Can an interface extend multiple interfaces in Java?

Java Inheritance Multiple Inheritance Extends

Java Problem Overview

Can an interface extend multiple interfaces in Java? This code appears valid in my IDE and it does compile:

interface Foo extends Runnable, Set, Comparator<String> { }

but I had heard that multiple inheritance was not allowed in Java. Why does there appear to be an exception for interfaces?

Java Solutions

Solution 1 - Java

Yes, you can do it. An interface can extend multiple interfaces, as shown here:

interface Maininterface extends inter1, inter2, inter3 {  
  // methods
}

A single class can also implement multiple interfaces. What if two interfaces have a method defining the same name and signature?

There is a tricky point:

interface A {
    void test();
}

interface B {
    void test();
}

class C implements A, B {

	@Override
	public void test() {
		    		
	}     
    
}

Then single implementation works for both :).

Read my complete post here:

http://codeinventions.blogspot.com/2014/07/can-interface-extend-multiple.html

Solution 2 - Java

An interface can extend multiple interfaces.

A class can implement multiple interfaces.

However, a class can only extend a single class.

Careful how you use the words extends and implements when talking about interface and class.

Solution 3 - Java

From the Oracle documentation page about multiple inheritance type,we can find the accurate answer here. Here we should first know the type of multiple inheritance in java:-

Multiple inheritance of state.
Multiple inheritance of implementation.
Multiple inheritance of type.

Java "doesn't support the multiple inheritance of state, but it support multiple inheritance of implementation with default methods since java 8 release and multiple inheritance of type with interfaces.

Then here the question arises for "diamond problem" and how Java deal with that:-

In case of multiple inheritance of implementation java compiler gives compilation error and asks the user to fix it by specifying the interface name. Example here:-

                interface A {
                    void method();
                }
                
                interface B extends A {
                    @Override
                    default void method() {
                        System.out.println("B");
                    }
                }
                
                interface C extends A { 
                    @Override
                    default void method() {
                        System.out.println("C");
                    }
                }
                
                interface D extends B, C {
                
                }

So here we will get error as:- interface D inherits unrelated defaults for method() from types B and C interface D extends B, C

You can fix it like:-

interface D extends B, C {
                @Override
                default void method() {
                    B.super.method();
                }
            }

2. In multiple inheritance of type java allows it because interface doesn't contain mutable fields and only one implementation will belong to the class so java doesn't give any issue and it allows you to do so.

In Conclusion we can say that java doesn't support multiple inheritance of state but it does support multiple inheritance of implementation and multiple inheritance of type.