HOME

TheInfoList



OR:

In the Java programming language, a keyword is any one of 67 reserved words that have a predefined meaning in the language. Because of this, programmers cannot use keywords in some contexts, such as names for variables,
methods Method ( grc, μέθοδος, methodos) literally means a pursuit of knowledge, investigation, mode of prosecuting such inquiry, or system. In recent centuries it more often means a prescribed process for completing a task. It may refer to: *Scien ...
, classes, or as any other
identifier An identifier is a name that identifies (that is, labels the identity of) either a unique object or a unique ''class'' of objects, where the "object" or class may be an idea, physical countable object (or class thereof), or physical noncountable ...
. Of these 67 keywords, 16 of them are only contextually reserved, and can sometimes be used as an identifier, unlike standard reserved words. Due to their special functions in the language, most integrated development environments for Java use syntax highlighting to display keywords in a different colour for easy identification.


List of JAVA keywords

;_ :Added in Java 9, the underscore has become a keyword and cannot be used as a variable name anymore. ; abstract : A method with no definition must be declared as abstract and the class containing it must be declared as abstract. Abstract classes cannot be instantiated. Abstract methods must be implemented in the sub classes. The abstract keyword cannot be used with variables or constructors. Note that an abstract class isn't required to have an abstract method at all. ;
assert Assertion or assert may refer to: Computing * Assertion (software development), a computer programming technique * assert.h, a header file in the standard library of the C programming language * Assertion definition language, a specification lan ...
(added in
J2SE 1.4 The Java language has undergone several changes since JDK 1.0 as well as numerous additions of classes and packages to the standard library. Since J2SE 1.4, the evolution of the Java language has been governed by the Java Community P ...
) :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. Assertions are disabled at runtime by default, but can be enabled through a command-line option or programmatically through a method on the class loader. : ;
boolean Any kind of logic, function, expression, or theory based on the work of George Boole is considered Boolean. Related to this, "Boolean" may refer to: * Boolean data type, a form of data with only two possible values (usually "true" and "false" ...
:Defines a boolean variable for the values "true" or "false" only. By default, the value of boolean primitive type is false. This keyword is also used to declare that a method returns a value of the primitive type
boolean Any kind of logic, function, expression, or theory based on the work of George Boole is considered Boolean. Related to this, "Boolean" may refer to: * Boolean data type, a form of data with only two possible values (usually "true" and "false" ...
. ;
break Break or Breaks or The Break may refer to: Time off from duties * Recess (break), time in which a group of people is temporarily dismissed from its duties * Break (work), time off during a shift/recess ** Coffee break, a short mid-morning rest ...
:Used to end the execution in the current loop body. :Used to break out of a
switch In electrical engineering, a switch is an electrical component that can disconnect or connect the conducting path in an electrical circuit, interrupting the electric current or diverting it from one conductor to another. The most common type of ...
block. ;
byte The byte is a unit of digital information that most commonly consists of eight bits. Historically, the byte was the number of bits used to encode a single character of text in a computer and for this reason it is the smallest addressable uni ...
:The byte keyword is used to declare a field that can hold an 8-bit signed two's complement integer. This keyword is also used to declare that a method returns a value of the primitive type byte. ;
case Case or CASE may refer to: Containers * Case (goods), a package of related merchandise * Cartridge case or casing, a firearm cartridge component * Bookcase, a piece of furniture used to store books * Briefcase or attaché case, a narrow box to ca ...
:A statement in the
switch In electrical engineering, a switch is an electrical component that can disconnect or connect the conducting path in an electrical circuit, interrupting the electric current or diverting it from one conductor to another. The most common type of ...
block can be labeled with one or more
case Case or CASE may refer to: Containers * Case (goods), a package of related merchandise * Cartridge case or casing, a firearm cartridge component * Bookcase, a piece of furniture used to store books * Briefcase or attaché case, a narrow box to ca ...
or default labels. The
switch In electrical engineering, a switch is an electrical component that can disconnect or connect the conducting path in an electrical circuit, interrupting the electric current or diverting it from one conductor to another. The most common type of ...
statement evaluates its expression, then executes all statements that follow the matching
case Case or CASE may refer to: Containers * Case (goods), a package of related merchandise * Cartridge case or casing, a firearm cartridge component * Bookcase, a piece of furniture used to store books * Briefcase or attaché case, a narrow box to ca ...
label; see
switch In electrical engineering, a switch is an electrical component that can disconnect or connect the conducting path in an electrical circuit, interrupting the electric current or diverting it from one conductor to another. The most common type of ...
. ;
catch Catch may refer to: In sports * Catch (game), children's game * Catch (baseball), a maneuver in baseball * Catch (cricket), a mode of dismissal in cricket * Catch or reception (gridiron football) * Catch, part of a rowing stroke In music * Catc ...
: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 Char may refer to: People * Char Fontane, American actress * Char Margolis, American spiritualist * René Char (1907–1988), French poet *The Char family of Colombia: ** Fuad Char, Colombian senator ** Alejandro Char Chaljub, mayor of Barranquill ...
:Defines a character variable capable of holding any character of the java source file's character set. ;
class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Class (philosophy), an analytical concept used differently ...
:A type that defines the implementation of a particular kind of object. A class definition defines instance and class fields,
methods Method ( grc, μέθοδος, methodos) literally means a pursuit of knowledge, investigation, mode of prosecuting such inquiry, or system. In recent centuries it more often means a prescribed process for completing a task. It may refer to: *Scien ...
, and inner classes as well as specifying the
interfaces Interface or interfacing may refer to: Academic journals * ''Interface'' (journal), by the Electrochemical Society * '' Interface, Journal of Applied Linguistics'', now merged with ''ITL International Journal of Applied Linguistics'' * '' Int ...
