Java Keywords &amp; Reserved Words

// ❌ WRONG - Using keyword as variable name
int class = 5;        // Error: 'class' is a keyword
String public = "hi"; // Error: 'public' is a keyword

// ✅ CORRECT
int studentClass = 5;
String publicMessage = "hi";

Case Sensitivity

Java

// Keywords are lowercase and case-sensitive
public class Example {  // ✅ CORRECT: 'public' is lowercase
    // Public class Example { } // ❌ WRONG: 'Public' is not a keyword
}

Java Keywords by Category

1. Access Modifiers (4 keywords)

Control visibility of classes, methods, and variables.

`public`

Accessible from anywhere.

Java

public class MyClass {
    public int publicVar = 10;
    
    public void publicMethod() {
        System.out.println("Accessible from anywhere");
    }
}

// Different package
MyClass obj = new MyClass();
obj.publicMethod();  // ✅ Works

`private`

Accessible only within the same class.

Java

public class BankAccount {
    private double balance = 1000.0;  // Only accessible inside this class
    
    private void updateBalance() {
        // Private method
    }
    
    public double getBalance() {
        return balance;  // Access private variable through public method
    }
}

// Usage
BankAccount account = new BankAccount();
// account.balance;  // ❌ Error: balance is private
account.getBalance();  // ✅ Works

`protected`

Accessible within same package and subclasses.

Java

// Parent class
public class Animal {
    protected String species;  // Accessible in subclasses
    
    protected void makeSound() {
        System.out.println("Some sound");
    }
}

// Subclass (even in different package)
public class Dog extends Animal {
    public void bark() {
        species = "Canine";  // ✅ Can access protected member
        makeSound();         // ✅ Can access protected method
    }
}

`default` (package-private)

No keyword - accessible only within same package.

Java

// No access modifier = package-private
class PackageClass {  // Only visible in same package
    int packageVar;   // Only accessible in same package
    
    void packageMethod() {
        // Only accessible in same package
    }
}

2. Class/Method/Variable Modifiers (7 keywords)

`static`

Belongs to class, not instance.

Java

public class Counter {
    static int count = 0;  // Shared across all instances
    
    static void increment() {
        count++;
    }
    
    public static void main(String[] args) {
        Counter.increment();  // Call without creating object
        System.out.println(Counter.count);  // Output: 1
    }
}

`final`

Cannot be changed (constant variable, non-overridable method, non-inheritable class).

Java

// Final variable (constant)
final int MAX_SIZE = 100;
// MAX_SIZE = 200;  // ❌ Error: cannot reassign

// Final method (cannot be overridden)
class Parent {
    final void display() {
        System.out.println("Cannot override this");
    }
}

class Child extends Parent {
    // void display() { }  // ❌ Error: cannot override final method
}

// Final class (cannot be extended)
final class Utility {
    // ...
}

// class MyUtility extends Utility { }  // ❌ Error: cannot extend final class

`abstract`

Incomplete class or method (must be overridden).

Java

abstract class Shape {
    abstract void draw();  // No implementation
    
    void display() {  // Can have concrete methods too
        System.out.println("This is a shape");
    }
}

class Circle extends Shape {
    @Override
    void draw() {  // Must implement abstract method
        System.out.println("Drawing circle");
    }
}

// Shape s = new Shape();  // ❌ Error: cannot instantiate abstract class
Shape s = new Circle();     // ✅ Works
s.draw();

`synchronized`

Thread-safe method or block.

Java

public class BankAccount {
    private int balance = 1000;
    
    // Only one thread can execute this at a time
    public synchronized void withdraw(int amount) {
        if (balance >= amount) {
            balance -= amount;
        }
    }
    
    // Synchronized block
    public void deposit(int amount) {
        synchronized(this) {
            balance += amount;
        }
    }
}

`volatile`

Variable may be modified by multiple threads.

Java

public class SharedData {
    // Ensures visibility across threads
    private volatile boolean flag = false;
    
    public void setFlag() {
        flag = true;  // Change visible to all threads immediately
    }
}

`transient`

Field not serialized.

