Default methods in Java - Step-by-Step Execution

Concept Flow - Default methods

Interface with default method

↓

Class implements interface

↓

Class inherits default method

↓

Optional: Class overrides default method

↓

Object calls method

↓

If overridden, use class method

↓

Else, use interface default method

This flow shows how a class implementing an interface can use or override a default method defined in that interface.

Execution Sample

Java

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

class MyClass implements MyInterface {}

public class Main {
    public static void main(String[] args) {
        new MyClass().greet();
    }
}

This code shows a class using a default method from an interface without overriding it.

Execution Table

Step	Action	Evaluation	Result
1	Define interface MyInterface with default method greet()	Method greet() available with default implementation	Interface ready
2	Define class MyClass implementing MyInterface	No override of greet() in MyClass	MyClass inherits default greet()
3	Create new MyClass object	Object created	Instance ready
4	Call greet() on MyClass instance	No override found in MyClass	Calls default greet() from MyInterface
5	Execute greet() method body	Print statement runs	Output: Hello from default method
6	Program ends	No more instructions	Execution stops

💡 Program ends after calling default method greet() from interface

Variable Tracker

Variable	Start	After Step 3	After Step 4	Final
MyClass instance	null	Object created	Same object calls greet()	Object exists until program ends

Key Moments - 2 Insights

Why does MyClass have the greet() method even though it is not defined there?

Because MyClass implements MyInterface, it inherits the default greet() method defined in the interface, as shown in step 4 of the execution_table.

What happens if MyClass defines its own greet() method?

If MyClass overrides greet(), the class method is called instead of the interface default, replacing the behavior seen in step 4.

Visual Quiz - 3 Questions

Look at the execution_table, what is printed when greet() is called on MyClass instance?

A Nothing is printed

B "Hello from MyClass"

C "Hello from default method"

D Compilation error

💡 Hint

Refer to step 5 in execution_table where the default greet() prints the message

At which step does the program create the MyClass object?

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

Check the action column in execution_table for object creation

If MyClass overrides greet(), which step changes in the execution_table?

A Step 4

B Step 5

C Step 3

D Step 1

💡 Hint

Step 4 shows method call resolution; overriding changes which method is called

Concept Snapshot

interface InterfaceName {
    default void methodName() {
        // default implementation
    }
}

class ClassName implements InterfaceName {
    // inherits default method or overrides it
}

- Default methods allow interfaces to provide method bodies.
- Classes implementing the interface inherit these methods.
- Classes can override default methods to change behavior.

Full Transcript

This example shows how Java interfaces can have default methods with implementations. A class implementing such an interface inherits the default method if it does not override it. When the method is called on an instance of the class, the default method runs, printing a message. If the class overrides the method, its version runs instead. This allows interfaces to add new methods without breaking existing classes.

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