Java Programming/Keywords

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Keywords are special tokens in the language which have reserved use in the language. Keywords may not be used as identifiers in Java - you cannot declare a field whose name is a keyword, for instance.

Examples of keywords are the primitive types, int and boolean; the control flow statements for and if; access modifiers such as public, and special words which mark the declaration and definition of Java classes, packages, and interfaces: class, package, interface.

Below are all the Java language keywords:

• abstract
• assert
• boolean
• break
• byte
• case
• catch
• char
• class
• const
• continue
• default
• do
• double
• else
• enum
• extends
• final
• finally
• float
• for
• goto
• if
• implements
• import
• instanceof
• int
• interface
• long
• native
• new
• package
• private
• protected
• public
• return
• short
• static
• strictfp
• super
• switch
• synchronized
• this
• throw
• throws
• transient
• try
• void
• volatile
• while

abstract

abstract is a Java keyword. ... It declares a class abstract, not all its methods are defined/implemented. Objects are not normally created from an abstract class. Rather, they are created from either a non-abstract subclass, or by having the functions declared alongside a new statement (which is not recommended).

It also declares a method abstract, that is, not implemented in the abstract class. A method cannot be declared abstract in a non abstract class.

Syntact:

public abstract class ClassName
or
abstract public class ClassName

and for methods in an abstract class :

public abstract void methodName();  // --- No body, no implementation ---
or
abstract public void methodName();  // --- No body, no implementation ---

For example:

public abstract ClassName { // --- This method does not have a body; it is abstract. --- public abstract void abstractMethod(); // --- This method does have a body; it is implemented in the abstract class and gives a default behavior. --- public void normalMethod() { ... } }

assert

assert is a Java keyword used to define an assert statement. An assert statement is used to declare an expected boolean condition in a program. If the program is running with assertions enabled, then the condition is checked at runtime. If the condition is false, the Java runtime system throws a AssertionError.

Assertions may be declared using the following syntax:

assert expression1 [: expression2];

expression1 is a boolean that will throw the assertion if it is false. When it is thrown, the assertion error exception is created with the parameter expression2 (if applicable).

An example:

assert list != null && list.size() > 0; Object value = list.get(0);

Assertions are usually used as a debugging aid. They should not be used instead of validating arguments to public methods, or in place of a more precise runtime error exception.

Assertions are enabled with the Java -ea or -enableassertions runtime option. See your Java environment documentation for additional options for controlling assertions.

boolean

boolean is a keyword which designates the boolean primitive type. There are only two possible boolean values: true and false. The default value for boolean fields is false.

The following is a declaration of a private boolean field named initialized, and its use in a method named synchronized.

private boolean initialized = false; public synchronized void {    if ( !initialized ) {       connection = connect();       initialized = true;    } }

Note that there is no automatic conversion between integer types (such as int) to boolean as is possible in some languages like C. Instead, one must use an equivalent expression such as (i != 0) which evaluates to true if i is not zero.

break

break is a Java keyword.

Jumps (breaks) out from a loop. Also used at switch statement.

For example:

for ( int i=0; i < maxLoopIter; i++ ) {    System.out.println("Iter=" +i);    if ( i == 5 ) {       break;  // -- 5 iteration is enough --    } }

See also:

• Java Programming/Keywords/switch

byte

byte is a keyword which designates the 8 bit signed integer primitive type.

The java.lang.Byte class is the nominal wrapper class when you need to store a byte value but an object reference is required.

Syntax:

byte <variable-name> = <integer-value>;

For example:

byte b = 65;

or

byte b = 'A'

The number 65 is the code for 'A' in ASCII.

See also:

• Java Programming/Primitive Types

case

case is a Java keyword.

This is part of the switch statement, to find if the value passed to the switch statement matches a value followed by case.