Java

import java.io.Serializable;

public class User implements Serializable {
    String username;
    transient String password;  // Won't be saved during serialization
    
    // When object is saved, password is not included
}

`native`

Method implemented in platform-specific code (C/C++).

Java

public class NativeExample {
    // Implementation in C/C++
    public native void nativeMethod();
    
    static {
        System.loadLibrary("nativelib");
    }
}

3. Control Flow (11 keywords)

`if`, `else`

Conditional execution.

Java

int age = 18;

if (age >= 18) {
    System.out.println("Adult");
} else if (age >= 13) {
    System.out.println("Teenager");
} else {
    System.out.println("Child");
}

`switch`, `case`, `default`

Multiple condition selection.

Java

int day = 3;

switch (day) {
    case 1:
        System.out.println("Monday");
        break;
    case 2:
        System.out.println("Tuesday");
        break;
    case 3:
        System.out.println("Wednesday");
        break;
    default:
        System.out.println("Other day");
}

`for`

Loop with initialization, condition, increment.

Java

// Traditional for loop
for (int i = 0; i < 5; i++) {
    System.out.println(i);
}

// Enhanced for loop (for-each)
int[] numbers = {1, 2, 3, 4, 5};
for (int num : numbers) {
    System.out.println(num);
}

`while`

Loop while condition is true.

Java

int count = 0;
while (count < 5) {
    System.out.println(count);
    count++;
}

`do`

Execute at least once, then check condition.

Java

int count = 0;
do {
    System.out.println(count);  // Executes at least once
    count++;
} while (count < 5);

`break`

Exit loop or switch.

Java

// Exit loop
for (int i = 0; i < 10; i++) {
    if (i == 5) {
        break;  // Exit loop when i is 5
    }
    System.out.println(i);
}

// Exit switch
switch (value) {
    case 1:
        doSomething();
        break;  // Exit switch
}

`continue`

Skip current iteration.

Java

for (int i = 0; i < 10; i++) {
    if (i % 2 == 0) {
        continue;  // Skip even numbers
    }
    System.out.println(i);  // Only prints odd numbers
}

`return`

Exit method and optionally return value.

Java

public int add(int a, int b) {
    return a + b;  // Return result
}

public void printMessage() {
    System.out.println("Hello");
    return;  // Optional for void methods
}

4. Exception Handling (6 keywords)

`try`, `catch`, `finally`

Handle exceptions.

Java

try {
    int result = 10 / 0;  // May throw ArithmeticException
} catch (ArithmeticException e) {
    System.out.println("Cannot divide by zero");
} finally {
    System.out.println("Always executes");
}

`throw`

Manually throw exception.

Java

public void validateAge(int age) {
    if (age < 0) {
        throw new IllegalArgumentException("Age cannot be negative");
    }
}

`throws`

Declare exceptions method may throw.

Java

public void readFile() throws IOException {
    FileReader file = new FileReader("file.txt");
    // May throw IOException
}

`assert`

Testing condition (disabled by default).

Java

public void divide(int a, int b) {
    assert b != 0 : "Divisor cannot be zero";
    int result = a / b;
}

// Enable with: java -ea ClassName

5. Primitive Data Types (8 keywords)

Java

// Integer types
byte smallNumber = 127;        // 8-bit: -128 to 127
short mediumNumber = 32000;    // 16-bit: -32,768 to 32,767
int number = 100000;           // 32-bit: -2^31 to 2^31-1
long bigNumber = 10000000000L; // 64-bit: -2^63 to 2^63-1

// Floating-point types
float decimal = 3.14f;         // 32-bit
double preciseDecimal = 3.14159265359; // 64-bit

// Character type
char letter = 'A';             // 16-bit Unicode

// Boolean type
boolean isTrue = true;         // true or false

6. Object-Oriented (7 keywords)

`class`

Define a class.

Java

public class Student {
    String name;
    int age;
    
    void study() {
        System.out.println(name + " is studying");
    }
}

`interface`

Define contract (abstract methods).

Java

public interface Drawable {
    void draw();  // Abstract method (no body)
    
    default void display() {  // Default method (Java 8+)
        System.out.println("Displaying");
    }
}

`extends`

Inherit from class.

