Multiple inheritance using interfaces in Java - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of multiple interface inheritance

What is the output of this Java program that uses multiple interfaces?

Java

interface A {
    default void show() {
        System.out.println("Interface A");
    }
}
interface B {
    default void show() {
        System.out.println("Interface B");
    }
}
class C implements A, B {
    public void show() {
        A.super.show();
        B.super.show();
        System.out.println("Class C");
    }
}
public class Test {
    public static void main(String[] args) {
        C obj = new C();
        obj.show();
    }
}

A

Interface A
Interface B
Class C

B

Class C
Interface A
Interface B

C

Interface B
Interface A
Class C

D Compilation error due to multiple inheritance conflict

💡 Hint

Look at how the show() method calls the default methods from both interfaces explicitly.

The class C overrides show() and calls both interface default methods using InterfaceName.super.show(), then prints its own message. So the output is Interface A, Interface B, then Class C.

❓ Predict Output

intermediate

Which method is called in multiple interface inheritance?

Given these interfaces and class, what will be printed when calling obj.display()?

Java

interface X {
    void display();
}
interface Y {
    default void display() {
        System.out.println("Y display");
    }
}
class Z implements X, Y {
    public void display() {
        System.out.println("Z display");
    }
}
public class Main {
    public static void main(String[] args) {
        Z obj = new Z();
        obj.display();
    }
}

A Runtime error

B Z display

C Compilation error because X has no default method

D Y display

💡 Hint

Class Z provides its own display() method, so it overrides the default.

Even though interface Y has a default display(), class Z overrides it with its own display() method, so "Z display" is printed.

🔧 Debug

advanced

Identify the error in multiple interface inheritance

What error will this code produce when compiled?

Java

interface P {
    void action();
}
interface Q {
    void action();
}
class R implements P, Q {
    // No action() method implemented
}
public class Demo {
    public static void main(String[] args) {
        R obj = new R();
        obj.action();
    }
}

A No error, prints nothing

B Runtime error: method action() not found

C Compilation error: duplicate method action() in interfaces

D Compilation error: class R must implement action()

💡 Hint

Check if class R provides implementation for all abstract methods from interfaces.

Both interfaces P and Q declare abstract method action(). Class R implements both but does not provide action(), so compilation fails.

📝 Syntax

advanced

Correct syntax for multiple interface inheritance

Which option correctly declares a class that inherits from two interfaces A and B?

A class MyClass inherits A, B {}

B class MyClass extends A, B {}

C class MyClass implements A, B {}

D class MyClass implements (A & B) {}

💡 Hint

In Java, classes use 'implements' keyword for interfaces and separate multiple interfaces with commas.

Java uses 'implements' keyword followed by comma-separated interface names to inherit multiple interfaces. 'extends' is for classes or interfaces extending interfaces.

🚀 Application

expert

Determine the number of methods in the resulting class

Given these interfaces and class, how many distinct methods does class D have after compilation?

Java

interface I1 {
    default void m1() {}
    void m2();
}
interface I2 {
    default void m1() {}
    void m3();
}
class D implements I1, I2 {
    public void m1() {}
    public void m2() {}
    public void m3() {}
}

A 3

B 2

C 4

D 5

💡 Hint

Count all methods declared or inherited, considering overrides and duplicates.

Class D overrides the conflicting default m1() from both I1 and I2, and implements the abstract m2() and m3(). Thus, it has three distinct methods: m1(), m2(), and m3().