List of Java Keywords

In the Java programming language, a keyword is one of 50 reserved words that have a predefined meaning in the language; because of this, programmers cannot use keywords as names for variables, methods, classes, or as any other identifier. Due to their special functions in the language, most integrated development environments such as NetBeans, Eclipse IDE for Java use syntax highlighting to display keywords in a different colour for easy identification.

 

Keywords Usage of Keywords
abstract In front of a `class` keyword, prevents this class to be directly instantiated. In front of a method signature, allows the implementation of this method to be deferred to an inheriting class.
assert Assert describes a predicate (a true–false statement) placed in a java-program to indicate that the developer thinks that the predicate is always true at that place. If an assertion evaluates to false at run-time, an assertion failure results, which typically causes execution to abort. Optionally enable by ClassLoader method.
boolean Defines a boolean variable for the values “true” or “false” only (NB: “null” as in class Boolean is not allowed).
break Used to end the execution in the current loop body.
byte Defines a byte variable representing a sequence of 8 bits. (NB: Only 1-byte-characters can be used, f.i. ‘€’ would produce an error).
case A statement in the switch block can be labeled with one or more case or default labels. The switch statement evaluates its expression, then executes all statements that follow the matching case label; see switch.
catch Used in conjunction with a try block and an optional finally block. The statements in the catch block specify what to do if a specific type of exception is thrown by the try block.
char Defines a character variable capable of holding any character of the java source file’s character set (NB: Physical storage two byte).
class A class is a group of objects that has common properties. It is a template or blueprint from which objects are created.
const Although reserved as a keyword in Java, Const is not used and has no function. For defining constants in java, see the ‘final’ reserved word.
continue Used to resume program execution at the end of the current loop body. If followed by a label, continue resumes execution at the end of the enclosing labeled loop body.
default The default keyword can optionally be used in a switch statement to label a block of statements to be executed if no case matches the specified value. It is also a type of access specifier.
do The do keyword is used in conjunction with while to create a do-while loop, which executes a block of statements associated with the loop and then tests a boolean expression associated with the while. If the expression evaluates to true, the block is executed again; this continues until the expression evaluates to false.
double The double keyword is used to declare a variable that can hold a 64-bit double precision floating-point number. This keyword is also used to declare that a method returns a value of the primitive type double.
else The else keyword is used in conjunction with if to create an if-else statement, which tests a boolean expression; if the expression evaluates to true, the block of statements associated with the if are evaluated; if it evaluates to false, the block of statements associated with the else are evaluated.
enum A Java keyword used to declare an enumerated type. Enumerations extend the base class.
extends The extends keyword indicates that you are making a new class that derives from an existing class. It is a concept from inheritance.
final Define an entity once that cannot be changed nor derived from later. More specifically: a final class cannot be subclasses, a final method cannot be overridden, and a final variable can occur at most once as a left-hand expression on an executed command. All methods in a final class are implicitly.
finally Used to define a block of statements for a block defined previously by the try keyword. The finally block is executed after execution exits the try block and any associated catch clauses regardless of whether an exception was thrown or caught, or execution left method in the middle of the try or catch blocks using the return keyword.
float The float keyword is used to declare a variable that can hold a 32-bit single precision floating-point number. This keyword is also used to declare that a method returns a value of the primitive type.
for The for keyword is used to create a for loop, which specifies a variable initialization, a boolean expression, and an incrementation. The variable initialization is performed first, and then the boolean expression is evaluated. If the expression evaluates to true, the block of statements associated with the loop are executed, and then the incrementation is performed. The boolean expression is then evaluated again; this continues until the expression evaluates to false.

The for keyword can also be used to create a so-called “enhanced for loop”, which specifies an array or Iterable object; each iteration of the loop executes the associated block of statements using a different element in the array or Iterable.

