Parent and child classes in Java - Mini Project: Build & Apply

Parent and Child Classes

📖 Scenario: You are creating a simple program to represent vehicles. You want to show how a child class can inherit properties from a parent class.

🎯 Goal: Build a parent class called Vehicle and a child class called Car that inherits from Vehicle. Then create an object of Car and print its details.

📋 What You'll Learn

Create a parent class named Vehicle with a String field called brand.

Create a child class named Car that extends Vehicle and adds an int field called year.

Create a constructor in Vehicle to set the brand.

Create a constructor in Car to set both brand and year.

Create a method displayInfo() in Car that prints the brand and year.

Create a Car object with brand "Toyota" and year 2020, then call displayInfo().

💡 Why This Matters

🌍 Real World

Understanding parent and child classes helps organize code by sharing common features and adding specific details, like different types of vehicles.

💼 Career

Inheritance is a key concept in object-oriented programming used in many software development jobs to build reusable and maintainable code.

1

Create the parent class Vehicle

Create a public class called Vehicle with a String field named brand. Add a constructor that takes a String brand parameter and sets the field.

Java

public class Vehicle {
    // Your code here
}

Hint

Think of Vehicle as a blueprint with a brand name. The constructor sets the brand.

2

Create the child class Car

Create a public class called Car that extends Vehicle. Add an int field named year. Create a constructor that takes String brand and int year parameters. Use super(brand) to call the parent constructor and set year.

Java

public class Vehicle {
    String brand;

    public Vehicle(String brand) {
        this.brand = brand;
    }
}

// Your code here

Hint

Use extends to inherit from Vehicle. Use super to call the parent constructor.

3

Add displayInfo method in Car

Inside the Car class, create a public method called displayInfo() that prints the brand and year in this format: "Brand: [brand], Year: [year]".

Java

public class Vehicle {
    String brand;

    public Vehicle(String brand) {
        this.brand = brand;
    }
}

public class Car extends Vehicle {
    int year;

    public Car(String brand, int year) {
        super(brand);
        this.year = year;
    }

    // Create displayInfo method here
    // Your code here
}

Hint

Use System.out.println to print the message. Access brand directly because it is inherited.

4

Create Car object and print info

In a public class called Main, create the main method. Inside it, create a Car object with brand "Toyota" and year 2020. Call the displayInfo() method on this object to print the details.

Java

public class Vehicle {
    String brand;

    public Vehicle(String brand) {
        this.brand = brand;
    }
}

public class Car extends Vehicle {
    int year;

    public Car(String brand, int year) {
        super(brand);
        this.year = year;
    }

    public void displayInfo() {
        System.out.println("Brand: " + brand + ", Year: " + year);
    }
}

// Your code here

Hint

Remember the main method signature and how to create objects with new.

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