Default methods in Java - Mini Project: Build & Apply

Using Default Methods in Java Interfaces

📖 Scenario: Imagine you are building a simple system for different types of vehicles. Each vehicle can describe itself. You want to provide a common description method that all vehicles can use, but also allow each vehicle to add its own details.

🎯 Goal: You will create an interface with a default method that provides a basic description. Then, you will create a class that implements this interface and uses the default method. Finally, you will print the description of the vehicle.

📋 What You'll Learn

Create an interface called Vehicle with a default method describe() that returns a String.

The default describe() method should return the String "This is a vehicle."

Create a class called Car that implements the Vehicle interface.

In the Car class, do not override the describe() method.

Create a main method to create an instance of Car and print the result of calling describe().

💡 Why This Matters

🌍 Real World

Default methods allow adding new functionality to interfaces without breaking existing code. This is useful when evolving APIs or libraries.

💼 Career

Understanding default methods is important for Java developers to maintain and extend interfaces safely in large projects.

1

Create the Vehicle interface with a default method

Create an interface called Vehicle with a default method named describe() that returns the String "This is a vehicle."

Java

// Your code here

Hint

Use the default keyword inside the interface to create a method with a body.

2

Create the Car class implementing Vehicle

Create a class called Car that implements the Vehicle interface. Do not override the describe() method.

Java

public interface Vehicle {
    default String describe() {
        return "This is a vehicle.";
    }
}

// Your code here

Hint

Just declare the class and implement the interface. No need to write the describe method again.

3

Add the main method to create Car and call describe

Add a main method inside the Car class. Inside main, create an instance of Car named myCar and call myCar.describe(), storing the result in a variable called description.

Java

public interface Vehicle {
    default String describe() {
        return "This is a vehicle.";
    }
}

public class Car implements Vehicle {
    // No override of describe()

    // Your code here
}

Hint

Remember the syntax for the main method and how to create an object and call a method.

4

Print the description

Inside the main method of the Car class, add a line to print the description variable using System.out.println().

Java

public interface Vehicle {
    default String describe() {
        return "This is a vehicle.";
    }
}

public class Car implements Vehicle {
    // No override of describe()

    public static void main(String[] args) {
        Car myCar = new Car();
        String description = myCar.describe();
        // Your code here
    }
}

Hint

Use System.out.println(description); to print the text.

Practice

1. What is the main purpose of default methods in Java interfaces?

easy

A. To enable multiple inheritance of classes

B. To declare abstract methods that must be implemented by classes

C. To create private helper methods inside interfaces

D. To allow interfaces to have method bodies without breaking existing implementations

Solution

1. Step 1: Understand interface method rules before Java 8

   Interfaces could only declare abstract methods without bodies, forcing all implementing classes to define them.

2. Step 2: Role of default methods

   Default methods allow interfaces to provide method bodies, so new methods can be added without breaking existing classes.

3. Final Answer:

   To allow interfaces to have method bodies without breaking existing implementations -> Option D

4. Quick Check:

   Default methods add bodies to interfaces safely [OK]

Hint: Default methods add code to interfaces without breaking old classes [OK]

Common Mistakes:

• Confusing default methods with abstract methods
• Thinking default methods enable multiple inheritance of classes
• Believing default methods are private helper methods

2. Which of the following is the correct syntax to declare a default method in a Java interface?

easy

A. default void show() { System.out.println("Hello"); }

B. void default show() { System.out.println("Hello"); }

C. void show() default { System.out.println("Hello"); }

D. default show() void { System.out.println("Hello"); }

Solution

1. Step 1: Recall default method syntax

   Default methods start with the keyword default, followed by the return type, method name, and body.

2. Step 2: Check each option

   default void show() { System.out.println("Hello"); } matches the correct syntax: default void show() { ... }. Others have incorrect order or keywords.

3. Final Answer:

   default void show() { System.out.println("Hello"); } -> Option A

4. Quick Check:

   default + return type + method name + body = correct syntax [OK]

Hint: default keyword comes before return type in interface methods [OK]

Common Mistakes:

• Placing default keyword after return type
• Using default as a method modifier incorrectly
• Omitting method body for default methods

3. What will be the output of the following code?

interface A {
    default void greet() {
        System.out.println("Hello from A");
    }
}

class B implements A {
    public void greet() {
        System.out.println("Hello from B");
    }
}

public class Test {
    public static void main(String[] args) {
        A obj = new B();
        obj.greet();
    }
}

medium

A. Hello from B

B. Hello from A

C. Compilation error

D. Runtime error

Solution

1. Step 1: Understand method overriding with default methods

   Class B overrides the default greet() method from interface A with its own implementation.

2. Step 2: Determine which greet() is called

   At runtime, the overridden method in class B is called, printing "Hello from B".

3. Final Answer:

   Hello from B -> Option A

4. Quick Check:

   Overridden method in class wins [OK]

Hint: Class method overrides interface default method [OK]

Common Mistakes:

• Assuming default method runs instead of overridden
• Expecting compilation error due to default method
• Confusing runtime and compile-time behavior

4. Identify the error in the following code snippet:

interface X {
    default void display() {
        System.out.println("X display");
    }
}

interface Y {
    default void display() {
        System.out.println("Y display");
    }
}

class Z implements X, Y {
    public void display() {
        // ???
    }
}

What should be done inside class Z's display() method to fix the error?

medium

A. Remove one interface from implements list

B. Call X.super.display() or Y.super.display() to resolve ambiguity

C. Make display() method abstract in class Z

D. No changes needed; code compiles fine

Solution

1. Step 1: Understand diamond problem with default methods

   Class Z implements two interfaces X and Y, both having default display() methods, causing ambiguity.

2. Step 2: Resolve ambiguity by overriding and calling specific interface method

   Class Z must override display() and explicitly call one interface's default method using X.super.display() or Y.super.display().

3. Final Answer:

   Call X.super.display() or Y.super.display() to resolve ambiguity -> Option B

4. Quick Check:

   Explicit super call fixes default method conflict [OK]

Hint: Use InterfaceName.super.method() to fix default method conflicts [OK]

Common Mistakes:

• Ignoring ambiguity and expecting code to compile
• Trying to remove interfaces instead of overriding
• Making method abstract in a concrete class

5. Given two interfaces with default methods, how can a class implement both and combine their behaviors in a single method?

interface Printer {
    default void print() {
        System.out.println("Printing document");
    }
}

interface Scanner {
    default void print() {
        System.out.println("Scanning document");
    }
}

class MultiFunctionDevice implements Printer, Scanner {
    public void print() {
        // Combine both behaviors here
    }
}

Which code inside print() correctly combines both default methods?

hard

A. super.print();

B. print(); print();

C. Printer.super.print(); Scanner.super.print();

D. Printer.print(); Scanner.print();

Solution

1. Step 1: Understand calling multiple default methods

   To combine behaviors, the class must explicitly call each interface's default method using InterfaceName.super.method().

2. Step 2: Check each option

   Printer.super.print(); Scanner.super.print(); correctly calls both default methods. print(); print(); causes infinite recursion. super.print(); is invalid syntax. Printer.print(); Scanner.print(); is invalid because interfaces cannot be called like classes.

3. Final Answer:

   Printer.super.print(); Scanner.super.print(); -> Option C

4. Quick Check:

   Use InterfaceName.super.method() to call multiple defaults [OK]

Hint: Call each interface default with InterfaceName.super.method() [OK]

Common Mistakes:

• Calling method recursively causing stack overflow
• Using super.print() without interface name
• Trying to call interface methods like static methods