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 for Java use syntax highlighting to display keywords in a different color for easy identification.
- 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 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.
- Defines a boolean variable for the values "true" or "false" only (NB: "null" as in class Boolean is not allowed).
- Used to end the execution in the current loop body.
- Defines a byte variable representing a sequence of 8 bits. (NB: Only 1-byte-characters can be used, f.i. '€' would produce an error).
- A statement in the
switchblock can be labeled with one or more
switchstatement evaluates its expression, then executes all statements that follow the matching
- Used in conjunction with a
tryblock and an optional
finallyblock. The statements in the
catchblock specify what to do if a specific type of exception is thrown by the
- Defines a character variable capable of holding any character of the java source file's character set (NB: Physical storage may exceed one byte).
- 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. 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().
- Although reserved as a keyword in Java,
constis not used and has no function. For defining constants in java, see the 'final' reserved word.
- Used to resume program execution at the end of the current loop body. If followed by a label,
continueresumes execution at the end of the enclosing labeled loop body.
defaultkeyword can optionally be used in a switch statement to label a block of statements to be executed if no
casematches the specified value; see
switch. Alternatively, the default keyword can also be used to declare default values in a Java annotation. From Java 8 onwards, the default keyword is also used to specify that a method in an interface provides the default implementation of an optional method.
dokeyword is used in conjunction with
whileto 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
doublekeyword is used to declare a variable that can hold a 64-bit double precision IEEE 754 floating-point number. This keyword is also used to declare that a method returns a value of the primitive type
elsekeyword is used in conjunction with
ifto create an if-else statement, which tests a boolean expression; if the expression evaluates to
true, the block of statements associated with the
ifare evaluated; if it evaluates to
false, the block of statements associated with the
enum(as of J2SE 5.0)
- A Java keyword used to declare an enumerated type. Enumerations extend the base class
- 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.
- 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
- Used to define a block of statements for a block defined previously by the
finallyblock is executed after execution exits the
tryblock and any associated
catchclauses regardless of whether an exception was thrown or caught, or execution left method in the middle of the
catchblocks using the
floatkeyword is used to declare a variable that can hold a 32-bit single precision IEEE 754 floating-point number. This keyword is also used to declare that a method returns a value of the primitive type
forkeyword 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
- As of J2SE 5.0, the
forkeyword can also be used to create a so-called "enhanced for loop", which specifies an array or
Iterableobject; each iteration of the loop executes the associated block of statements using a different element in the array or
ifkeyword 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
- 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.
- 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,
importstatements can import
staticmembers of a class.
- 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
instanceofoperator evaluates to true if and only if the runtime type of the object is assignment compatible with the class or interface.
intkeyword 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
- Used to declare a special type of class that only contains abstract methods, constant (
static final) fields and
staticinterfaces. It can later be implemented by classes that declare the interface with the
longkeyword 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
- Used in method declarations to specify that the method is not implemented in the same Java source file, but rather in another language.
- 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).
- A group of types. Packages are declared with the
privatekeyword 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.
protectedkeyword 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.
publickeyword 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.
- 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.
shortkeyword 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
- 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.
staticalso 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
staticmembers 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.
- 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
superkeyword 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.
switchkeyword is used in conjunction with
defaultto 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
casematches the value, the optional block labelled by
defaultis executed, if included.
- 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.
- Used to represent an instance of the class in which it appears.
thiscan be used to access class members and as a reference to the current instance. The
thiskeyword is also used to forward a call from one constructor in a class to another constructor in the same class.
- Causes the declared exception instance to be thrown. This causes execution to continue with the first enclosing exception handler declared by the
catchkeyword 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.
- 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
RuntimeExceptionmust be declared using the
- 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
transientkeywords are ignored.
- Defines a block of statements that have exception handling. If an exception is thrown inside the
tryblock, an optional
catchblock can handle declared exception types. Also, an optional
finallyblock can be declared that will be executed when execution exits the
catchclauses, regardless of whether an exception is thrown or not. A
tryblock must have at least one
catchclause or a
- 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.
whilekeyword 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
Reserved words for literal values
- A boolean literal value.
- A reference literal value.
- A boolean literal value.
- Flanagan 2005, p. 20.
- "Java Language Specification - Section 3.9: Keywords". The Java Language Specification. Oracle. February 13, 2012. Retrieved 2015-05-15.
- "The switch Statement". The Java Tutorials. Sun Microsystems, Inc. February 14, 2008. Retrieved 2014-12-18.
- Flanagan 2005, pp. 46-48.
- "The while and do-while Statements". The Java Tutorials. Sun Microsystems, Inc. February 14, 2008. Retrieved 2008-12-03.
- Flanagan 2005, pp. 48-49.
- "Primitive Data Types". The Java Tutorials. Sun Microsystems, Inc. February 14, 2008. Retrieved 2008-12-03.
- Flanagan 2005, p. 22.
- "Returning a Value from a Method". The Java Tutorials. Sun Microsystems, Inc. February 14, 2008. Retrieved 2008-12-03.
- Flanagan 2005, pp. 66-67.
- "The if-then and if-then-else Statements". The Java Tutorials. Sun Microsystems, Inc. February 14, 2008. Retrieved 2008-12-03.
- Flanagan 2005, pp. 44-46.
- "The for Statement". The Java Tutorials. Sun Microsystems, Inc. February 14, 2008. Retrieved 2008-12-03.
- Flanagan 2005, pp. 50-54.
- "Controlling Access to Members of a Class". The Java Tutorials. Sun Microsystems, Inc. February 14, 2008. Retrieved 2008-12-03.
- "Java Object Serialization Specification version 1.5.0". Sun/Oracle. 2004. 1.5 Defining Serializable Fields for a Class. Retrieved 2010-09-16.
- Grosso, William (November 21, 2001). "Java RMI: Serialization". ONJava. O'Reilly Media. Declaring serialPersistentFields. Retrieved 2010-09-16.
- Gosling, James; Joy, Bill; Steele, Guy; Bracha, Gilad (June 2005). Java Language Specification (Third ed.). Addison-Wesley Professional. ISBN 978-0-321-24678-3. Retrieved 2008-12-03.
- Flanagan, David (March 2005). Java in a Nutshell (Fifth ed.). O'Reilly Media. ISBN 978-0-596-00773-7. Retrieved 2010-03-03.