For example:

int i = 3; switch(i) { case 1:    System.out.println("The number is 1.");    break; case 2:    System.out.println("The number is 2.");    break; case 3:    System.out.println("The number is 3."); // this line will print    break; case 4:    System.out.println("The number is 4.");    break; case 5:    System.out.println("The number is 5.");    break; default:   System.out.println("The number is not 1, 2, 3, 4, or 5."); }

catch

catch is a keyword.

It's part of a try block. If an exception is thrown inside a try block, the exception will be compared to any of the catch part of the block. If the exception match with one of the exception in the catch part, the exception will be handled there.

For example:

try {    //...      throw new MyException_1();    //... } catch ( MyException_1 e ) {    // --- Handle the Exception_1 here -- } catch ( MyException_2 e ) {    // --- Handle the Exception_2 here -- }

See also:

• Java Programming/Keywords/try

char

char is a keyword. It defines a character primitive type.

The java.lang.Character class is the nominal wrapper class when you need to store a char value but an object reference is required.

Syntax:

char <variable-name> = '<character>';

For example:

char oneChar1 = 'A'; char oneChar2 = 65;

65 is the numeric representation of character 'A' , or its ASCII code.

System.out.println( oneChar1 ); System.out.println( oneChar2 );

outputs the following:

A
A

See also:

• Java Programming/Primitive Types

class

class is a Java keyword which begins the declaration and definition of a class.

The general syntax of a class declaration, using Extended Backus-Naur Form, is

class-declaration ::= [access-modifiers] class identifier
                      [extends-clause] [implements-clause] 
                      class-body

extends-clause ::= extends class-name
implements-clause ::= implements interface-names 
interface-names ::= interface-name [, interface-names]
class-body ::= { [member-declarations] }
member-declarations = member-declaration [member-declarations]
member-declaration = field-declaration 
                     | initializer 
                     | constructor 
                     | method-declaration
                     | class-declaration

The extends word is optional. If omitted, the class extends the Object class, as all Java classes inherit from it.

See also:

• Java Programming/Keywords/new

const

const is a reserved keyword, presently not being used.

In other programming languages, such as C, const is often used to declare a constant. However, in Java, final is used instead.

continue

continue is a Java keyword. It skips the remainder of the loop and continues with the next iteration.

For example:

for ( int i=0; i < maxLoopIter; i++ ) {    if ( i == 5 ) {       continue;  // -- 5 iteration is skipped --    }    System.println("Iteration = " +i); }

See also:

• Java Programming/Statements

default

default is a Java keyword.

This is an optional part of the switch statement, which only executes if none of the above cases are matched.

See also:

• Java Programming/Keywords/switch

do

do is a Java keyword.

It starts a do-while looping block. The do-while loop is functionally similar to the while loop, except the condition is evaluated after the statements execute

Syntax:

do { //statements; } while (condition);

For example:

do { i++; } while ( i < maxLoopIter );

See also:

• Java Programming/Statements
• Java Programming/Keywords/for
• Java Programming/Keywords/while

double

double is a keyword which designates the 64 bit float primitive type.

The java.lang.Double class is the nominal wrapper class when you need to store a double value but an object reference is required.

Syntax:

double <variable-name> = <float-value>;

For example:

double d = 65.55;

See also:

• Java Programming/Primitive Types

else

else is a Java keyword. It is an optional part of a branching statement. It starts the 'false' statement block.

The general syntax of a if, using Extended Backus-Naur Form, is

branching-statement ::= if condition-clause 
                                    single-statement | block-statement
                       [ else 
                                    single-statement | block-statement ]
 
condition-clause    ::= ( Boolean Expression )
single-statement    ::= Statement
block-statement     ::= { Statement [ Statement ] }

For example:

if ( expression ) {    System.out.println("'True' statement block"); } else {    System.out.println("'False' statement block"); }

See also:

• Java Programming/Keywords/if

enum

enum is a keyword since Java 1.5.

This keyword defines a group of constants.