the class implements and the immediate superclass of the class. If the superclass is not explicitly specified, the superclass is implicitly . The class keyword can also be used in the form Class.class to get a Class object without needing an instance of that class. For example, String.class can be used instead of doing new String().getClass(). ; const :Unused but reserved. ; 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 In computer programming languages, a switch statement is a type of selection control mechanism used to allow the value of a variable or expression to change the control flow of program execution via search and map. Switch statements function s ...
to label a block of statements to be executed if no case matches the specified value; see ''
switch In electrical engineering, a switch is an electrical component that can disconnect or connect the conducting path in an electrical circuit, interrupting the electric current or diverting it from one conductor to another. The most common type of ...
''. Alternatively, the default keyword can also be used to declare default values in a Java annotation. From Java 8 onwards, the default keyword can be used to allow an interface to provide an implementation of a method. ; 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 A double is a look-alike or doppelgänger; one person or being that resembles another. Double, The Double or Dubble may also refer to: Film and television * Double (filmmaking), someone who substitutes for the credited actor of a character * ...
:The double keyword is used to declare a variable that can hold a 64-bit
double precision Double-precision floating-point format (sometimes called FP64 or float64) is a floating-point number format, usually occupying 64 bits in computer memory; it represents a wide dynamic range of numeric values by using a floating radix point. Flo ...
IEEE 754 The IEEE Standard for Floating-Point Arithmetic (IEEE 754) is a technical standard for floating-point arithmetic established in 1985 by the Institute of Electrical and Electronics Engineers (IEEE). The standard addressed many problems found ...
floating-point number In computing, floating-point arithmetic (FP) is arithmetic that represents real numbers approximately, using an integer with a fixed precision, called the significand, scaled by an integer exponent of a fixed base. For example, 12.345 can be r ...
. 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 In computer science, a Boolean expression is an expression used in programming languages that produces a Boolean value when evaluated. A Boolean value is either true or false. A Boolean expression may be composed of a combination of the Boolean c ...
; 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 Telephone number mapping is a system of unifying the international telephone number system of the public switched telephone network with the Internet addressing and identification name spaces. Internationally, telephone numbers are systematic ...
(added in
J2SE 5.0 The Java (programming language), Java language has undergone several changes since Java Development Kit, JDK 1.0 as well as numerous additions of class (computer science), classes and packages to the standard library (computer science), li ...
) :A Java keyword used to declare an
enumerated type In computer programming, an enumerated type (also called enumeration, enum, or factor in the R programming language, and a categorical variable in statistics) is a data type consisting of a set of named values called ''elements'', ''members'', '' ...
. Enumerations extend the base class . ; 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 Final, Finals or The Final may refer to: *Final (competition), the last or championship round of a sporting competition, match, game, or other contest which decides a winner for an event ** Another term for playoffs, describing a sequence of con ...
: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 on an executed command. All methods in a final class are implicitly final. ;
finally Finally may refer to: Albums * ''Finally'' (Namie Amuro album) (2017) * ''Finally'' (Blackstreet album) or its title song * ''Finally'' (Sean Ensign album) or its title song "It's My Life (Finally)" * ''Finally'' (Layzie Bone & A.K. album) ...
: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 Single-precision floating-point format (sometimes called FP32 or float32) is a computer number format, usually occupying 32 bits in computer memory; it represents a wide dynamic range of numeric values by using a floating radix point. A floatin ...
IEEE 754 floating-point number. This keyword is also used to declare that a method returns a value of the primitive type float. ; for :The for keyword is used to create a
for loop In computer science a for-loop or for loop is a control flow statement for specifying iteration. Specifically, a for loop functions by running a section of code repeatedly until a certain condition has been satisfied. For-loops have two par ...
, which specifies a variable initialization, a
boolean expression In computer science, a Boolean expression is an expression used in programming languages that produces a Boolean value when evaluated. A Boolean value is either true or false. A Boolean expression may be composed of a combination of the Boolean c ...
, 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. :As of
J2SE 5.0 The Java (programming language), Java language has undergone several changes since Java Development Kit, JDK 1.0 as well as numerous additions of class (computer science), classes and packages to the standard library (computer science), li ...
, the for keyword can also be used to create a so-called " enhanced for loop", which specifies an
array An array is a systematic arrangement of similar objects, usually in rows and columns. Things called an array include: {{TOC right Music * In twelve-tone and serial composition, the presentation of simultaneous twelve-tone sets such that the ...
or object; each iteration of the loop executes the associated block of statements using a different element in the array or Iterable. ;
goto GoTo (goto, GOTO, GO TO or other case combinations, depending on the programming language) is a statement found in many computer programming languages. It performs a one-way transfer of control to another line of code; in contrast a function c ...
:Unused but reserved. ; if :The if keyword is used to create an if statement, which tests a
boolean expression In computer science, a Boolean expression is an expression used in programming languages that produces a Boolean value when evaluated. A Boolean value is either true or false. A Boolean expression may be composed of a combination of the Boolean c ...
; 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; see '' else''. ;implements :Included in a class declaration to specify one or more
interfaces Interface or interfacing may refer to: Academic journals * ''Interface'' (journal), by the Electrochemical Society * '' Interface, Journal of Applied Linguistics'', now merged with ''ITL International Journal of Applied Linguistics'' * '' Int ...
