Implementing interfaces in Java - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an interface in Java?

An interface in Java is a blueprint that defines methods without implementations. Classes can implement interfaces to promise they provide those methods.

beginner

How do you declare that a class implements an interface?

Use the keyword <code>implements</code> after the class name, followed by the interface name. For example: <code>class Dog implements Animal</code>.

intermediate

Can a class implement multiple interfaces in Java?

Yes, a class can implement multiple interfaces by listing them separated by commas. For example: <code>class Robot implements Walkable, Talkable</code>.

intermediate

What happens if a class does not implement all methods of an interface?

The class must be declared abstract, or else the compiler will give an error because it did not fulfill the interface contract.

advanced

Why use interfaces instead of abstract classes?

Interfaces allow multiple inheritance of type, letting a class promise to provide many behaviors. Abstract classes are for shared code and single inheritance.

Which keyword is used to make a class follow an interface in Java?

A inherits

B extends

C implements

D uses

The keyword implements is used for a class to follow an interface.

If a class implements an interface, what must it do?

A Nothing, methods are optional

B Implement all methods declared in the interface

C Declare the class as abstract

D Only implement some methods

A class must implement all methods declared in the interface unless it is abstract.

Can a Java class implement more than one interface?

A Yes, by listing interfaces separated by commas

B No, only one interface is allowed

C Yes, but only if interfaces extend each other

D No, Java does not support interfaces

Java allows multiple interfaces to be implemented by a class, separated by commas.

What is the main purpose of interfaces?

A To store data fields

B To create objects

C To provide method implementations

D To define a contract of methods a class must provide

Interfaces define a contract of methods without implementations that classes promise to provide.

If a class does not implement all interface methods, what must it be declared as?

A abstract

B final

C static

D public

The class must be declared abstract if it does not implement all interface methods.

Explain how to implement an interface in a Java class and what rules must be followed.

Think about the syntax and what the compiler expects.

Describe the difference between interfaces and abstract classes in Java.

Consider how Java supports multiple behaviors.

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