Default methods in Java - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a default method in an interface.

Java

public interface MyInterface {
    default void [1]() {
        System.out.println("Default method");
    }
}

Drag options to blanks, or click blank then click option'

Avoid

Bdefault

CdefaultMethod

Dmethod

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'default' as the method name causes a syntax error.

Omitting the method name entirely.

The method name should be defaultMethod. The keyword default is already used before the return type.

2fill in blank

medium

Complete the code to call the default method from an interface inside a class.

Java

public class MyClass implements MyInterface {
    public void callDefault() {
        [1].super.defaultMethod();
    }
}

Drag options to blanks, or click blank then click option'

Ainterface

BMyInterface

Cthis

Dsuper

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'super' alone causes a compile error.

Using 'this' does not call the interface default method.

To call a default method from an interface, use InterfaceName.super.methodName(). Here, MyInterface.defaultMethod() is incorrect; it should be MyInterface.super.defaultMethod(), but since the blank is before the dot, the correct choice is MyInterface to complete the call.

3fill in blank

hard

Fix the error in the code to correctly override a default method in a class.

Java

public class MyClass implements MyInterface {
    @Override
    public void [1]() {
        System.out.println("Overridden method");
    }
}

Drag options to blanks, or click blank then click option'

Adefaultmethod

Bdefault_method

CDefaultMethod

DdefaultMethod

Attempts:

3 left

💡 Hint

Common Mistakes

Changing the case of the method name causes it not to override.

Adding underscores or other characters changes the method signature.

The method name must exactly match the default method name in the interface, which is defaultMethod with correct case.

4fill in blank

hard

Fill both blanks to define a default method with a return type and a return statement.

Java

public interface Calculator {
    default int [1](int a, int b) {
        return a [2] b;
    }
}

Drag options to blanks, or click blank then click option'

Aadd

B+

C-

D*

Attempts:

3 left

💡 Hint

Common Mistakes

Using subtraction or multiplication operator instead of addition.

Using a method name that does not match the operation.

The method name is add and the operator to add two numbers is +.

5fill in blank

hard

Fill all three blanks to implement a default method that checks if a number is even.

Java

public interface NumberCheck {
    default boolean [1](int num) {
        return num [2] 2 [3] 0;
    }
}

Drag options to blanks, or click blank then click option'

AisEven

B%

C==

D!=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '!=' instead of '==' causes incorrect logic.

Using wrong method names or operators.

The method name is isEven. The modulo operator % checks the remainder, and == compares it to zero to determine evenness.

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