 

goto Although reserved as a keyword in Java, goto is not used and has no function.
if The if keyword is used to create an if statement, which tests a boolean expression; if the expression evaluates to true, the block of statements associated with the if statement is executed. This keyword can also be used to create an if-else statement.
implements Included in a class declaration to specify one or more interfaces that are implemented by the current class. A class inherits the types and abstract methods declared by the interfaces.
import Used at the beginning of a source file to specify classes or entire Java packages to be referred to later without including their package names in the reference. The import statements can import static members of a class.
instanceof A binary operator that takes an object reference as its first operand and a class or interface as its second operand and produces a boolean result. The instanceof operator evaluates to true if and only if the runtime type of the object is assignment compatible with the class or interface.
int The int keyword is used to declare a variable that can hold a 32-bit signed two’s complement integer. This keyword is also used to declare that a method returns a value of the primitive type int.
interface Used to declare a special type of class that only contains abstract methods, constant (static final) fields and static interfaces. It can later be implemented by classes that declare the interface with the implements keyword.
long The long keyword is used to declare a variable that can hold a 64-bit signed two’s complement integer. This keyword is also used to declare that a method returns a value of the primitive type long.
native Used in method declarations to specify that the method is not implemented in the same Java source file, but rather in another language.
new Used to create an instance of a class or array object. Using keyword for this end is not completely necessary (as exemplified by Scala), though it serves two purposes: it enables the existence of different namespace for methods and class names, it defines statically and locally that a fresh object is indeed created, and of what runtime type it is (arguably introducing dependency into the code).
package A group of types. Packages are declared with the package keyword.
private The private keyword is used in the declaration of a method, field, or inner class; private members can only be accessed by other members of their own class.
protected The protected keyword is used in the declaration of a method, field, or inner class; protected members can only be accessed by members of their own class, that class’s subclasses or classes from the same package.
public The public keyword is used in the declaration of a class, method, or field; public classes, methods, and fields can be accessed by the members of any class.
return Used to finish the execution of a method. It can be followed by a value required by the method definition that is returned to the caller.
short The short keyword is used to declare a field that can hold a 16-bit signed two’s complement integer. This keyword is also used to declare that a method returns a value of the primitive type short.
static Used to declare a field, method, or inner class as a class field. Classes maintain one copy of class fields regardless of how many instances exist of that class. static also is used to define a method as a class method. Class methods are bound to the class instead of to a specific instance, and can only operate on class fields. (Classes and interfaces declared as static members of another class or interface are actually top-level classes and are not inner classes.)
strictfp A Java keyword used to restrict the precision and rounding of floating point calculations to ensure portability.
super Used to access members of a class inherited by the class in which it appears. Allows a subclass to access overridden methods and hidden members of its superclass. The super keyword is also used to forward a call from a constructor to a constructor in the superclass.
switch The switch keyword is used in conjunction with case and default to create a switch statement, which evaluates a variable, matches its value to a specific case, and executes the block of statements associated with that case. If no case matches the value, the optional block labelled by default is executed, if included.
synchronized Used in the declaration of a method or code block to acquire the mutex lock for an object while the current thread executes the code. For static methods, the object locked is the class’s Class. Guarantees that at most one thread at a time operating on the same object executes that code. The mutex lock is automatically released when execution exits the synchronized code. Fields, classes and interfaces cannot be declared as synchronized.
this Used to represent an instance of the class in which it appears. this can be used to access class members and as a reference to the current instance. The this keyword is also used to forward a call from one constructor in a class to another constructor in the same class.
throws Used in method declarations to specify which exceptions are not handled within the method but rather passed to the next higher level of the program. All uncaught exceptions in a method that are not instances of RuntimeException must be declared using the throws keyword.
throw Causes the declared exception instance to be thrown. This causes execution to continue with the first enclosing exception handler declared by the catch keyword to handle an assignment compatible exception type. If no such exception handler is found in the current method, then the method returns and the process is repeated in the calling method. If no exception handler is found in any method call on the stack, then the exception is passed to the thread’s uncaught exception handler.
transient Declares that an instance field is not part of the default serialized form of an object. When an object is serialized, only the values of its non-transient instance fields are included in the default serial representation. When an object is deserialized, transient fields are initialized only to their default value. If the default form is not used, e.g. when a serialPersistentFields table is declared in the class hierarchy, all transient keywords are ignored.
try Defines a block of statements that have exception handling. If an exception is thrown inside the try block, an optional catch block can handle declared exception types. Also, an optional finally block can be declared that will be executed when execution exits the try block and catch clauses, regardless of whether an exception is thrown or not. A try block must have at least one catch clause or a finally block.
void The void keyword is used to declare that a method does not return any value.
volatile Used in field declarations to specify that the variable is modified asynchronously by concurrently running threads. Methods, classes and interfaces thus cannot be declared volatile, nor can local variables or parameters.
while The while keyword is used to create a while loop, which tests a boolean expression and executes the block of statements associated with the loop if the expression evaluates to true; this continues until the expression evaluates to false. This keyword can also be used to create a do-while loop.
   

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