Interface with default implementations in Kotlin - Step-by-Step Execution

Concept Flow - Interface with default implementations

Define Interface

↓

Add Default Method

↓

Implement Interface in Class

↓

Use Class Instance

↓

Call Method

↓

Class Override?

Yes→Use Override

No↓

Use Default Implementation

This flow shows how an interface with a default method is defined, implemented by a class, and how method calls use either the class override or the default implementation.

Execution Sample

Kotlin

interface Greeter {
    fun greet() {
        println("Hello from default!")
    }
}

class Friendly : Greeter {
    override fun greet() {
        println("Hi there!")
    }
}

fun main() {
    val f = Friendly()
    f.greet()
}

This code defines an interface with a default greet method, a class that overrides it, and calls greet on the class instance.

Execution Table

Step	Action	Evaluation	Output
1	Define interface Greeter with default greet()	Interface created with default greet()
2	Define class Friendly implementing Greeter	Class Friendly ready, overrides greet()
3	Create instance f of Friendly	f is Friendly instance
4	Call f.greet()	Check if Friendly overrides greet() -> Yes	Hi there!
5	End of main()	Program ends

💡 Program ends after calling overridden greet() method on Friendly instance

Variable Tracker

Variable	Start	After Step 3	Final
f	undefined	Friendly instance created	Friendly instance

Key Moments - 2 Insights

Why does calling greet() on Friendly print "Hi there!" instead of the default message?

Because Friendly overrides the greet() method, so the call uses the class's version, as shown in execution_table step 4.

What happens if Friendly does not override greet()?

The default greet() from the interface is used, printing "Hello from default!". This is the fallback behavior if no override exists.

Visual Quiz - 3 Questions

Look at the execution_table at step 4, what output is produced when f.greet() is called?

A "Hello from default!"

B No output

C "Hi there!"

D Compilation error

💡 Hint

Check the Output column at step 4 in the execution_table.

If the class Friendly did not override greet(), what would be the output at step 4?

A "Hi there!"

B "Hello from default!"

C No output

D Runtime error

💡 Hint

Refer to key_moments answer 2 about default method usage.

In variable_tracker, what is the value of variable 'f' after step 3?

A Friendly instance

B null

C undefined

D Greeter instance

💡 Hint

Look at variable_tracker row for 'f' after step 3.

Concept Snapshot

interface InterfaceName {
    fun method() { // default implementation
        // code
    }
}

class MyClass : InterfaceName {
    override fun method() { // optional override
        // code
    }
}

- Interfaces can have methods with default code.
- Classes can override these methods or use defaults.
- Calling the method uses override if present, else default.

Full Transcript

This example shows how Kotlin interfaces can have default method implementations. The interface Greeter defines greet() with a default print statement. The class Friendly implements Greeter and overrides greet() with its own print statement. When we create an instance of Friendly and call greet(), the overridden method runs, printing "Hi there!". If Friendly did not override greet(), the default method from the interface would run instead. This allows interfaces to provide common behavior while letting classes customize it. The execution table traces each step: defining interface and class, creating instance, calling method, and output. The variable tracker shows the instance creation. Key moments clarify why the overridden method runs and what happens without override. The quiz tests understanding of output and variable states. The snapshot summarizes syntax and behavior for quick reference.