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().

Practice

1. What is the main reason Java uses interfaces for multiple inheritance instead of classes?

easy

A. To avoid the diamond problem and allow safe multiple inheritance

B. Because Java does not support inheritance at all

C. To make code run faster

D. To prevent any method overriding

Solution

1. Step 1: Understand Java inheritance limitations

   Java does not allow multiple inheritance with classes to avoid ambiguity known as the diamond problem.

2. Step 2: Role of interfaces in multiple inheritance

   Interfaces allow multiple inheritance of type without implementation conflicts, enabling safe multiple inheritance.

3. Final Answer:

   To avoid the diamond problem and allow safe multiple inheritance -> Option A

4. Quick Check:

   Interfaces solve diamond problem = A [OK]

Hint: Interfaces enable multiple inheritance safely in Java [OK]

Common Mistakes:

• Thinking Java supports multiple class inheritance
• Believing interfaces improve speed
• Confusing method overriding prevention

2. Which of the following is the correct syntax for a class Car implementing two interfaces Engine and Wheels?

easy

A. public class Car implements Engine & Wheels {}

B. public class Car extends Engine, Wheels {}

C. public class Car implements Engine, Wheels {}

D. public class Car extends Engine implements Wheels {}

Solution

1. Step 1: Recall Java syntax for implementing multiple interfaces

   Java uses the keyword implements followed by a comma-separated list of interfaces.

2. Step 2: Check each option's syntax

   public class Car implements Engine, Wheels {} correctly uses implements Engine, Wheels. Options B and D incorrectly use extends for interfaces or mix keywords wrongly. public class Car implements Engine & Wheels {} uses an invalid '&' symbol.

3. Final Answer:

   public class Car implements Engine, Wheels {} -> Option C

4. Quick Check:

   Multiple interfaces use comma with implements = A [OK]

Hint: Use 'implements' with commas for multiple interfaces [OK]

Common Mistakes:

• Using extends instead of implements for interfaces
• Using '&' instead of commas
• Mixing extends and implements incorrectly

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

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

medium

A. B\nA

B. A\nB

C. Compilation error due to method conflict

D. No output

Solution

1. Step 1: Understand default methods in interfaces

   Both interfaces A and B have a default method show(). Class C implements both and overrides show().

2. Step 2: Analyze method calls inside C's show()

   C's show() calls A.super.show() then B.super.show(), so it prints "A" then "B" on separate lines.

3. Final Answer:

   A B -> Option B

4. Quick Check:

   Calling both interface methods prints A then B [OK]

Hint: Use InterfaceName.super.method() to call specific default methods [OK]

Common Mistakes:

• Expecting compilation error without override
• Confusing order of prints
• Missing super calls causing ambiguity

4. Identify the error in the following code snippet:

interface X {
    void display();
}
interface Y {
    void display();
}
class Z implements X, Y {
    // No display() method implemented
}

medium

A. Class Z must implement display() method or be declared abstract

B. No error, code compiles fine

C. Interfaces cannot have methods with same name

D. Class Z should extend interfaces, not implement

Solution

1. Step 1: Check interface method requirements

   Interfaces X and Y both declare display() method without implementation.

2. Step 2: Check class Z implementation

   Class Z implements both interfaces but does not provide display() method, so it must be abstract or implement the method.

3. Final Answer:

   Class Z must implement display() method or be declared abstract -> Option A

4. Quick Check:

   Implement all interface methods or declare abstract = D [OK]

Hint: Implement all interface methods or declare class abstract [OK]

Common Mistakes:

• Assuming no implementation needed if methods have same name
• Thinking interfaces cannot share method names
• Using extends instead of implements for interfaces

5. Given interfaces Readable and Writable with methods read() and write() respectively, how can you design a class FileHandler that implements both interfaces and provides default logging before each operation? Choose the best approach.

hard

A. Implement both interfaces in FileHandler and override methods to add logging then call interface methods

B. Make FileHandler extend both interfaces and implement methods without logging

C. Use abstract class instead of interfaces for multiple inheritance

D. Implement interfaces and add logging inside overridden methods in FileHandler

Solution

1. Step 1: Understand interface implementation and method overriding

   FileHandler must implement both read() and write() methods from interfaces.

2. Step 2: Add logging before operations

   Override methods in FileHandler to add logging code before performing the actual operation.

3. Step 3: Choose best approach

   Implement interfaces and add logging inside overridden methods in FileHandler correctly describes implementing interfaces and adding logging inside overridden methods. Implement both interfaces in FileHandler and override methods to add logging then call interface methods incorrectly suggests calling interface methods which have no implementation. Make FileHandler extend both interfaces and implement methods without logging uses wrong keyword extend. Use abstract class instead of interfaces for multiple inheritance avoids interfaces and is not best for multiple inheritance.

4. Final Answer:

   Implement interfaces and add logging inside overridden methods in FileHandler -> Option D

5. Quick Check:

   Override interface methods to add behavior = C [OK]

Hint: Override interface methods to add extra behavior like logging [OK]

Common Mistakes:

• Trying to extend interfaces instead of implement
• Calling interface methods directly which have no body
• Using abstract class when interfaces are better for multiple inheritance