. With generics, it became possible to work in a similar manner with different types without declaring the exact types. The main purpose of generics is to ensure type safety and to detect runtime errors during compilation. Unlike C#, information on the used parameters is not available at runtime due to type erasure.[12]
Classes can be parameterized by adding a type variable inside angle brackets (< and >) following the class name. It makes possible the use of this type variable in class members instead of actual types. There can be more than one type variable, in which case they are declared in a comma-separated list.
It is possible to limit a type variable to a subtype of some specific class or declare an interface that must be implemented by the type. In this case the type variable is appended by the extends keyword followed by a name of the class or the interface. If the variable is constrained by both class and interface or if there are several interfaces, the class name is written first, followed by interface names with & sign used as the delimiter.
/* This class has two type variables, T and V. T must be a subtype of ArrayList and implement Formattable interface */publicclassMapper<TextendsArrayList&Formattable,V>{publicvoidadd(Tarray,Vitem){// array has add method because it is an ArrayList subclassarray.add(item);}}
When a variable of a parameterized type is declared or an instance is created, its type is written exactly in the same format as in the class header, except the actual type is written in the place of the type variable declaration.
/* Mapper is created with CustomList as T and Integer as V.CustomList must be a subclass of ArrayList and implement Formattable */Mapper<CustomList,Integer>mapper=newMapper<CustomList,Integer>();
Since Java SE 7, it is possible to use a diamond (<>) in place of type arguments, in which case the latter will be inferred. The following code in Java SE 7 is equivalent to the code in the previous example:
Mapper<CustomList,Integer>mapper=newMapper<>();
When declaring a variable for a parameterized type, it is possible to use wildcards instead of explicit type names. Wildcards are expressed by writing ? sign instead of the actual type. It is possible to limit possible types to the subclasses or superclasses of some specific class by writing the extends keyword or the super keyword correspondingly followed by the class name.
/* Any Mapper instance with CustomList as the first parametermay be used regardless of the second one.*/Mapper<CustomList,?>mapper;mapper=newMapper<CustomList,Boolean>();mapper=newMapper<CustomList,Integer>();/* Will not accept types that use anything buta subclass of Number as the second parameter */voidaddMapper(Mapper<?,?extendsNumber>mapper){}
Generic methods and constructors
Usage of generics may be limited to some particular methods, this concept applies to constructors as well. To declare a parameterized method, type variables are written before the return type of the method in the same format as for the generic classes. In the case of constructor, type variables are declared before the constructor name.
classMapper{// The class itself is not generic, the constructor is<T,V>Mapper(Tarray,Vitem){}}/* This method will accept only arrays of the same type asthe searched item type or its subtype*/static<T,VextendsT>booleancontains(Titem,V[]arr){for(TcurrentItem:arr){if(item.equals(currentItem)){returntrue;}}returnfalse;}
Generic interfaces
Interfaces can be parameterized in the similar manner as the classes.
interfaceExpandable<TextendsNumber>{voidaddItem(Titem);}// This class is parameterizedclassArray<TextendsNumber>implementsExpandable<T>{voidaddItem(Titem){}}// And this is not and uses an explicit type insteadclassIntegerArrayimplementsExpandable<Integer>{voidaddItem(Integeritem){}}
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