Java

class Animal {
    void eat() {
        System.out.println("Eating");
    }
}

class Dog extends Animal {  // Dog inherits from Animal
    void bark() {
        System.out.println("Barking");
    }
}

`implements`

Implement interface.

Java

interface Flyable {
    void fly();
}

class Bird implements Flyable {
    @Override
    public void fly() {
        System.out.println("Flying");
    }
}

`new`

Create object instance.

Java

Student student = new Student();  // Create new object
int[] array = new int[10];        // Create new array

`this`

Reference to current object.

Java

public class Person {
    String name;
    
    public Person(String name) {
        this.name = name;  // Distinguish field from parameter
    }
    
    public void display() {
        this.printName();  // Call method on current object
    }
    
    private void printName() {
        System.out.println(this.name);
    }
}

`super`

Reference to parent class.

Java

class Animal {
    void makeSound() {
        System.out.println("Some sound");
    }
}

class Dog extends Animal {
    @Override
    void makeSound() {
        super.makeSound();  // Call parent method
        System.out.println("Bark");
    }
}

7. Package Management (2 keywords)

`package`

Declare package.

Java

package com.example.myapp;  // Must be first statement

public class MyClass {
    // ...
}

`import`

Import classes from other packages.

Java

import java.util.ArrayList;     // Import single class
import java.util.*;             // Import all classes
import static java.lang.Math.*; // Static import

public class Example {
    public static void main(String[] args) {
        ArrayList<String> list = new ArrayList<>();
        System.out.println(sqrt(16));  // Static import allows this
    }
}

8. Special Keywords (4 keywords)

`void`

Method returns no value.

Java

public void printMessage() {
    System.out.println("Hello");
    // No return statement needed
}

`instanceof`

Check object type.

Java

Object obj = "Hello";

if (obj instanceof String) {
    String str = (String) obj;  // Safe to cast
    System.out.println(str.toUpperCase());
}

`null`

Null reference (absence of value).

Java

String text = null;  // No object assigned

if (text == null) {
    System.out.println("Text is null");
}

// text.length();  // ❌ NullPointerException

`enum`

Define enumeration (Java 5+).

Java

public enum Day {
    MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY, SUNDAY
}

// Usage
Day today = Day.MONDAY;

switch (today) {
    case MONDAY:
        System.out.println("Start of week");
        break;
    case FRIDAY:
        System.out.println("Almost weekend");
        break;
}

9. Reserved but Unused (2 keywords)

`goto`

Reserved but not implemented.

Java

// goto is reserved but cannot be used in Java
// goto label;  // ❌ Compile error

`const`

Reserved but not implemented (use final instead).

Java

// const is reserved but cannot be used
// const int MAX = 100;  // ❌ Compile error

// Use final instead
final int MAX = 100;  // ✅ Correct

Complete Keyword List (51 Total)

Access Modifiers (4)

public, private, protected, default (no keyword)

Modifiers (7)

static, final, abstract, synchronized, volatile, transient, native

Control Flow (11)

if, else, switch, case, default, for, while, do, break, continue, return

Exception Handling (6)

try, catch, finally, throw, throws, assert

Primitive Types (8)

byte, short, int, long, float, double, char, boolean

Object-Oriented (7)

class, interface, extends, implements, new, this, super

Package (2)

package, import

Special (4)

void, instanceof, null, enum

Reserved (2)

goto, const

Common Mistakes

Mistake 1: Using Keywords as Identifiers

Java

// ❌ WRONG
int class = 5;
String return = "value";
void private() { }

// ✅ CORRECT
int studentClass = 5;
String returnValue = "value";
void makePrivate() { }

Mistake 2: Case Errors

Java

// ❌ WRONG
Public class MyClass { }  // 'Public' is not a keyword
STATIC int count;         // 'STATIC' is not a keyword

// ✅ CORRECT
public class MyClass { }  // Lowercase
static int count;         // Lowercase

Mistake 3: Confusing `final`, `finally`, `finalize`

Java

// final - constant/non-overridable
final int MAX = 100;

// finally - always executes in try-catch
try {
    // code
} finally {
    // always runs
}

