List of Java keywords explained

In the Java programming language, a keyword is any one of 68 reserved words^[1] that have a predefined meaning in the language. Because of this, programmers cannot use keywords in some contexts, such as names for variables, methods, classes, or as any other identifier.^[2] Of these 68 keywords, 17 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.

Reserved keywords

The following words are reserved keywords and cannot be used as identifiers under any circumstances.

_

Added in Java 9, the underscore has become a keyword and cannot be used as a variable name anymore.^[3]

[[abstract type|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.

[[assertion (software development)|assert]] (added in J2SE 1.4)^[4]

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 data type|boolean]]

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 data type|boolean]]. In most other languages, the Boolean type is usually simply called bool.

[[break statement|break]]

Used to end the execution in the current loop body.

Used to break out of a [[#switch|switch]] block.

[[byte]]

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.

[[switch statement|case]]

A statement in the [[#switch|switch]] block can be labeled with one or more [[#case|case]] or [[#default|default]] labels. The [[#switch|switch]] statement evaluates its expression, then executes all statements that follow the matching [[#case|case]] label; see [[#switch|switch]].

[[exception handling syntax#Java|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.

[[character (computing)|char]]

Defines a character variable capable of holding any character of the Java source file's character set.

[[class (computer science)#Java|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 . 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.

[[continue (Java)|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; see [[#switch|switch]].^[5] 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 while loop|do]]

The do keyword is used in conjunction with [[#while|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.^[6]

[[double precision|double]]

The double keyword is used to declare a variable that can hold a 64-bit double precision IEEE 754 floating-point number.^[7] This keyword is also used to declare that a method returns a value of the primitive type double.^[8]

[[conditional (programming)|else]]

The else keyword is used in conjunction with [[#if|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.^[9]

[[enumerated type|enum]] (added in J2SE 5.0)^[4]

A Java keyword used to declare an enumerated type. Enumerations extend the base class .

[[inheritance (object-oriented programming)|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 (Java)|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 on an executed command. All methods in a final class are implicitly final.

[[exception handling syntax#Java|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.

[[single precision|float]]

The float keyword is used to declare a variable that can hold a 32-bit single precision IEEE 754 floating-point number.^[7] This keyword is also used to declare that a method returns a value of the primitive type float.^[8]

[[for loop|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.^[10]

As of J2SE 5.0, the for keyword can also be used to create a so-called "enhanced for loop", which specifies an array or object; each iteration of the loop executes the associated block of statements using a different element in the array or Iterable.^[10]

[[if statement|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; see [[#else|else]].^[9]

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 import static members of a class. A Java module may itself be imported (by writing import module), automatically importing all exported packages.^[11]

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.

[[integer (computer science)|int]]

The int keyword is used to declare a variable that can hold a 32-bit signed two's complement integer.^[7] This keyword is also used to declare that a method returns a value of the primitive type int.^[8]

[[interface (Java)|interface]]

Used to declare an interface 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 is not allowed in Java, interfaces are used to circumvent it. An interface can be defined within another interface.

[[long integer|long]]

The long keyword is used to declare a variable that can hold a 64-bit signed two's complement integer.^[7] This keyword is also used to declare that a method returns a value of the primitive type long.^[8]

[[Java Native Interface|native]]

Used in method declarations to specify that the method is not implemented in the same Java source file, but rather in another language.

[[object lifetime#Java|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).

[[Java package|package]]

Java package is a group of similar classes and interfaces. Packages are declared with the package keyword.

[[b:Java Programming/Classes, Objects and Types|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.^[12]

[[b:Java Programming/Classes, Objects and Types|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.^[12]

[[b:Java Programming/Classes, Objects and Types|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.^[12]

[[method (computer science)|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 integer|short]]

The short keyword is used to declare a field that can hold a 16-bit signed two's complement integer.^[7] This keyword is also used to declare that a method returns a value of the primitive type short.^[8]

[[static variable|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 behaviorally top-level classes.^[13]

[[inheritance (object-oriented programming)|super]]

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 statement|switch]]

The switch keyword is used in conjunction with [[#case|case]] and [[#default|default]] to create a switch statement, which evaluates a variable, matches its value to a specific case (including patterns), 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.^[5] The switch keyword can also be used with the non-reserved keyword yield to create switch expressions.

[[mutual exclusion|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 (Java)|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.

[[exception handling syntax#Java|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.

[[exception handling syntax#Java|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 checked exceptions in a method (extend Exception but not RuntimeException) must be declared using the throws keyword.

[[transient (computer programming)|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.^[14] ^[15]

[[exception handling syntax#Java|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 type|void]]

The void keyword is used to declare that a method does not return any value.^[8]

[[volatile variable|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.^[16] Methods, classes and interfaces thus cannot be declared volatile, nor can local variables or parameters.

[[do while loop|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; see [[#do|do]].^[6]

Unused

The following words are reserved as keywords, but currently have no use or purpose.

[[constant (programming)|const]]

Although reserved as a keyword in Java, const is not used and has no function.^[2] In other languages, const is typically used to define constants. For defining constants in Java, see the final keyword.

