Java Keywords
Java Keywords
Java keywords are reserved words that have a specific meaning in the Java programming language. These keywords are predefined and cannot be used as identifiers, such as variable names, method names, or class names. Understanding these keywords is essential for writing correct and efficient Java code. This tutorial will cover the Java keywords, their categories, and examples of how they are used in programs.
1. What are Java Keywords?
Java keywords are special words that the Java compiler recognizes and interprets as specific instructions. Since these keywords have a predefined meaning, you cannot use them for anything other than their intended purpose. For example, you cannot name a variable class or static because these are reserved keywords in Java.
2. List of Java Keywords
Java has 50 reserved keywords, which are categorized based on their functionality. Below is a detailed table listing the most commonly used keywords, their descriptions, and examples of how they are used in Java code.
2.1 Data Type Keywords
These keywords define the data types of variables and methods.
Keyword
Description
Example
byte
Defines an 8-bit integer.
byte b = 10;
short
Defines a 16-bit integer.
short s = 1000;
int
Defines a 32-bit integer.
int num = 12345;
long
Defines a 64-bit integer.
long l = 123456789L;
float
Defines a 32-bit floating-point number.
float f = 3.14f;
double
Defines a 64-bit floating-point number.
double d = 3.14159;
char
Defines a single 16-bit Unicode character.
char c = 'A';
boolean
Defines a variable with two possible values: true or false.
boolean isJavaFun = true;
void
Specifies that a method does not return any value.
void methodName() { }
2.2 Control Flow Keywords
These keywords control the flow of the program by defining loops, conditional statements, and decision-making structures.
Keyword
Description
Example
if
Executes a block of code if a condition is true.
if (condition) { }
else
Executes a block of code if the condition in the if statement is false.
else { }
switch
Allows multiple possible execution paths based on a variable's value.
switch (value) { }
case
Defines a block of code in a switch statement.
case 1: { }
default
Defines a block of code in a switch statement if no case matches.
default: { }
while
Repeatedly executes a block of code while a condition is true.
while (condition) { }
do
Executes a block of code once, then repeats the loop while a condition is true.
do { } while (condition);
for
Repeatedly executes a block of code a specified number of times.
for (int i = 0; i < n; i++) { }
break
Exits a loop or switch statement prematurely.
break;
continue
Skips the current iteration of a loop and proceeds to the next iteration.
continue;
return
Exits from a method and optionally returns a value.
return value;
2.3 Access Modifier Keywords
These keywords control the visibility and accessibility of classes, methods, and variables.
Keyword
Description
Example
public
The code is accessible from any other class.
public class MyClass { }
protected
The code is accessible within the same package and subclasses.
protected void method() { }
private
The code is accessible only within the same class.
private int number = 10;
2.4 Class and Object Keywords
These keywords are used to define classes, objects, and methods.
Keyword
Description
Example
class
Defines a class.
class MyClass { }
interface
Defines an interface, which is a contract that classes can implement.
interface MyInterface { }
extends
Indicates that a class is inheriting from a superclass.
class MyClass extends SuperClass { }
implements
Indicates that a class is implementing an interface.
class MyClass implements MyInterface { }
new
Creates new objects.
MyClass obj = new MyClass();
this
Refers to the current instance of a class.
this.variable = value;
super
Refers to the parent class and is used to call the parent class's methods or constructors.
super.method();
abstract
Defines a class or method that cannot be instantiated and must be subclassed.
abstract class MyClass { }
static
Indicates that a method or variable belongs to the class rather than any instance of the class.
static int count = 0;
final
Prevents a class from being subclassed, a method from being overridden, or a variable from being modified.
final int MAX = 100;
2.5 Exception Handling Keywords
These keywords are used to handle exceptions (errors) in Java.
Keyword
Description
Example
try
Defines a block of code to test for errors.
try { }
catch
Defines a block of code to handle errors if they occur.
catch (Exception e) { }
finally
Defines a block of code that will execute regardless of whether an error occurs.
finally { }
throw
Throws an exception manually.
throw new Exception();
throws
Indicates that a method may throw an exception.
void method() throws Exception { }
2.6 Other Important Keywords
These keywords have specific uses in Java programming.
Keyword
Description
Example
enum
Defines a set of named constants.
enum Day { MONDAY, TUESDAY, ... }
instanceof
Tests whether an object is an instance of a specific class or subclass.
if (obj instanceof MyClass) { }
synchronized
Ensures that a method or block of code is executed by only one thread at a time.
synchronized void method() { }
volatile
Indicates that a variable's value may be changed by different threads.
volatile int flag;
transient
Prevents a variable from being serialized.
transient int temp;
native
Indicates that a method is implemented in a language other than Java (e.g., C or C++).
native void method();
assert
Used for debugging purposes to test assumptions in the code.
assert condition;
strictfp
Ensures consistent floating-point calculations across different platforms.
strictfp class MyClass { }
package
Defines a package (a namespace that organizes classes).
package com.example;
import
Includes classes or entire packages into your program.
import java.util.Scanner;
3. Reserved Keywords (Not Used)
Java also reserves certain keywords for potential future use. These keywords are currently not used in the language but cannot be used as identifiers.
Keyword
Description
const
Reserved for future use; cannot be used as a variable name.
goto
Reserved for future use; cannot be used as a variable name.
Conclusion
Java keywords are the backbone of the language's syntax and serve specific roles in your programs. Understanding these keywords is crucial for writing correct and efficient Java code. Since these words have predefined meanings, they cannot be used for naming variables, classes, or methods.
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