Implementing interfaces in Java - Step-by-Step Execution

Concept Flow - Implementing interfaces

Define Interface

↓

Create Class

↓

Use 'implements' keyword

↓

Override Interface Methods

↓

Create Object of Class

↓

Call Overridden Methods

↓

Program Runs

This flow shows how a class uses an interface by implementing its methods and then running the program.

Execution Sample

Java

interface Animal {
    void sound();
}

class Dog implements Animal {
    public void sound() {
        System.out.println("Woof");
    }
}

public class Main {
    public static void main(String[] args) {
        Animal myDog = new Dog();
        myDog.sound();
    }
}

This code defines an interface Animal with a method sound, then a Dog class implements it and prints 'Woof'.

Execution Table

Step	Action	Evaluation	Result
1	Define interface Animal with method sound()	Interface created	Animal interface ready
2	Define class Dog implementing Animal	Dog must override sound()	Dog class ready with sound() method
3	Create Dog object assigned to Animal reference	Object created	myDog points to Dog instance
4	Call myDog.sound()	Dog's sound() runs	Prints 'Woof'
5	Program ends	No more instructions	Execution stops

💡 Program ends after printing 'Woof' from Dog's sound() method

Variable Tracker

Variable	Start	After Step 3	After Step 4	Final
myDog	null	Dog instance	Dog instance	Dog instance

Key Moments - 3 Insights

Why do we use 'implements' keyword in the Dog class?

Because Dog promises to provide code for all methods declared in the Animal interface, as shown in step 2 of the execution_table.

What happens if Dog does not override the sound() method?

The code will not compile because the interface requires sound() to be implemented, as explained in step 2.

Why can we assign a Dog object to an Animal variable?

Because Dog implements Animal, so a Dog is an Animal type, shown in step 3 where myDog is Animal type but holds Dog instance.

Visual Quiz - 3 Questions

Look at the execution_table, what is printed when myDog.sound() is called at step 4?

A "Meow"

B "Woof"

C Nothing is printed

D Compilation error

💡 Hint

Check step 4 in execution_table where Dog's sound() prints 'Woof'

At which step does the Dog class promise to provide the sound() method implementation?

A Step 3

B Step 1

C Step 2

D Step 4

💡 Hint

Step 2 shows Dog implements Animal and must override sound()

If we remove 'implements Animal' from Dog, what happens?

A Dog class cannot be assigned to Animal variable

B Dog class must override sound() anyway

C Dog still compiles and runs

D Program prints 'Woof' twice

💡 Hint

Refer to variable_tracker and step 3 in execution_table about type assignment

Concept Snapshot

interface InterfaceName {
    void methodName();
}

class ClassName implements InterfaceName {
    public void methodName() {
        // method body
    }
}

- Use 'implements' to promise methods
- Must override all interface methods
- Can assign object to interface type variable

Full Transcript

This example shows how to implement interfaces in Java. First, we define an interface Animal with a method sound(). Then, the Dog class uses 'implements Animal' to promise it will provide the sound() method. Dog overrides sound() to print 'Woof'. In main, we create a Dog object but assign it to an Animal variable. When we call sound() on this variable, Dog's version runs and prints 'Woof'. The program ends after this. Key points: 'implements' means the class must provide all interface methods. You can use an interface type variable to hold any object of classes that implement it.

Practice

1. What keyword does a Java class use to follow an interface?

easy

A. uses

B. extends

C. inherits

D. implements

Solution

1. Step 1: Understand Java class and interface relationship

   In Java, a class follows an interface by using a specific keyword to promise it will provide all methods declared in the interface.

2. Step 2: Identify the correct keyword

   The keyword to make a class follow an interface is implements, not extends which is for classes inheriting other classes.

3. Final Answer:

   implements -> Option D

4. Quick Check:

   Class follows interface = implements [OK]

Hint: Remember: classes use implements for interfaces, extends for classes [OK]

Common Mistakes:

• Using extends instead of implements for interfaces
• Confusing inherits keyword which doesn't exist in Java
• Using uses keyword which is invalid

2. Which of the following is the correct syntax to declare a class Car that implements interface Vehicle?

easy

A. class Car extends Vehicle {}

B. class Car implements Vehicle {}

C. interface Car implements Vehicle {}

D. class Car uses Vehicle {}

Solution

1. Step 1: Identify class and interface keywords

   A class is declared with class, and interfaces with interface. Here, Car is a class, Vehicle is an interface.