[[GOTO|goto]]

Although reserved as a keyword in Java, goto is not used and has no function.^[2] In other languages, goto is typically used as a one-way control statement to jump to a label at another line of code.

[[strictfp]] (added in J2SE 1.2)

Although reserved as a keyword in Java, strictfp is obsolete, and no longer has any function.^[17] Previously this keyword was used to restrict the precision and rounding of floating point calculations to ensure portability.

Contextual keywords

The following identifiers are contextual keywords, and are only restricted in some contexts:

exports

Used in a module declaration to specify which packages are available to other modules.

[[Java Platform Module System|module]]

Declares a module (a collection of related packages and resources that can be treated as a unit), used to encapsulate and expose only the public API of a library.

non-sealed

Used to declare that a class or interface which extends a sealed class can be extended by unknown classes.^[18]

open

Indicates that all classes in a package are accessible via reflection by other modules.

opens

Used to open a specific package for reflection to other modules.

permits

The permits clause specifies the classes that are permitted to extend a sealed class.

provides

Used to declare that a module provides an implementation of a service interface.

[[Record (computer science)|record]]

A special kind of class that acts as a transparent carrier of immutable data, automatically providing .equals, .hashCode, and .toString methods.

requires

Used in a module declaration to specify that the module depends on another module.

sealed

A sealed class or interface can only be extended or implemented by classes and interfaces permitted to do so.

to

Used with the opens directive to specify which module is allowed to reflectively access the package.

transitive

Used with the requires directive to indicate that a module not only requires another module but also makes that module's dependencies available to modules that depend on it.

uses

Used in a module to declare that the module is using a service (i.e. it will consume a service provided by other modules).

[[Type inference|var]]

A special identifier that cannot be used as a type name (since Java 10).^[19] Used to declare a variable without explicitly specifying the type, rather relying on the compiler to infer the type based on the initialiser.

when

Used as an additional check for a case statement. ^[20]

with

Used with the provides directive to specify which implementation of a service is provided by the module.

yield

Used to set a value for a switch expression, when using labelled statement groups (for example, case L:).^[21]

Reserved words for literal values

The following words refer to literal values used by the language.

[[truth value|true]]

A boolean literal value.

[[truth value|false]]

A boolean literal value.

[[null pointer|null]]

A reference literal value.

External links

Book: Gosling . James . James Gosling . Joy . Bill . Bill Joy . Steele . Guy . Guy Steele . Bracha . Gilad . Gilad Bracha . Java Language Specification . Third . Addison-Wesley Professional . June 2005 . 978-0-321-24678-3 . 2008-12-03.
Book: Flanagan, David . David Flanagan

. David Flanagan . Java in a Nutshell . Fifth . . March 2005 . 978-0-596-00773-7 . 2010-03-03 . registration.

Notes and References

Web site: Java Platform, Standard Edition Java API Reference.
Web site: Java Language Specification - Section 3.9: Keywords . The Java Language Specification . . 2018-08-21 . 2018-12-25.
Web site: Treatment of underscores. JEP 302: Lambda Leftovers .
Web site: Java Language Keywords . The Java Tutorials . Sun Microsystems, Inc. . 2017-07-24.
Web site: The switch Statement . The Java Tutorials . Sun Microsystems, Inc. . February 14, 2008 . 2014-12-18.
Web site: The while and do-while Statements . The Java Tutorials . Sun Microsystems, Inc. . February 14, 2008 . 2008-12-03.
Web site: Primitive Data Types . The Java Tutorials . Sun Microsystems, Inc. . February 14, 2008 . 2008-12-03.
Web site: Returning a Value from a Method . The Java Tutorials . Sun Microsystems, Inc. . February 14, 2008 . 2008-12-03.
Web site: The if-then and if-then-else Statements . The Java Tutorials . Sun Microsystems, Inc. . February 14, 2008 . 2008-12-03.
Web site: The for Statement . The Java Tutorials . Sun Microsystems, Inc. . February 14, 2008 . 2008-12-03.
Web site: JEP 494: Module Import Declarations (Second Preview). openjdk.org.
Web site: Controlling Access to Members of a Class . The Java Tutorials . Sun Microsystems, Inc. . February 14, 2008 . 2008-12-03.
Web site: Nested Classes (The Java™ Tutorials > Learning the Java Language > Classes and Objects) .
Web site: Java Object Serialization Specification version 1.5.0 . 1.5 Defining Serializable Fields for a Class . Sun/Oracle . 2004 . 2010-09-16 .
Web site: William . Grosso . Java RMI: Serialization . Declaring serialPersistentFields . ONJava . O'Reilly Media . November 21, 2001 . 2010-09-16.
Web site: Java Volatile Keyword .
Web site: JEP 306: Restore Always-Strict Floating-Point Semantics.
Web site: Sealed Classes. 2021-08-07. docs.oracle.com. Oracle Corporation.
Web site: Chapter 3. Lexical Structure. docs.oracle.com. 2018-12-25.
Web site: Chapter 14. Blocks, Statements, and Patterns.
Web site: Switch Expressions . Oracle Corporation . docs.oracle.com . 2020-12-27.