For example:

/** Grades of courses */   enum Grade { A, B, C, D, F };   // ...   private Grade gradeA = Grade.A;

This enum constant then can be passed in to methods:

student.assignGrade(gradeA);   /**    * Assigns the grade for this course to the student    * @param GRADE  Grade to be assigned    */   public void assignGrade(final Grade GRADE) {     grade = GRADE;   }

An enum may also have parameters:

public enum DayOfWeek {   /** Enumeration constants */   MONDAY(1), TUESDAY(2), WEDNESDAY(3), THURSDAY(4), FRIDAY(5), SATURDAY(6), SUNDAY(0);   /** Code for the days of the week */   private byte dayCode = 0;   /**    * Private constructor    * @param VALUE  Value that stands for a day of the week.    */   private DayOfWeek(final byte VALUE) {     dayCode = java.lang.Math.abs(VALUE%7);   }     /**    * Gets the day code    * @return  The day code    */   public byte getDayCode() {     return dayCode;   } }

An enum may also implement interfaces besides java.lang.Comparable and java.io.Serializable, which it already implicitly implements:

public enum DayOfWeek implements Runnable {   MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY, SUNDAY;     /**    * Run method prints all elements    */   public void run() {     System.out.println("name() = " + name() +       ", toString() = \"" + toString() + "\"");   } }

extends

extends is a Java keyword.

Used in class and interface definition to declare the class or interface that is to be extended.

Syntax:

public class MyClass extends SuperClass {   //... }   public interface MyInterface extends SuperInterface {   //... }

In Java 1.5 and later, the "extends" keyword is also used to specify an upper bound on a type parameter in Generics.

class Foo<T extends Number> { /*...*/ }

See also:

• Java Programming/Creating Objects
• Java Programming/Keywords/class

final

final is a keyword.

It has more than one meaning depending whether it used for a class, a method, or for a variable.

• It is denoting that a variable is to be constant. This is similar to const in other languages. A variable declared with the final keyword cannot be modified by the program after initialization. This is useful for universal constants, such as pi, or other values that the programmer would like to keep constant throughout the code.
• It marks a method final, meaning that subclasses can not override this method. The compiler checks and gives an error if you try to override the method.
• It marks a class final, meaning that the class can not be subclassed.

For example:

static final double PI = 3.1415926;   final public void method() {    //... }   final public class MyClass {    //... }

Note that final is always placed before the variable type in a constant, before the return type of a method, and before the keyword class in a class header.

See also:

• Java Programming/Access Modifiers

finally

finally is a keyword.

It is an optional part of a try block. The code inside the finally block will always be executed. This is also true for cases when there is an exception or even executed return statement in the try block.

Three things can happen in a try block:

• No exception is thrown. In such a case, the following are executed:
  • code in the try block
  • code in the finally block
  • code after the try-catch block
• An exception is thrown and a matching catch block found. In such a case, the following are executed:
  • code in the try block until where the exception occurred
  • code in the matched catch block
  • code in the finally block
  • code after the try-catch block
• An exception is thrown and no matching catch block exists. In such a case, the following are executed:
  • code in the try block until where the exception occurred
  • code in the finally block

Note: in that third and final case, NO CODE after the try-catch block is executed.

For example:

public void method() throws NoMatchedException {    try {    //...      throw new MyException_1();    //...    } catch ( MyException_1 e ) {      // --- Handle the Exception_1 here --    } catch ( MyException_2 e ) {      // --- Handle the Exception_2 here --    } finally {      // --- This will always be executed no matter what --    }    // --- Code after the try-catch block }

Note: if there is an exception that happens before the try-catch block, the finally block is not executed.

If return statement is used inside finally, it overrides the return statement in the try-catch block. For instance, the construct

try {    return 11;  }  finally {    return 12;  }

will return 12, not 11. Professional code almost never contains statements that alter execution order (like return, break, continue) inside the finally block, as such code is more difficult to read and maintain.

