Why Parent and child classes in Java? - Purpose & Use Cases

The Big Idea

What if you could write common code once and magically share it with many different things?

The Scenario

Imagine you have to write separate code for every type of vehicle: cars, trucks, and motorcycles. Each has some common features like speed and color, but also unique ones. Writing all these from scratch every time is tiring and confusing.

The Problem

Manually copying and changing code for each vehicle type leads to mistakes and wastes time. If you want to update a common feature, you must change it everywhere, risking errors and inconsistencies.

The Solution

Parent and child classes let you write shared features once in a parent class. Child classes then add or change only what is unique. This saves time, reduces errors, and keeps code organized.

Before vs After

✗ Before

class Car { int speed; String color; void drive() { /* code */ } } class Truck { int speed; String color; void drive() { /* code */ } }

✓ After

class Vehicle { int speed; String color; void drive() { /* code */ } } class Car extends Vehicle { /* unique code */ } class Truck extends Vehicle { /* unique code */ }

What It Enables

This concept makes it easy to build complex programs by reusing and customizing code efficiently.

Real Life Example

Think of a game where many characters share common moves but have special powers. Parent and child classes let you create a base character and then add unique powers to each type without repeating code.

Key Takeaways

Parent classes hold shared features.

Child classes add or change unique parts.

This saves time and reduces mistakes.

Practice

1.

What keyword is used in Java to create a child class from a parent class?

easy

A. extends

B. implements

C. inherits

D. super

Solution

1. Step 1: Understand class inheritance in Java

   Java uses a specific keyword to link a child class to a parent class, allowing reuse of code.

2. Step 2: Identify the correct keyword

   The keyword extends is used to create a child class that inherits from a parent class.

3. Final Answer:

   extends -> Option A

4. Quick Check:

   Inheritance keyword = extends [OK]

Hint: Remember: child class extends parent class [OK]

Common Mistakes:

• Using 'implements' which is for interfaces
• Using 'inherits' which is not a Java keyword
• Confusing 'super' keyword with inheritance declaration

2.

Which of the following is the correct syntax to declare a child class Dog that inherits from a parent class Animal?

?

easy

A. class Dog implements Animal {}

B. class Dog inherits Animal {}

C. class Dog extends Animal {}

D. class Dog Animal {}

Solution

1. Step 1: Recall Java class inheritance syntax

   In Java, the child class uses the keyword extends followed by the parent class name.

2. Step 2: Match the correct syntax

   Only class Dog extends Animal {} is valid syntax for inheritance.

3. Final Answer:

   class Dog extends Animal {} -> Option C

4. Quick Check:

   Syntax for inheritance = extends [OK]

Hint: Use 'extends' keyword to link child and parent classes [OK]

Common Mistakes:

• Using 'inherits' which is not a Java keyword
• Using 'implements' which is for interfaces
• Omitting the keyword between class names

3.

What will be the output of the following Java code?

class Parent {
    void show() {
        System.out.println("Parent class");
    }
}
class Child extends Parent {
    void show() {
        System.out.println("Child class");
    }
}
public class Test {
    public static void main(String[] args) {
        Parent obj = new Child();
        obj.show();
    }
}

medium

A. Parent class

B. Runtime error

C. Compilation error

D. Child class

Solution

1. Step 1: Understand method overriding and polymorphism

   The child class overrides the show() method of the parent class. The object is declared as parent type but created as child type.

2. Step 2: Determine which method runs at runtime

   Java uses runtime polymorphism, so the child class's show() method is called.

3. Final Answer:

   Child class -> Option D

4. Quick Check:

   Overridden method runs from child class [OK]

Hint: Object type decides method at runtime, not reference type [OK]

Common Mistakes:

• Thinking parent method runs because of reference type
• Expecting compilation or runtime errors
• Ignoring method overriding rules

4.

Find the error in the following Java code snippet:

class Parent {
    void greet() {
        System.out.println("Hello from Parent");
    }
}
class Child extends Parent {
    void greet() {
        System.out.println("Hello from Child");
    }
}
public class Test {
    public static void main(String[] args) {
        Child obj = new Parent();
        obj.greet();
    }
}

medium

A. Cannot assign Parent object to Child reference

B. Method greet() is missing return type

C. Child cannot extend Parent

D. No error, code runs fine

Solution

1. Step 1: Analyze object assignment compatibility

   In Java, a parent class object cannot be assigned to a child class reference because the parent may lack child-specific features.

2. Step 2: Identify the error in the code

   The line Child obj = new Parent(); causes a compile-time error due to incompatible types.

3. Final Answer:

   Cannot assign Parent object to Child reference -> Option A

4. Quick Check:

   Parent object cannot be assigned to child variable [OK]

Hint: Child reference needs child or subclass object [OK]

Common Mistakes:

• Thinking parent object can be assigned to child reference
• Confusing method return types with assignment errors
• Ignoring Java type compatibility rules

5.

Given the classes below, what will be the output when running new Child().display();?

class Parent {
    void display() {
        System.out.println("Parent display");
    }
}
class Child extends Parent {
    void display() {
        super.display();
        System.out.println("Child display");
    }
}

hard

A. Child display

B. Parent display
Child display

C. Parent display

D. Compilation error

Solution

1. Step 1: Understand use of super in child method

   The child class's display() method calls super.display(), which runs the parent class's display() method first.

2. Step 2: Determine output sequence

   First, "Parent display" is printed, then "Child display" is printed on the next line.

3. Final Answer:

   Parent display Child display -> Option B

4. Quick Check:

   super calls parent method before child output [OK]

Hint: super.method() runs parent method inside child method [OK]

Common Mistakes:

• Ignoring the call to super.display()
• Expecting only child output
• Thinking super causes error without constructor