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 In computing, source code, or simply code, is any collection of code, with or without comments, written using a human-readable programming language, usually as plain text. The source code of a program is specially designed to facilitate the wo ...
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 import static members of a class. ;instanceof :A
binary operator In mathematics, a binary operation or dyadic operation is a rule for combining two elements (called operands) to produce another element. More formally, a binary operation is an operation of arity two. More specifically, an internal binary o ...
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 Interface or interfacing may refer to: Academic journals * ''Interface'' (journal), by the Electrochemical Society * '' Interface, Journal of Applied Linguistics'', now merged with ''ITL International Journal of Applied Linguistics'' * '' Int ...
:Used to declare a special type of class that only contains abstract or default methods, constant (static final) fields and static interfaces. It can later be implemented by classes that declare the interface with the implements keyword. As
multiple inheritance Multiple inheritance is a feature of some object-oriented computer programming languages in which an object or class can inherit features from more than one parent object or parent class. It is distinct from single inheritance, where an object or ...
is not allowed in Java, interfaces are used to circumvent it. An interface can be defined within another interface. ;
long Long may refer to: Measurement * Long, characteristic of something of great duration * Long, characteristic of something of great length * Longitude (abbreviation: long.), a geographic coordinate * Longa (music), note value in early music mensu ...
: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 Native may refer to: People * Jus soli, citizenship by right of birth * Indigenous peoples, peoples with a set of specific rights based on their historical ties to a particular territory ** Native Americans (disambiguation) In arts and entert ...
:Used in method declarations to specify that the method is not implemented in the same Java source file, but rather in another language. ;
new New is an adjective referring to something recently made, discovered, or created. New or NEW may refer to: Music * New, singer of K-pop group The Boyz Albums and EPs * ''New'' (album), by Paul McCartney, 2013 * ''New'' (EP), by Regurgitator, ...
: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 :Java package is a group of similar classes and interfaces. Packages are declared with the package keyword. ;
private Private or privates may refer to: Music * " In Private", by Dusty Springfield from the 1990 album ''Reputation'' * Private (band), a Denmark-based band * "Private" (Ryōko Hirosue song), from the 1999 album ''Private'', written and also recorde ...
: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 Protection is any measure taken to guard a thing against damage caused by outside forces. Protection can be provided to physical objects, including organisms, to systems, and to intangible things like civil and political rights. Although th ...
: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 In public relations and communication science, publics are groups of individual people, and the public (a.k.a. the general public) is the totality of such groupings. This is a different concept to the sociology, sociological concept of the ''Öf ...
: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 Return may refer to: In business, economics, and finance * Return on investment (ROI), the financial gain after an expense. * Rate of return, the financial term for the profit or loss derived from an investment * Tax return, a blank document or t ...
: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 Static may refer to: Places *Static Nunatak, a nunatak in Antarctica United States * Static, Kentucky and Tennessee *Static Peak, a mountain in Wyoming **Static Peak Divide, a mountain pass near the peak Science and technology Physics *Static el ...
: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. ;
super Super may refer to: Computing * SUPER (computer program), or Simplified Universal Player Encoder & Renderer, a video converter / player * Super (computer science), a keyword in object-oriented programming languages * Super key (keyboard butt ...
:Inheritance basically used to achieve dynamic binding or run-time polymorphism in java. 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 In electrical engineering, a switch is an electrical component that can disconnect or connect the conducting path in an electrical circuit, interrupting the electric current or diverting it from one conductor to another. The most common type of ...
:The switch keyword is used in conjunction with
case Case or CASE may refer to: Containers * Case (goods), a package of related merchandise * Cartridge case or casing, a firearm cartridge component * Bookcase, a piece of furniture used to store books * Briefcase or attaché case, a narrow box to ca ...
and default to create a
switch statement In computer programming languages, a switch statement is a type of selection control mechanism used to allow the value of a variable or expression to change the control flow of program execution via search and map. Switch statements function s ...
, 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 In computer science, a lock or mutex (from mutual exclusion) is a synchronization primitive: a mechanism that enforces limits on access to a resource when there are many threads of execution. A lock is designed to enforce a mutual exclusion concur ...
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. ;
throw Throwing is an action which consists in accelerating a projectile and then releasing it so that it follows a ballistic trajectory, usually with the aim of impacting a remote target. This action is best characterized for animals with prehensile ...
: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. ; 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. ;
transient ECHELON, originally a secret government code name, is a surveillance program (signals intelligence/SIGINT collection and analysis network) operated by the five signatory states to the UKUSA Security Agreement:Given the 5 dialects that us ...
: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 guarantee visibility of changes to variables across threads. Every read of a volatile variable will be read from main memory, and not from the CPU cache, and that every write to a volatile variable will be written to main memory, and not just to the CPU cache. 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 In most computer programming languages, a while loop is a control flow statement that allows code to be executed repeatedly based on a given Boolean condition. The ''while'' loop can be thought of as a repeating if statement. Overview The ' ...
, which tests a
boolean expression In computer science, a Boolean expression is an expression used in programming languages that produces a Boolean value when evaluated. A Boolean value is either true or false. A Boolean expression may be composed of a combination of the Boolean c ...
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; see '' do''.