See also:

• Java Programming/Keywords/try
• Java Programming/Keywords/catch
• Java Programming/Throwing and Catching Exceptions#Catching Rule

float

float is a keyword which designates the 32 bit float primitive type.

The java.lang.Float class is the nominal wrapper class when you need to store a float value but an object reference is required.

Syntax:

float <variable-name> = <float-value>;

For example:

float price = 49.95;

See also:

• Java Programming/Primitive Types

for

for is a Java keyword.

It starts a looping block.

The general syntax of a for, using Extended Backus-Naur Form, is

for-looping-statement ::= for condition-clause 
                                    single-statement | block-statement
 
condition-clause    ::= ( before-statement;  Boolean Expression ; after-statement )
single-statement    ::= Statement
block-statement     ::= { Statement [ Statement ] }

For example:

for ( int i=0; i < maxLoopIter; i++ ) {     System.println("Iter: " +i); }

See also:

• Java Programming/Keywords/while
• Java Programming/Keywords/do

goto

goto is a reserved keyword, presently not being used.

if

if is a Java keyword. It starts a branching statement.

The general syntax of a if, using Extended Backus-Naur Form, is

branching-statement ::= if condition-clause 
                                    single-statement | block-statement
                       [ else 
                                    single-statement | block-statement ]
 
condition-clause    ::= ( Boolean Expression )
single-statement    ::= Statement
block-statement     ::= { Statement [ Statements ] }

For example:

if ( boolean Expression ) {    System.out.println("'True' statement block"); } else {    System.out.println("'False' statement block"); }

See also:

• Java Programming/Keywords/else

implements

implements is a Java keyword.

Used in class definition to declare the Interfaces that are to be implemented by the class.

Syntax:

public class MyClass implements MyInterface1, MyInterface2  {    ...  }

See also:

• Java Programming/Creating Objects
• Java Programming/Keywords/class
• Java Programming/Keywords/interface

import

import is a Java keyword.

It declares a Java class to use in the code below the import statement. Once a Java class is declared, then the class name can be used in the code without specifying the package the class belongs to.

Use the '*' character to declare all the classes belonging to the package.

Syntax:

import package.JavaClass; import package.*;

The static import construct allows unqualified access to static members without inheriting from the type containing the static members:

   import static java.lang.Math.PI;

Once the static members have been imported, they may be used without qualification:

   double r = cos(PI * theta);

Caveat: use static import very sparingly to avoid polluting the program's namespace!

See also:

• Java Programming/Packages

instanceof

instanceof is a keyword.

It checks if an object reference is an instance of a type, and returns a boolean value;

The <object-reference> instanceof Object will return true for all non-null object references, since all Java objects are inherited from Object. instanceof will always return false if <object-reference> is null.

Syntax:

<object-reference> instanceof TypeName

For example:

class Fruit  {   //...  }  class Apple extends Fruit  {   //...  }  class Orange extends Fruit  {   //...  }  public class Test  {     public static void main(String[] args)     {        Collection<Object> coll = new ArrayList<Object>();          Apple app1 = new Apple();        Apple app2 = new Apple();        coll.add(app1);        coll.add(app2);          Orange or1 = new Orange();        Orange or2 = new Orange();        coll.add(or1);        coll.add(or2);          printColl(coll);     }       private static String printColl( Collection<?> coll )     {        for (Object obj : coll)        {           if ( obj instanceof Object )           {              System.out.print("It is a Java Object and");           }           if ( obj instanceof Fruit )           {              System.out.print("It is a Fruit and");           }           if ( obj instanceof Apple )           {              System.out.println("it is an Apple");           }           if ( obj instanceof Orange )           {              System.out.println("it is an Orange");           }        }     }  }

Run the program:

java Test

The output:

"It is a Java Object and It is a Fruit and it is an Apple"
"It is a Java Object and It is a Fruit and it is an Apple"
"It is a Java Object and It is a Fruit and it is an Orange"
"It is a Java Object and It is a Fruit and it is an Orange"

Note that the instanceof operator can also be applied to interfaces. For example, if the example above was enhanced with the interface

interface Edible {  //... }

and the the classes modified such that they implemented this interface

class Orange extends Fruit implements Edible  {   ...  }

we could ask if our object were edible.

if ( obj instanceof Edible )  {    System.out.println("it is edible");  }

int

int is a keyword which designates the 32 bit signed integer primitive type.

