Interface declaration in Java - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an interface in Java?

An interface in Java is a blueprint that defines a set of abstract methods (methods without a body) that a class can implement. It specifies what a class must do, but not how.

beginner

How do you declare an interface in Java?

You declare an interface using the keyword interface followed by the interface name and a block containing method signatures. Example:

public interface MyInterface {
  void myMethod();
}

intermediate

Can interfaces have method bodies in Java?

Since Java 8, interfaces can have default methods with bodies using the default keyword, and static methods with bodies. But regular abstract methods do not have bodies.

beginner

What keyword does a class use to implement an interface?

A class uses the <code>implements</code> keyword to promise it will provide code for all the methods declared in the interface.

intermediate

Can an interface extend another interface?

Yes, an interface can extend one or more other interfaces using the extends keyword. This allows combining multiple interfaces into one.

Which keyword is used to declare an interface in Java?

A interface

B class

C implements

D extends

The keyword interface is used to declare an interface.

How does a class promise to follow an interface's rules?

A overrides

B extends

C inherits

D implements

A class uses implements to promise it will provide all methods from the interface.

Can interfaces have method bodies in Java 8 and later?

A Only for abstract methods

B No, never

C Yes, for default and static methods

D Only for private methods

Since Java 8, interfaces can have default and static methods with bodies.

Which of these is true about interfaces?

A Interfaces define method signatures without bodies by default

B Interfaces can contain constructors

C Interfaces cannot extend other interfaces

D Interfaces can be instantiated directly

Interfaces define method signatures without bodies by default.

What keyword allows an interface to inherit from another interface?

A implements

B extends

C inherits

D uses

Interfaces use extends to inherit from other interfaces.

Explain how to declare an interface and how a class uses it.

Think about the keywords and what each part means.

Describe the difference between abstract methods and default methods in interfaces.

Consider what changed in Java 8.

Practice

1. What is the main purpose of an interface in Java?

easy

A. To inherit code from multiple classes

B. To store data values like variables

C. To create objects directly

D. To declare methods that a class must implement without providing their body

Solution

1. Step 1: Understand what an interface declares

   An interface only declares method signatures without any implementation.

2. Step 2: Compare with other options

   Interfaces do not store data, create objects, or inherit code from classes.

3. Final Answer:

   To declare methods that a class must implement without providing their body -> Option D

4. Quick Check:

   Interface purpose = declare methods only [OK]

Hint: Interfaces declare methods without bodies [OK]

Common Mistakes:

• Thinking interfaces can store variables
• Confusing interfaces with classes
• Believing interfaces create objects

2. Which of the following is the correct way to declare an interface named Vehicle in Java?

easy

A. interface Vehicle {}

B. class Vehicle interface {}

C. interface Vehicle() {}

D. Vehicle interface {}

Solution

1. Step 1: Recall Java interface syntax

   Interfaces are declared with the keyword interface followed by the name and curly braces.

2. Step 2: Check each option

   interface Vehicle {} matches correct syntax: interface Vehicle {}. Others have syntax errors like misplaced parentheses or keywords.

3. Final Answer:

   interface Vehicle {} -> Option A

4. Quick Check:

   Correct interface syntax = interface Name {} [OK]

Hint: Use 'interface Name {}' to declare interfaces [OK]

Common Mistakes:

• Adding parentheses after interface name
• Mixing class and interface keywords
• Omitting the 'interface' keyword

3. What will be the output of the following code?

interface Animal {
    void sound();
}

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

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

medium

A. Compilation error

B. sound

C. Bark

D. No output

Solution

1. Step 1: Understand interface implementation

   The class Dog implements Animal and provides the method sound() which prints "Bark".

2. Step 2: Trace the main method execution

   In main, an Animal reference points to a Dog object, calling sound() prints "Bark".

3. Final Answer:

   Bark -> Option C

4. Quick Check:

   Interface method called prints 'Bark' [OK]

Hint: Implemented method runs when called via interface reference [OK]

Common Mistakes:

• Expecting interface to print something
• Missing 'public' in method implementation causing error
• Thinking no output occurs

4. Identify the error in this interface declaration:

interface Calculator {
    int add(int a, int b) {
        return a + b;
    }
}

medium

A. Interfaces cannot have method bodies unless default or static

B. Method name 'add' is invalid in interfaces

C. Return type 'int' is not allowed in interfaces

D. Interface name must start with lowercase

Solution

1. Step 1: Check method body rules in interfaces

   In Java, interface methods cannot have bodies unless marked as default or static.

2. Step 2: Analyze the given method

   The method add has a body but no default or static keyword, causing a syntax error.

3. Final Answer:

   Interfaces cannot have method bodies unless default or static -> Option A

4. Quick Check:

   Method bodies in interfaces need default/static [OK]

Hint: Interface methods need default/static for bodies [OK]

Common Mistakes:

• Adding method bodies without default/static
• Thinking method names are restricted
• Ignoring Java naming conventions

5. Given two interfaces:

interface Printable {
    void print();
}

interface Showable {
    void show();
}

class Document implements Printable, Showable {
    public void print() {
        System.out.println("Printing document");
    }
    public void show() {
        System.out.println("Showing document");
    }
}

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

What is the output when running Test?

hard

A. Compilation error due to multiple interfaces

B. Printing document\nShowing document

C. Printing document only

D. No output

Solution

1. Step 1: Understand multiple interface implementation

   Java allows a class to implement multiple interfaces and must provide all their methods.

2. Step 2: Trace method calls in main

   The Document class implements both methods. Calling print() and show() prints both messages.

3. Final Answer:

   Printing document\nShowing document -> Option B

4. Quick Check:

   Multiple interfaces implemented, all methods run [OK]

Hint: Classes can implement many interfaces, all methods must be defined [OK]

Common Mistakes:

• Thinking multiple interfaces cause errors
• Forgetting to implement all interface methods
• Expecting only one method to run