Reserved Identifiers

The following identifiers are contextual keywords, and are only restricted in some contexts: exports module ;non-sealed :Used to declare that a class or interface which extends a sealed class can be extended by unknown classes. ;open ;opens ;permits :The permits clause specifies the classes that are permitted to extend a sealed class. ;provides ;record requires ;sealed :A sealed class or interface can only be extended or implemented by classes and interfaces permitted to do so. ;to ;transitive ;uses ;var :A special identifier that cannot be used as a type name (since Java 10). ;with ;yield :Used to set a value for a switch expression, when using labelled statement groups (for example, case L:).


Reserved words for literal values

;
true True most commonly refers to truth, the state of being in congruence with fact or reality. True may also refer to: Places * True, West Virginia, an unincorporated community in the United States * True, Wisconsin, a town in the United States * ...
:A boolean literal value. ; false :A boolean literal value. ;
null Null may refer to: Science, technology, and mathematics Computing * Null (SQL) (or NULL), a special marker and keyword in SQL indicating that something has no value * Null character, the zero-valued ASCII character, also designated by , often use ...
:A reference literal value.


Unused

; const :Although reserved as a keyword in Java, const is not used and has no function. For defining constants in Java, see the final keyword. ;
goto GoTo (goto, GOTO, GO TO or other case combinations, depending on the programming language) is a statement found in many computer programming languages. It performs a one-way transfer of control to another line of code; in contrast a function c ...
:Although reserved as a keyword in Java, goto is not used and has no function. ; strictfp (added in J2SE 1.2) :Although reserved as a keyword in Java, strictfp is obsolete, and no longer has any function. Previously this keyword was used to restrict the precision and rounding of floating point calculations to ensure portability.


See also

* Java annotation


References


External links

* * {{Refend Java (programming language) Java keywords