What are keywords in java

Java Keywords

Java has a set of keywords that are reserved words that cannot be used as variables, methods, classes, or any other identifiers:

Keyword	Description
abstract	A non-access modifier. Used for classes and methods: An abstract class cannot be used to create objects (to access it, it must be inherited from another class). An abstract method can only be used in an abstract class, and it does not have a body. The body is provided by the subclass (inherited from)
assert	For debugging
boolean	A data type that can only store true and false values
break	Breaks out of a loop or a switch block
byte	A data type that can store whole numbers from -128 and 127
case	Marks a block of code in switch statements
catch	Catches exceptions generated by try statements
char	A data type that is used to store a single character
class	Defines a class
continue	Continues to the next iteration of a loop
const	Defines a constant. Not in use — use final instead
default	Specifies the default block of code in a switch statement
do	Used together with while to create a do-while loop
double	A data type that can store whole numbers from 1.7e−308 to 1.7e+308
else	Used in conditional statements
enum	Declares an enumerated (unchangeable) type
exports	Exports a package with a module. New in Java 9
extends	Extends a class (indicates that a class is inherited from another class)
final	A non-access modifier used for classes, attributes and methods, which makes them non-changeable (impossible to inherit or override)
finally	Used with exceptions, a block of code that will be executed no matter if there is an exception or not
float	A data type that can store whole numbers from 3.4e−038 to 3.4e+038
for	Create a for loop
goto	Not in use, and has no function
if	Makes a conditional statement
implements	Implements an interface
import	Used to import a package, class or interface
instanceof	Checks whether an object is an instance of a specific class or an interface
int	A data type that can store whole numbers from -2147483648 to 2147483647
interface	Used to declare a special type of class that only contains abstract methods
long	A data type that can store whole numbers from -9223372036854775808 to 9223372036854775808
module	Declares a module. New in Java 9
native	Specifies that a method is not implemented in the same Java source file (but in another language)
new	Creates new objects
package	Declares a package
private	An access modifier used for attributes, methods and constructors, making them only accessible within the declared class
protected	An access modifier used for attributes, methods and constructors, making them accessible in the same package and subclasses
public	An access modifier used for classes, attributes, methods and constructors, making them accessible by any other class
requires	Specifies required libraries inside a module. New in Java 9
return	Finished the execution of a method, and can be used to return a value from a method
short	A data type that can store whole numbers from -32768 to 32767
static	A non-access modifier used for methods and attributes. Static methods/attributes can be accessed without creating an object of a class
strictfp	Restrict the precision and rounding of floating point calculations
super	Refers to superclass (parent) objects
switch	Selects one of many code blocks to be executed
synchronized	A non-access modifier, which specifies that methods can only be accessed by one thread at a time
this	Refers to the current object in a method or constructor
throw	Creates a custom error
throws	Indicates what exceptions may be thrown by a method
transient	A non-accesss modifier, which specifies that an attribute is not part of an object’s persistent state
try	Creates a try. catch statement
var	Declares a variable. New in Java 10
void	Specifies that a method should not have a return value
volatile	Indicates that an attribute is not cached thread-locally, and is always read from the «main memory»
while	Creates a while loop

Note: true , false , and null are not keywords, but they are literals and reserved words that cannot be used as identifiers.

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Java Reserved and Contextual Keywords

Java has 51 reserved words that have very specific meanings and cannot be used as identifiers in the application code. Also, 16 contextual keywords are treated as keywords when found in a specific context. Programmers should not use these keywords for anything other than what they are meant to be.

1. What is a Keyword in Java?

The keywords are predefined, reserved words that have a very specific meaning for the compiler. These keywords cannot be used as variables, methods, classes, or any other identifiers.

In the following statement, int is a keyword that indicates that the variable age is of integer type (32-bit signed two’s complement integer). We can’t use int as a variable name etc. Using keywords as identifiers will result in compilation errors.

• The keywords const and goto are reserved, even though they are not currently used.
• true , false , and null might seem like keywords, but they are literal; we cannot use them as identifiers in our programs.
• strictfp was added in JDK 1.2.
• assert was added in JDK 1.4.
• enum was added in JDK 1.5.
• Later versions of features such as sealed classes, records and JPMS added few more contextual keywords.

The following 51 keywords cannot be used as identifiers.