The java.lang.Integer class is the nominal wrapper class when you need to store an int value but an object reference is required.

Syntax:

int <variable-name> = <integer-value>;

For example:

int i = 65;

See also:

• Java Programming/Primitive Types

interface

interface is a Java keyword. It starts the declaration of a Java Interface.

For example:

public interface SampleInterface {    public void method1();    //... }

See also:

• Java Programming/Keywords/new

long

long is a keyword which designates the 64 bit signed integer primitive type.

The java.lang.Long class is the nominal wrapper class when you need to store a long value but an object reference is required.

Syntax:

long <variable-name> = <integer-value>;

For example:

long timestamp = 1269898201;

See also:

• Java Programming/Primitive Types

native

native is a java keyword.

It marks a method, that it will be implemented in other languages, not in Java. It works together with JNI(Java Native Interface)

Syntax:

[public|protected|private] native method();

Native methods were used in the past to write performance critical sections but with java getting faster this is now less common. Native methods are currently needed when

• You need to call from java a library, written in other language.
• You need to access system or hardware resources that are only reachable from the other language (typically C). Actually, many system functions that interact with real computer (disk and network IO, for instance) can only do this because they call native code.

To complete writing native method, you need to process your class with javah tool that will generate a header code in C. You then need to provide implementation of the header code, produce dynamically loadable library (.so under Linux, .dll under Windows) and load it (in the simplest case with System.load(library_file_name) . The code completion is trivial if only primitive types like integers are passed but gets more complex if it is needed to exchange strings or objects from the C code. In general, everything can be on C level, including creation of the new objects and calling back methods, written in java.

To call the code in some other language (including C++), you need to write a bridge from C to that language. This is usually trivial as most of languages are callable from C.

See also

• [1] - JNI programming tutorial.
• [2] - JNI specification.

new

new is a Java keyword. It creates a Java object and allocates memory for it on the heap. new is also used for array creation, as arrays are also objects.

Syntax:

<JavaType> <variable> = new <JavaObject>();

For example:

LinkedList list = new LinkedList(); int[] intArray = new int[10]; String[][] stringMatrix = new String[5][10];

See also:

• Java Programming/Creating Objects

package

package is a Java keyword. It declares a 'name space' for the Java class. It must be put at the top of the Java file, it should be the first Java statement line.

To ensure that the package name will be unique across vendors, usually the company url is used starting in backword.

Syntax:

package package;

For example:

package com.mycompany.myapplication.mymodule;

See also:

• Java Programming/Packages
• Java Programming/Keywords/import

private

private is a Java keyword which declares a member's access as private. That is, the member is only visible within the class, not from any other class (including subclasses). The visibility of private members extends to nested classes.

Please note: Because access modifiers are not handled at instance level but at class level, private members of an object are visible from other instances of the same class!

Syntact:

private void method();

See also:

• Java Programming/Access Modifiers

protected

protected is a Java keyword.

This keyword is an access modifier, used before a method or other class member to signify that the method or variable can not only be accessed by elements residing in its own class or classes in the same package (as it would be for the default visibility level) but moreover from subclasses of its own class, including subclasses in foreign packages (if the access is made on an expression, whose type is the type of this subclass).

Syntax:

protected <returnType> <methodName>(<parameters>);

For example:

protected int getAge(); protected void setYearOfBirth(int year);

See also:

• Java Programming/Access Modifiers

public

public is a Java keyword which declares a member's access as public. Public members are visible to all other classes. This means that any other class can access a public field or method. Further, other classes can modify public fields unless the field is declared as final.

