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.
int age;- The keywords
constandgotoare reserved, even though they are not currently used. true,false, andnullmight seem like keywords, but they are literal; we cannot use them as identifiers in our programs.strictfpwas added in JDK 1.2.assertwas added in JDK 1.4.enumwas added in JDK 1.5.- Later versions of features such as sealed classes, records and JPMS added few more contextual keywords.
2. Java 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. |
3. Contextual Keywords
The following 16 words can be interpreted as keywords or as other tokens, depending on the context in which they appear.
| Keyword | Description |
|---|---|
| exports | used for importing and exporting the modules. |
| module | used for declaring modules. |
| non-sealed | used to define sealed classes and interfaces. |
| open | used for declaring modules. |
| opens | used for importing and exporting the modules. |
| permits | used to define sealed classes and interfaces. |
| provides | used for importing and exporting the modules. |
| record | used to define new |
| requires | used for importing and exporting the modules. |
| sealed | used to define sealed classes and interfaces. |
| to | used for importing and exporting the modules. |
| transitive | recognized as a terminal in a RequiresModifier. |
| uses | used for importing and exporting the modules. |
| var | used to infer local variable types. |
| with | used for importing and exporting the modules. |
| yield | used to yield a value in a switch statement. |
- The
moduleandopenare used for declaring modules. - The
exports,opens,provides,requires,to,uses, andwithare used for importing and exporting the modules. - The
transitiveis recognized as a terminal in a RequiresModifier. - The
varis used to infer local variable types. - The
yieldis used to yield a value in a switch statement. - The
recordis used to define new record types. - The
non-sealed,permits, andsealedare 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.
Happy Learning !!
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