The following are brief definitions of the keywords for the Java programming language. Most IDEs use syntax highlighting to display keywords in a different color for easy identification.
- abstract
- Used in a class declaration to specify that a class is not to be
instantiated, but rather extended by other classes. Used in a method
declaration to declare a method without providing the implementation.
An abstract class can have abstract methods that are not implemented in
the abstract class, but in subclasses. All methods declared in aninterface
are implicitlyabstract
.
- assert (as of J2SE 1.4)
- A keyword used to make the assumed value of a condition explicit. If the condition is not true, an
AssertionError
is thrown.
- boolean
- Refers to an expression or variable that can have only a true or false value. Java provides the
boolean
type and the literal valuestrue
andfalse
.
- break
- Used to resume program execution at the statement immediately
following the current enclosing block or statement. If followed by a label, the program resumes execution at the statement immediately following the enclosing labeled statement or block.
- byte
- An 8-bit integer. A keyword used to declare an expression, method return value, or variable of type byte.
- case
- Defines a group of statements to execute if the specified value matches the value defined by the enclosing
switch
statement.
- catch
- Defines an exception handler—a group of statements that are executed if an exception is thrown in the block defined by a preceding
try
keyword. The code is executed only if the class of the thrown exception
is assignment compatible with the exception class declared by thecatch
clause.
- char
- A 16-bit Unicode character. A Java keyword used to declare an expression, method return value, or variable of type character.
- class
- A type that defines the implementation of a particular kind of object. A class definition defines instance and class fields, methods, and inner classes as well as specifying the interfaces the class implements and the immediate superclass of the class. If the superclass is not explicitly specified, the superclass is implicitly
Object
.
- const (reserved without use)
- This keyword is not used by current versions of the Java programming language.
- 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
- Defines a group of statements to begin executing if the value defined by the enclosing
switch
statement does not match any value specified by acase
keyword in theswitch
statement.
- do
- Used to declare a loop that will iterate a block of statements. The loop’s exit condition is specified with the
while
keyword. The loop will execute once before evaluating the exit condition.
- double
- A 64-bit floating point value. A Java keyword used to declare an expression, method return value, or variable of type double-precision floating point number.
- else
- Used to define a statement or block of statements that are executed in the case that the test condition specified by the
if
keyword evaluates to false.
- enum (as of J2SE 5.0)
- A Java keyword used to declare an enumerated type. Enumerations extend the base class
Enum
.
- extends
- Used in a class declaration to specify the superclass; used in an
interface declaration to specify one or more superinterfaces. Class X
extends class Y to add functionality, either by adding fields or
methods to class Y, or by overriding methods of class Y. An interface Z
extends one or more interfaces by adding methods. Class X is said to be
a subclass of class Y; Interface Z is said to be a subinterface of the
interfaces it extends. - Also used to specify an upper bound on a type parameter in Generics.
- final
- Define an entity once that cannot be changed nor derived from
later. More specifically: a final class cannot be subclassed, a final
method cannot be overridden, and a final variable can occur at most
once as a left-hand expression. All methods in a final class are
implicitlyfinal
.
- finally
- Used to define a block of statements for a block defined previously by the
try
keyword. Thefinally
block is executed after execution exits thetry
block and any associatedcatch
clauses regardless of whether an exception was thrown or caught, or execution left method in the middle of thetry
orcatch
blocks using thereturn
keyword.
- float
- A 32-bit floating point value. A Java keyword used to declare an
expression, method return value, or variable of type single-precision
floating point number.
- for
- Used to define a loop that reiterates statements. The
for
loop specifies the statements to be executed, exit condition, and
initialization variables for the loop. The exit condition is evaluated
before the first iteration of the loop. Since J2SE 5.0, a form of thefor
loop specifies anIterable
object where each iteration of the loop processes one of its contained elements.
- goto (reserved without use)
- This keyword is not used by current versions of the Java programming language.
- if
- Used to conduct a conditional test and execute a block of
statements if the test evaluates to true. If an optional block is
defined with theelse
keyword, then theelse
block is executed if the test evaluates to false.
- 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. Since J2SE 5.0,
import
statements can importstatic
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. Theinstanceof
operator evaluates to true if and only if the runtime type of the object is assignment compatible with the class or interface.
- int
- A 32-bit integer value. A Java keyword used to declare an expression, method return value, or variable of type integer.
- interface
- Used to declare a special type of class that only contains abstract methods, constant (
static final
) fields andstatic
interfaces. It can later be implemented by classes that declare the interface with theimplements
keyword.
- long
- A 64-bit integer value. A Java keyword used to declare an expression, method return value, or variable of type long integer.
- 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.
- package
- A group of types. Packages are declared with the
package
keyword.
- private
- An access modifier used in a method, field or inner class
declaration. It signifies that the member can only be accessed by other
elements of its class.
- protected
- An access modifier used in a method, field or inner class
declaration. It signifies that the member can only be accessed by
elements residing in its class, subclasses, or classes in the same
package.
- public
- An access modifier used in a class, method or field declaration. It
signifies that the class, method or variable can be accessed by
elements residing in other classes or packages. All members declared in
aninterface
are implicitlypublic
.
- 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
- A 16-bit integer value. Used to declare an expression, method return value, or variable of type short integer.
- 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 asstatic
members of another class or interface are actually top-level classes and are not inner classes.)
- strictfp (as of J2SE 1.2)
- 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. - Also used to specify a lower bound on a type parameter in Generics.
- switch
- Used to evaluate a variable that can later be matched with a value specified by the
case
keyword in order to execute a group of statements.
- 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’
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. Thethis
keyword is also used to forward a call from one constructor in a class to another constructor in the same class.
- throw
- Causes the declared exception instance to be thrown. This causes
execution to continue with the first enclosing exception handler
declared by thecatch
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.
- 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
ofRuntimeException
must be declared using thethrows
keyword.
- 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.
- try
- Defines a block of statements that have exception handling. If an exception is thrown inside the
try
block, an optionalcatch
block can handle declared exception types. Also, an optionalfinally
block can be declared that will be executed when execution exits thetry
block andcatch
clauses, regardless of whether an exception is thrown or not. Atry
block must have at least onecatch
clause or afinally
block.
- void
- Used in method declarations to specify that the method does not return any value.
void
can also be used as a nonfunctional statement.
- 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.
- while
- Used to declare a loop that iterates a block of statements. The
loop’s exit condition is specified as part of the while statement. Ifwhile
appears before the body of the loop, the exit condition is evaluated before the first iteration. Ifwhile
appears after the loop body then thedo
keyword designates the beginning of the loop body which is executed once before evaluating the exit condition.
- false
- A boolean literal value.
- null
- A reference literal value.
- true
- A boolean literal value.