A best practice is to give fields private access and reserve public access to only the set of methods and final fields that define the class' public constants. This helps with encapsulation and information hiding, since it allows you to change the implementation of a class without affecting the consumers who use only the public API of the class.

Below is an example of an immutable public class named Length which maintains private instance fields named units and magnitude but provides a public constructor and two public accessor methods.

package org.wikibooks.java; public class Length {    private double magnitude;    private String units;    public Length(double magnitude, String units) {       if (units == null || units.trim().length() == 0) {           throw new IllegalArgumentException("non-null, non-empty units required.");       }       this.magnitute = magnitude;       this.units = units;    }    public double getMagnitude() {       return magnitude;    }    public String getUnits() {       return units;    } }

return

return is a Java keyword.

Returns a primitive value, or an object reference, or nothing(void). It does not return object values, only object references.

Syntax:

return variable;  // --- Returns variable
or
return;           // --- Returns nothing

short

short is a keyword. It defines a 16 bit signed integer primitive type.

Syntax:

short <variable-name> = <integer-value>;

For example:

short age = 65;

See also:

• Java Programming/Primitive Types

static

static is a keyword that states that all instances of a given class are to share the same variable/method. This is used for a constant variable or a method that is the same for every instance of a class, such as the methods in the Math class. The main method of a class is generally labelled static. The static keyword can also be used in the context of defining inner classes. These classes are not bound to an instance of the outer defining class.

No object needs to be created to use static variables or call static methods. Just put the class name before the static variable or method to use them.

Static methods cannot call non static methods. The this current object reference is also not available in static methods.

Syntax for variables:

public static variableName;

Syntax for methods :

public static void methodName() { /*...*/ }

To access them :

ClassName.variableName = 10;
ClassName.methodName();

For example:

public static final double pi = 3.14159; public static void main(String[] args) {    //... }

strictfp

strictfp is a java keyword, since Java 1.2 .

It makes sure that floating point calculations result precisely the same regardless of the underlying operating system and hardware platform, even if more precision could be obtained. This is compatible with the earlier version of Java 1.1 . If you need that use it.

Syntax for classes:

public strictfp class MyClass 
{ 
  //...
}

Syntax for methods:

public strictfp void method() 
{ 
  ...
}

See also:

• http://en.wikipedia.org/wiki/Strictfp

super

super is a keyword.

• It is used inside a sub-class method definition to call a method defined in the super class. Private methods of the super-class can not be called. Only public and protected methods can be called by the super keyword.
• It is also used by class constructors to invoke constructurs of its parent class.

Syntax:

super.<method-name>();

For example:

public class SuperClass  {     public void printHello()     {        System.out.println( <code><font style="color:DarkGreen; text-decoration: none; font-weight: bold;">&quot;Hello from SuperClass&quot;</font></code> );       return;     }  }  //...  public class SubClass extends SuperClass  {     public void printHello()     {        super.printHello();        System.out.println( <code><font style="color:DarkGreen; text-decoration: none; font-weight: bold;">&quot;Hello from SubClass&quot;</font></code> );       return;     }       public static main( String[] args )     {        SubClass obj = new SubClass();        obj.printHello();     }  }

Running the above program:

Java SubClass

The output:

"Hello from SuperClass"
"Hello from SubClass"

See also:

• Java Programming/Keywords/extends

In Java 1.5 and later, the "super" keyword is also used to specify a lower bound on a wildcard type parameter in Generics.

public void sort(Comparator<? super T> comp) { ... }

switch

switch is a Java keyword.

It is a branching operation, based on a number. The 'number' must be either char, byte, short, or int primitive type.

Syntax:

switch ( <integer-var> )
{
   case <label1>: <statements>;
   case <label2>: <statements>;
   ...
   case <labeln>: <statements>;
   default: <statements>;
}

When the <integer-var> value match one of the <label>, then: The statements after the matched label will be executed including the following label's statements, until the end of the switch block, or until a break keyword is reached.