2. Step 2: Use correct syntax for implementing interface

   The class Car must use implements keyword to follow Vehicle interface. So class Car implements Vehicle {} is correct.

3. Final Answer:

   class Car implements Vehicle {} -> Option B

4. Quick Check:

   Class + implements + Interface = correct syntax [OK]

Hint: Class implements interface with 'implements' keyword [OK]

Common Mistakes:

• Using extends instead of implements for interfaces
• Declaring class as interface
• Using invalid keyword uses

3. What will be the output of this code?

interface Printer {
    void print();
}

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

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

medium

A. Printing document

B. Compilation error: print() not implemented

C. Runtime error

D. No output

Solution

1. Step 1: Check if interface method is implemented

   The interface Printer declares method print(). The class Document implements Printer and provides public void print() method, so no error.

2. Step 2: Trace main method execution

   Main creates Printer reference p to new Document object and calls p.print(). This calls Document's print() which prints "Printing document".

3. Final Answer:

   Printing document -> Option A

4. Quick Check:

   Implemented method runs and prints output [OK]

Hint: Implemented interface methods run normally when called [OK]

Common Mistakes:

• Forgetting to make print() public causes compile error
• Assuming interface methods run automatically without implementation
• Confusing runtime error with compile error

4. Identify the error in this code:

interface Animal {
    void sound();
}

class Dog implements Animal {
    void sound() {
        System.out.println("Bark");
    }
}

medium

A. No error, code is correct

B. Dog should extend Animal, not implement

C. Method sound() must be public in Dog class

D. Interface Animal cannot have methods

Solution

1. Step 1: Check method visibility in interface implementation

   Interface methods are implicitly public. When implementing, the method must be declared public in the class.

2. Step 2: Identify method declaration in Dog class

   Dog's sound() method has default (package-private) visibility, missing public keyword, causing compile error.

3. Final Answer:

   Method sound() must be public in Dog class -> Option C

4. Quick Check:

   Interface methods require public implementation [OK]

Hint: Interface methods must be public in implementing class [OK]

Common Mistakes:

• Omitting public keyword on implemented methods
• Using extends instead of implements for interfaces
• Thinking interface methods can be private

5. Given interface Calculator with methods add(int a, int b) and subtract(int a, int b), which class correctly implements it to return the sum and difference respectively?

hard

A. class Calc implements Calculator {
public int add(int a, int b) { return a + b; }
public int subtract(int a, int b) { return a - b; }
}

B. class Calc implements Calculator {
int add(int a, int b) { return a + b; }
int subtract(int a, int b) { return a - b; }
}

C. class Calc extends Calculator {
public int add(int a, int b) { return a + b; }
public int subtract(int a, int b) { return a - b; }
}

D. class Calc implements Calculator {
public void add(int a, int b) { System.out.println(a + b); }
public void subtract(int a, int b) { System.out.println(a - b); }
}

Solution

1. Step 1: Check method signatures and visibility

   Interface methods are public and return int. So implementing methods must be public and return int with same parameters.

2. Step 2: Analyze each option

   class Calc implements Calculator { public int add(int a, int b) { return a + b; } public int subtract(int a, int b) { return a - b; } } matches signatures exactly with public int return type. class Calc implements Calculator { int add(int a, int b) { return a + b; } int subtract(int a, int b) { return a - b; } } misses public keyword. class Calc extends Calculator { public int add(int a, int b) { return a + b; } public int subtract(int a, int b) { return a - b; } } uses extends which is invalid for interfaces. class Calc implements Calculator { public void add(int a, int b) { System.out.println(a + b); } public void subtract(int a, int b) { System.out.println(a - b); } } changes return type to void, which is incorrect.

3. Final Answer:

   class Calc implements Calculator { public int add(int a, int b) { return a + b; } public int subtract(int a, int b) { return a - b; } } -> Option A

4. Quick Check:

   Match method signatures exactly with public and return type [OK]

Hint: Implemented methods must match interface signatures exactly [OK]

Common Mistakes:

• Omitting public keyword on methods
• Using extends instead of implements for interfaces
• Changing return types or parameters