Keyword	Description
abstract	Used with classes and methods. An abstract class cannot be instantiated. An abstract method is incomplete without the body and must be implemented in the child class to create an instance of the child class.
assert	enables us to test the assumptions about our program.
boolean	represents only one of two possible values i.e. either true or false .
break	is used to terminate for , while , or do-while loop. It may also be used to terminate a switch statement as well.
byte	can store whole numbers from -128 and 127.
case	represents a block of code in switch statements.
catch	follows the try block and handles the checked exceptions thrown by try block and any possible unchecked exceptions.
char	used to store a single character.
class	defines a class.
const	is a reserved keyword for constant values. Use final instead.
continue	skips the current iteration of a for, while, or do-while loops and jumps to the next iteration.
default	used to specify the default block in a switch statement and default methods in functional interfaces.
do	used to contain the statements to execute repeatedly until the condition in the while statement is true.
double	used to declare a variable that can hold 64-bit floating-point number.
else	used to indicate the alternative branches in an if statement.
enum	is a type whose fields consist of a fixed set of constants.
extends	used for extending a class.
final	used with class variables, methods or classes. A final variable cannot be assigned another value after it has been initialized. A final method cannot be overridden in the child class. No class can subclass a final class.
finally	contains code to be executed everytime a try-catch block is completed – either with errors or without any error.
float	used to declare a variable that can hold a 32-bit floating-point number.
for	start a loop to execute a set of instructions repeatedly when a condition is true. If the number of iterations is known, it is recommended to use for loop.
goto	Currently, not in use.
if	used for writing conditional statements.
implements	used for implementing an interface.
import	import a package, class or interface to the current class.
instanceof	Checks whether an object is an instance of a specific class or an interface.
int	used to store a 32-bit integer value.
interface	declares an interface.
long	used to store a 64-bit integer value.
native	indicates native code (platform-specific).
new	creates a new object of the specified class.
package	declares a package for storing the related classes.
private	access modifier to indicate that a method or variable may be accessed only in the class in which it is declared.
protected	access modifier to indicate that a class, method or variable may be accessed only in the current package, or inherited outside the current package.
public	access modifier to indicate that a class, method or variable is accessible everywhere.
return	used to return from a method when its execution is complete.
short	used to store a 16-bit integer value.
static	indicates that a variable or method belongs to the class object, not to the individual instances of that class.
strictfp	used to restrict the floating-point calculations to ensure portability.
super	used to refer to parent class objects.
switch	help in providing multiple possible execution paths for a program.
synchronized	marks a block or method a critical section where one and only one thread is executing at a time.
this	used to refer to the current object.
throw	used to explicitly throw an exception from a method or constructor.
throws	used to declare the list of exceptions that may be thrown by that method or constructor.
transient	used on class attributes/variables to indicate that the serialization process of this class should ignore such variables.
try	contains the application code which is expected to work in normal conditions.
void	specifies that a method should not have a return value.
volatile	indicates that an attribute is not cached thread-locally, and is always read from the “main memory”.
while	continually executes a block of statements until a particular condition evaluates to true
_ (Underscore)	added in Java 9, to prevent writing underscores as an unused lambda, method, or catch formal parameter.

The following 16 words can be interpreted as keywords or as other tokens, depending on the context in which they appear.

• The module and open are used for declaring modules.
• The exports , opens , provides , requires , to , uses , and with are used for importing and exporting the modules.
• The transitive is recognized as a terminal in a RequiresModifier.
• The var is used to infer local variable types.
• The yield is used to yield a value in a switch statement.
• The record is used to define new record types.
• The non-sealed , permits , and sealed are used to define sealed classes and interfaces.

Learning about all keywords on a single page is not possible. We will learn about each Java keyword in its dedicated tutorial.

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Java Language Keywords

Here is a list of keywords in the Java programming language. You cannot use any of the following as identifiers in your programs. The keywords const and goto are reserved, even though they are not currently used. true , false , and null might seem like keywords, but they are actually literals; you cannot use them as identifiers in your programs.

abstract	continue	for	new	switch
assert ***	default	goto *	package	synchronized
boolean	do	if	private	this
break	double	implements	protected	throw
byte	else	import	public	throws
case	enum ****	instanceof	return	transient
catch	extends	int	short	try
char	final	interface	static	void
class	finally	long	strictfp **	volatile
const *	float	native	super	while

*	not used
**	added in 1.2
***	added in 1.4
****	added in 5.0

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