Default methods in Java - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of default method call in interface

What is the output of this Java code?

Java

interface Speaker {
    default void sayHello() {
        System.out.println("Hello from Speaker");
    }
}

class Person implements Speaker {
}

public class Main {
    public static void main(String[] args) {
        Person p = new Person();
        p.sayHello();
    }
}

A Compilation error: sayHello() not implemented

B No output

C Runtime error: NullPointerException

D Hello from Speaker

💡 Hint

Remember that default methods in interfaces provide an implementation that classes can inherit.

The interface Speaker has a default method sayHello(). The class Person implements Speaker but does not override sayHello(), so the default method is called, printing "Hello from Speaker".

❓ Predict Output

intermediate

Output when overriding default method

What will be printed when running this code?

Java

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

class Document implements Printer {
    public void print() {
        System.out.println("Printing from Document");
    }
}

public class Main {
    public static void main(String[] args) {
        Document doc = new Document();
        doc.print();
    }
}

A Printing from Printer

B Printing from Document

C Runtime error: MethodNotFoundException

D Compilation error: print() must be public

💡 Hint

Check if the class overrides the default method.

The class Document overrides the default print() method from Printer interface. So when print() is called on Document instance, the overridden method runs, printing "Printing from Document".

❓ Predict Output

advanced

Resolving conflict between two default methods

What is the output of this code?

Java

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

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

class C implements A, B {
    public void show() {
        A.super.show();
    }
}

public class Main {
    public static void main(String[] args) {
        C c = new C();
        c.show();
    }
}

A Compilation error: class C must override show()

B B show

C A show

D Runtime error: AmbiguousMethodException

💡 Hint

When two interfaces have the same default method, the implementing class must resolve the conflict explicitly.

Class C implements both interfaces A and B which have default show() methods. To resolve the conflict, C overrides show() and calls A's version explicitly using A.super.show(). So output is "A show".

🔧 Debug

advanced

Identify the compilation error in default method usage

This code does not compile. What is the cause of the compilation error?

Java

interface X {
    default void doSomething() {
        System.out.println("Doing something");
    }
}

class Y implements X {
    public void doSomething() {
        System.out.println("Doing something else");
    }
}

public class Main {
    public static void main(String[] args) {
        Y y = new Y();
        y.doSomething();
    }
}

A Method doSomething() in class Y must be public to override interface method

B Interface X cannot have default methods

C Class Y must be abstract

D Main class must implement interface X

💡 Hint

Check the access modifier of the overriding method compared to the interface method.

Interface methods are implicitly public. When overriding, the method in the class must be public or more accessible. Here, doSomething() in Y has default (package-private) access, causing a compilation error.

🧠 Conceptual

expert

Behavior of default methods with multiple inheritance and diamond problem

Consider interfaces I1 and I2 both have a default method foo(). Class D implements both interfaces but does NOT override foo(). What happens when foo() is called on an instance of D?

A Compilation error due to diamond problem; class D must override foo()

B Runtime error due to ambiguous method call

C The foo() method from I2 is called by default

D The foo() method from I1 is called by default

💡 Hint

Java requires explicit resolution when multiple interfaces provide the same default method.

When a class implements two interfaces that have the same default method, Java forces the class to override that method to resolve the conflict. Otherwise, it causes a compilation error.

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