For example:

int var = 3; switch ( var ) {    case 1:       System.out.println( "Case: 1" );       System.out.println( "Execute until break" );       break;                        case 2:       System.out.println( "Case: 2" );       System.out.println( "Execute until break" );       break;    case 3:       System.out.println( "Case: 3" );       System.out.println( "Execute until break" );       break;        case 4:       System.out.println( "Case: 4" );       System.out.println( "Execute until break" );       break;          default:       System.out.println( "Case: default" );       System.out.println( "Execute until break" );       break;           }

The output from the above code is:

Case: 3
Execute until break

The same code can be written with if-else blocks":

int var = 3; if ( var == 1 ) {    System.out.println( "Case: 1" );    System.out.println( "Execute until break" ); } else if ( var == 2 ) {    System.out.println( "Case: 2" );    System.out.println( "Execute until break" ); } else if ( var == 3 ) {    System.out.println( "Case: 3" );    System.out.println( "Execute until break" ); } else if ( var == 4 ) {    System.out.println( "Case: 4" );    System.out.println( "Execute until break" ); } else {    // -- This is the default part --    System.out.println( "Case: default" );    System.out.println( "Execute until break" ); }

See also:

• Java Programming/Keywords/if

synchronized

synchronized is a keyword.

It marks a 'critical section'. A 'critical section' is where one and only one thread is executing. So to enter into the marked code the threads are 'synchronized', only one can enter, the others have to wait. For more information see Synchronizing Threads Methods or [3].

The synchronized keyword can be used in two ways:

• Mark a method synchronized
• Create a synchronized block

The syntax to mark a method synchronized is

public synchronized void method()
{
  // Thread.currentThread() has a lock on this object, i.e. a synchronized method is the same as 
  // calling { synchronized(this) {…} }.
}

A synchronized block is marked as

synchronized ( <object_reference> )
{
  // Thread.currentThread() has a lock on object_reference. All other threads trying to access it will
  // be blocked until the current thread releases the lock.
}

Singleton example

As an example, we can show a thread-safe version of a singleton:

/**
 * The singleton class that can be instantiated only once with lazy instantiation
 */
public class Singleton {
    /** Static class instance */
    private volatile static Singleton instance = null;
 
    /**
     * Standard private constructor
     */
    private Singleton() {
        // Some initialisation
    }
 
    /**
     * Getter of the singleton instance
     * @return The only instance
     */
    public static Singleton getInstance() {
        if (instance == null) {
            // If the instance does not exist, go in time-consuming
            // section:
            synchronized (Singleton.class) {
                if (instance == null) instance = new Singleton();
            }
        }
 
        return instance;
    }
 }

this

this is a Java keyword. It contains the current object reference.

Syntax:

this.method();
or
this.variable;

Example #1:

public class MyClass  {     //...     private String _memberVar;     //...     public void setMemberVar( String value )     {         this._memberVar= value;     }  }

Example #2:

public class MyClass  {     MyClass(int a, int b) {         System.out.println("int a: " + a);         System.out.println("int b: " + b);     }     MyClass(int a) {         this(a, 0);     }     //...     public static void main(String[] args) {         new MyClass(1, 2);         new MyClass(5);     }  }

throw

throw is a keyword. It 'throws' an exception.

Syntax:

throw <Exception Ref>;

For example:

public Customer findCustomer( String name ) throws '''CustomerNotFoundException'''  {     Customer custRet = null;       Iterator iter = _customerList.iterator();     while ( iter.hasNext() )     {         Customer cust = (Customer) iter.next();         if ( cust.getName().equals( name ) )         {            // --- Customer find --            custRet = cust;            break;         }      }      if ( custRet == null )      {         // --- Customer not found ---         throw new '''CustomerNotFoundException'''( "Customer "+ name + "was not found" );      }       return custRet   }

See also:

• Java Programming/Keywords/throws

throws

throws is a Java keyword. It is used in a method definition to declare the Exceptions to be thrown by the method.

