Overview
Default methods were introduced in Java 8 as a means to enhance interfaces without breaking the existing implementation of classes. This feature allows developers to add new methods to interfaces with an implementation, thus providing backward compatibility for older interface versions.
Key Concepts
- Backward Compatibility: Default methods enable interfaces to evolve without breaking existing implementations.
- Multiple Inheritance of Behavior: They allow an object to inherit methods from multiple sources, aiding in the simulation of multiple inheritance.
- Functional Programming Support: Default methods support lambda expressions by allowing methods in interfaces to have a body, which is essential for functional programming patterns.
Common Interview Questions
Basic Level
- What is a default method in an interface in Java 8?
- How do you define a default method in a Java interface?
Intermediate Level
- How does the introduction of default methods affect multiple inheritance in Java?
Advanced Level
- How do you resolve conflicts arising from multiple interfaces containing default methods with the same signature?
Detailed Answers
1. What is a default method in an interface in Java 8?
Answer: A default method is a method in an interface that has an implementation. It allows an interface to provide a "default" behavior for classes that do not implement this method, ensuring backward compatibility and avoiding the necessity for the implementing classes to define the method.
Key Points:
- Introduced in Java 8 to enhance interfaces without breaking existing code.
- Helps in adding new functionality to interfaces in a backward-compatible way.
- Supports lambda expressions by providing body for methods in interfaces.
Example:
interface MyInterface {
// Default method with an implementation
default void show() {
System.out.println("Default Method Executed");
}
}
2. How do you define a default method in a Java interface?
Answer: To define a default method in a Java interface, use the default keyword followed by the method signature and its body. The default method can be called by any class that implements the interface without the need for an explicit method implementation.
Key Points:
- Use the default keyword before the method.
- Must provide an implementation body for the default method.
- Helps in adding methods to interfaces without affecting implementing classes.
Example:
interface Vehicle {
// Default method
default void print() {
System.out.println("I am a vehicle!");
}
}
3. How does the introduction of default methods affect multiple inheritance in Java?
Answer: The introduction of default methods adds a form of multiple inheritance of behavior to Java, as a class can inherit behavior from multiple interfaces with default methods. However, Java still doesn't support multiple inheritance of state (i.e., extending multiple classes) due to ambiguity and complexity. In cases where a class implements multiple interfaces with conflicting default methods, the compiler requires the class to explicitly specify which default method to use.
Key Points:
- Enables multiple inheritance of behavior, not state.
- Can lead to the "diamond problem" where a class inherits the same default method from multiple interfaces.
- Requires explicit resolution of method conflicts.
Example:
interface FirstInterface {
default void show() {
System.out.println("FirstInterface's show");
}
}
interface SecondInterface {
default void show() {
System.out.println("SecondInterface's show");
}
}
// Implementing both interfaces in a class
class MyClass implements FirstInterface, SecondInterface {
// Resolving conflict by overriding the default method
public void show() {
FirstInterface.super.show(); // Chooses FirstInterface's version of show()
}
}
4. How do you resolve conflicts arising from multiple interfaces containing default methods with the same signature?
Answer: When a class implements multiple interfaces that contain default methods with the same signature, Java requires the class to explicitly override the conflicting method. Within the overridden method, the class can explicitly choose which interface's default method to invoke using InterfaceName.super.methodName() syntax, or it can provide a completely new implementation.
Key Points:
- Conflicts must be resolved manually in the implementing class.
- Use InterfaceName.super.methodName() to specify which interface's default method to use.
- Provides a clear path to resolve the diamond problem.
Example:
interface Alpha {
default void display() {
System.out.println("Alpha display");
}
}
interface Beta {
default void display() {
System.out.println("Beta display");
}
}
class Gamma implements Alpha, Beta {
// Overriding the conflicting method
@Override
public void display() {
Alpha.super.display(); // Explicitly using Alpha's display method
}
}
This structure ensures a comprehensive preparation guide for understanding and implementing default methods in interfaces introduced in Java 8, catering to interview questions ranging from basic to advanced levels.
