Inheritance in Java

What is Inheritance? Inheritance is a mechanism in Java where one class (subclass or child) can inherit the fields and methods of another class (superclass or parent).

This promotes code reuse, hierarchical classification, and polymorphism.

Overview:

• SuperClass ans SubClass
• Common relationships
• Functions in a hierarchy
• Instanceof operator
• Casting

A. SuperClass ans SubClass

| Concept | Description | | ———— | —————————————— | | Inheritance | One class inherits another | | Superclass | Provides common functionality | | Subclass | Specializes or extends superclass behavior | | extends | Keyword for inheritance | | Constructors | Not inherited | | Access | Controlled via access modifiers |

1. Basic Example

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// Superclass
class Animal {
    void makeSound() {
        System.out.println("Animal makes sound");
    }
}

// Subclass
class Dog extends Animal {
    void bark() {
        System.out.println("Dog barks");
    }
}

public class Main {
    public static void main(String[] args) {
        Dog d = new Dog();
        d.makeSound();  // Inherited from Animal
        d.bark();       // Specific to Dog
    }
}

Dog inherits makeSound() from Animal. It also defines its own method bark().

2. Multilevel Inheritance

Java supports multilevel (not multiple) inheritance:

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

class Dog extends Animal {
    void bark() { System.out.println("barking..."); }
}

class Puppy extends Dog {
    void weep() { System.out.println("weeping..."); }
}

3. No Multiple Inheritance (of classes)

Java does not support multiple inheritance for classes to avoid ambiguity (known as the diamond problem).

However, OOP has multiple inheritance properties.

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class A {}
class B {}
// class C extends A, B {} // => Compilation Error

Java handles this via interfaces.

4. Access Modifiers and Inheritance

5. Constructor Behavior

• Constructors are not inherited.
• A subclass constructor must call a superclass constructor (implicitly or explicitly).

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class Animal {
    Animal() {
        System.out.println("Animal constructor");
    }
}

class Dog extends Animal {
    Dog() {
        super(); // optional if superclass has no-arg constructor
        System.out.println("Dog constructor");
    }
}

B. Common Relationships in Inheritance

Inheritance is used to model “is-a” relationships in object-oriented design. This section covers how classes relate to each other in Java using inheritance, including practical examples and guidelines.

1. The “is-a” Relationship

• In Java, inheritance represents an “is-a” relationship.
• If class B extends class A, then B is-a A.

Example:

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class Animal { }
class Dog extends Animal { }

Dog d = new Dog();
System.out.println(d instanceof Animal); // true

A Dog is an Animal

2. Common Real-World Examples

Each subclass inherits shared behavior and adds specific behavior.

3. Inheritance Design Patterns

a) Generalization to Specialization

Start with a general concept and refine it.

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class Vehicle {
    void move() { System.out.println("Moving..."); }
}

class Car extends Vehicle {
    void playMusic() { System.out.println("Playing music..."); }
}

b) Common Interface Extraction

When multiple classes share behavior, extract a superclass or interface:

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class Dog {
    void bark() { }
}

class Cat {
    void meow() { }
}

// Both can be made subclasses of Animal
class Animal {
    void makeSound() { }
}

4. “Has-a” vs “Is-a”

Relationship Meaning Implementation is-a One class is a type of another Use extends / implements has-a One class contains another Use object composition

Example:

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class Engine { }

class Car {
    private Engine engine; // has-a relationship
}

5. When to Use Inheritance

Use inheritance when:

• There’s a clear “is-a” relationship.
• You want to reuse common logic.
• You plan to use polymorphism (override methods in subclasses).

Avoid inheritance if:

• The relationship is “has-a” (prefer composition).
• It would create tight coupling or reduce flexibility.

6. UML Class Diagram Example

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           Animal
           /    \
        Dog     Cat

• Dog and Cat both extend Animal
• Both can override methods like makeSound()

C. Functions in a Hierarchy

When using inheritance, functions (methods) play a central role in how behavior is shared and customized across a class hierarchy. Java allows you to inherit, override, and extend methods in subclasses.

1. Inheriting Methods

If a superclass has a method, the subclass automatically inherits it — unless it overrides the method.