// finalize() - garbage collection (deprecated)
@Override
protected void finalize() {
    // Called before object is garbage collected
}

Mistake 4: Misusing `static`

Java

public class Example {
    int instanceVar = 10;
    static int staticVar = 20;
    
    // ❌ WRONG - Cannot access instance variable from static method
    public static void staticMethod() {
        // System.out.println(instanceVar);  // Error
        System.out.println(staticVar);  // ✅ OK
    }
    
    // ✅ CORRECT - Instance method can access both
    public void instanceMethod() {
        System.out.println(instanceVar);  // ✅ OK
        System.out.println(staticVar);    // ✅ OK
    }
}

Mistake 5: Breaking Without Label

Java

// ❌ WRONG - Trying to break outer loop
for (int i = 0; i < 5; i++) {
    for (int j = 0; j < 5; j++) {
        if (j == 2) {
            break;  // Only breaks inner loop
        }
    }
}

// ✅ CORRECT - Using labeled break
outer:
for (int i = 0; i < 5; i++) {
    for (int j = 0; j < 5; j++) {
        if (j == 2) {
            break outer;  // Breaks outer loop
        }
    }
}

Keywords by Frequency of Use

Very Common (Learn First)

public, private, static, void, class, new, if, else, for, while, return, int, String, this, null

Common

protected, final, abstract, try, catch, throw, throws, switch, case, break, continue, extends, implements, super, import, package

Less Common

synchronized, volatile, transient, native, instanceof, enum, assert, default, interface, do, byte, short, long, float, double, char, boolean

Rare

goto (reserved), const (reserved)

Interview Tips

Interview Questions About Keywords:

Q: What's the difference between final, finally, and finalize?

›final: Makes variable constant, method non-overridable, class non-extendable
›finally: Block that always executes in try-catch
›finalize(): Method called before garbage collection (deprecated)

Q: When to use static?

›Utility methods that don't need object state
›Constants shared across all instances
›Factory methods
›Main method

Q: What's the difference between abstract class and interface?

›Abstract class: Can have both abstract and concrete methods, constructors, fields
›Interface: Only abstract methods (before Java 8), no constructors, only constants

Q: What's transient used for?

›Mark fields that shouldn't be serialized
›Example: Passwords, temporary data

Q: Can you use goto in Java?

›No, it's reserved but not implemented
›Use labeled break/continue instead

Key Takeaways

Java Keywords Summary:

Total Keywords: 51 reserved words

Categories:

›Access modifiers: Control visibility
›Class modifiers: Define behavior (static, final, abstract)
›Control flow: Direct program execution
›Exception handling: Manage errors
›Primitive types: Basic data types
›OOP: Classes, interfaces, inheritance
›Package: Organize code

Most Important:

›public, private, protected: Access control
›static: Class-level members
›final: Constants and immutability
›abstract: Incomplete classes/methods
›class, interface: Define types
›extends, implements: Inheritance
›this, super: Object references
›new: Create objects
›if, for, while: Control flow
›try, catch: Exception handling

Remember:

›Keywords are case-sensitive (lowercase only)
›Cannot be used as identifiers
›goto and const are reserved but unused
›8 primitive types, 6 exception keywords, 11 control flow

Common Mistakes to Avoid:

›Using keywords as variable names
›Capitalizing keywords (Public instead of public)
›Confusing final/finally/finalize
›Accessing instance members from static context

Congratulations! You now know all Java keywords and how to use them correctly!

What to Read Next

Continue Learning Basics

Next Step

Data Types

Deep dive into Java's primitive and reference types.

Variables

Learn variable declaration and scope basics.

Access Modifiers

Go deeper on access control keywords like public and private.

static Keyword

Master static members — one of Java's most-used keywords.

Practice Programs

Put this topic into practice — each program has full code, output and a step-by-step explanation you can run online.

this Keyword

Uses this to distinguish a constructor parameter from a field of the same name.

super Keyword

Uses super to call a parent class's constructor and its overridden method.

Final Variable

A final variable assigned once and never reassigned.

Browse all Java programs →

Static Variables

Uses a static field to track a count shared across every object of a class.

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