Syntax:

public myMethod() throws MyException1, MyException2
{
  ...
}

Example:

class MyDefinedException extends Exception  {   public MyDefinedException(String str)   {      super(str);   }    }  public class MyClass  {     public static void showMyName(String str) throws MyDefinedException     {           if(str.equals("What is your Name?"))                 throw new MyDefinedException("My name is Blah Blah");     }     public static void main(String a[])     {        try        {           showMyName("What is your Name?");        }        catch(MyDefinedException mde)        {           mde.printStackTrace();        }      }  }

transient

transient is a Java keyword which marks a member variable not to be serialized when it is persisted to streams of bytes. When an object is transferred through the network, the object needs to be 'serialized'. Serialization converts the object state to serial bytes. Those bytes are sent over the network and the object is recreated from those bytes. Member variables marked by the java transient keyword are not transferred, they are lost intentionally.

Syntax:

private transient <member-variable>;
or 
transient private <member-variable>;

For example:

public class Foo  {    private String saveMe;    private transient String dontSaveMe;    private transient String password;    //...  }

See also:

• Java language specification reference: jls
• Serializable Interface. Serializable

try

try is a keyword.

It starts a try block. If an Exception is thrown inside a try block, the Exception will be compared any of the catch part of the block. If the Exception match with one of the Exception in the catch part, the exception will be handled there.

Three things can happen in a try block:

• No exception is thrown:
  • the code in the try block
  • plus the code in the finally block will be executed
  • plus the code after the try-catch block is executed
• An exception is thrown and a match is found among the catch blocks:
  • the code in the try block until the exception occurred is executed
  • plus the matched catch block is executed
  • plus the finally block is executed
  • plus the code after the try-catch block is executed
• An exception is thrown and no match found among the catch blocks:
  • the code in the try block until the exception occurred is executed
  • plus the finally block is executed
  • NO CODE after the try-catch block is executed

For example:

public void method() throws NoMatchedException  {    try {      //...      throw new '''MyException_1'''();      //...    } catch ( MyException_1 e ) {      // --- '''Handle the Exception_1 here''' --    } catch ( MyException_2 e ) {      // --- Handle the Exception_2 here --    } finally {      // --- This will always be executed no matter what --    }    // --- Code after the try-catch block  }

How the catch-blocks are evaluated see Catching Rule

See also:

• Java Programming/Keywords/catch
• Java Programming/Keywords/finally
• Java Programming/Throwing and Catching Exceptions#Catching Rule

void

void is a Java keyword.

Used at method declaration and definition to specify that the method does not return any type, the method returns void. It is not a type and there is no void references/pointers as in C/C++.

For example:

public void method()  {    //...    return;   // -- In this case the return is optional  }

See also:

• Java Programming/Keywords/return

volatile

volatile is a keyword.

When member variables are marked with this keyword, it changes the runtime behavior in a way that is noticeable when multiple threads access these variables. Without the volatile keyword, one thread could observe another thread update member variables in an order that is not consistent with what is specified in sourcecode. Unlike the synchronized keyword, concurrent access to a volatile field is allowed.

Syntax:

private volatile <member-variable>;
or 
volatile private <member-variable>;

For example:

private volatile changingVar;

See also:

• Java Programming/Keywords/synchronized

while

while is a Java keyword.

It starts a looping block.

The general syntax of a while, using Extended Backus-Naur Form, is

while-looping-statement ::= while condition-clause 
                                    single-statement | block-statement
 
condition-clause    ::= ( Boolean Expression )
single-statement    ::= Statement
block-statement     ::= { Statement [ Statements ] }

For example:

while ( i < maxLoopIter )  {     System.println("Iter=" +i++);  }

See also:

• Java Programming/Statements
• Java Programming/Keywords/for
• Java Programming/Keywords/do