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

class Dog extends Animal {
    // Inherits makeSound() by default
}

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Dog d = new Dog();
d.makeSound(); // prints "Some animal sound"

2. Overriding Methods

A subclass can override a method to provide its own version of the behavior.

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

class Dog extends Animal {
    @Override
    void makeSound() {
        System.out.println("Bark!");
    }
}

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Animal a = new Dog();
a.makeSound(); // prints "Bark!" – Dog's version

Overriding enables runtime polymorphism

3. Rules of Method Overriding

• The method name, return type, and parameters must match exactly.
• The overridden method must not be less accessible than the superclass method.
• You can use the @Override annotation for clarity and safety.
• Only non-static methods can be overridden.

4. Calling Superclass Method with super

You can call the original version of the method from the superclass using super.

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

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

5. Method Accessibility in Hierarchy

6. Abstract Methods (in Abstract Classes)

An abstract method is a method declared without an implementation in an abstract class. Subclasses must implement it.

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abstract class Animal {
    abstract void makeSound();
}

class Cat extends Animal {
    @Override
    void makeSound() {
        System.out.println("Meow");
    }
}

7. Method Overloading vs Overriding

D. instanceof Operator

1. What is instanceof?

The instanceof operator is used to test whether an object is an instance of a specific class, subclass, or interface.

It returns a boolean value:

• true → the object is an instance of the specified type.
• false → it is not.

Syntax:

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object instanceof ClassName

Example:

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class Animal {}
class Dog extends Animal {}

public class Main {
    public static void main(String[] args) {
        Animal a = new Dog();

        System.out.println(a instanceof Dog);     // true
        System.out.println(a instanceof Animal);  // true
        System.out.println(a instanceof Object);  // true
    }
}

A Dog is an Animal and also an Object, so all checks return true.

2. Null-Safe Behavior

If the object is null, instanceof always returns false.

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Dog d = null;
System.out.println(d instanceof Dog); // false

3. Use Cases

4. With Interfaces

You can use instanceof to check if an object implements an interface:

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interface Pet {}

class Cat implements Pet {}

public class Main {
    public static void main(String[] args) {
        Pet p = new Cat();
        System.out.println(p instanceof Cat); // true
        System.out.println(p instanceof Pet); // true
    }
}

5. Pattern Matching

Since Java 16, instanceof supports pattern matching, which simplifies syntax:

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if (obj instanceof Dog d) {
    d.bark(); // No cast needed
}

This avoids explicit casting and is type-safe.

6. instanceof vs Bad Design

Overuse of instanceof may signal bad object-oriented design. Prefer polymorphism:

[NO] Instead of:

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if (shape instanceof Circle) {
    // special logic
}

[Yes] Use method overriding:

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shape.draw();

Let each subclass define its own behavior.

E. Casting in Inheritance

In Java, casting is used to convert one object reference to another type—typically between classes in the same inheritance hierarchy. It is essential when working with polymorphism.

1. Types of Casting

There are two main types:

2. Upcasting (Safe & Implicit)

Assigning a subclass object to a superclass reference.

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

class Dog extends Animal {
    void bark() { System.out.println("Bark!"); }
}

public class Main {
    public static void main(String[] args) {
        Dog dog = new Dog();
        Animal a = dog; // Upcasting
        a.makeSound();  // OK
        // a.bark();    // Not accessible
    }
}

Upcasting is always safe. The reference type (Animal) controls accessible methods.

3. Downcasting (Explicit & Risky)

Assigning a superclass reference to a subclass type. Requires explicit casting and is only safe if the object is really an instance of the subclass.

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Animal a = new Dog(); // Upcast
Dog d = (Dog) a;       // Downcast
d.bark();              // Now accessible

Unsafe Downcasting:

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Animal a = new Animal(); 
Dog d = (Dog) a;  // ClassCastException at runtime!

4. Safe Downcasting with instanceof

Always check type before downcasting to avoid runtime errors:

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if (a instanceof Dog) {
    Dog d = (Dog) a;
    d.bark();
} else {
    System.out.println("Not a Dog");
}

5. Common Mistake: Wrong Assumptions

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Animal a = new Animal();
Cat c = (Cat) a; // Compiles, but runtime error!

This throws a ClassCastException because a is not actually a Cat.