What is the difference between static and default methods in a Java interface?

I was learning through interfaces when I noticed that you can now define static and default methods in an interface.

public interface interfacesample2 {
	public static void method() {
    	System.out.println("hello world");
	}

	public default void menthod3() {
    	System.out.println("default print");
	}
}

Kindly explain the difference of the two and also if there's an example of when we would use this would be nice. A little confused on Interfaces.

Differences between static and default methods in Java 8:

1. Default methods can be overriden in implementing class, while static cannot.

2. Static method belongs only to Interface class, so you can only invoke static method on Interface class, not on class implementing this Interface, see:

   public interface MyInterface { default void defaultMethod(){ System.out.println("Default"); }

    static void staticMethod(){
        System.out.println("Static");
    }    
   

   }

   public class MyClass implements MyInterface {

    public static void main(String[] args) {
   
        MyClass.staticMethod(); //not valid - static method may be invoked on containing interface class only
        MyInterface.staticMethod(); //valid
    }
   

   }

3. Both class and interface can have static methods with same names, and neither overrides other!

   public class MyClass implements MyInterface {

    public static void main(String[] args) {
   
        //both are valid and have different behaviour
        MyClass.staticMethod();
        MyInterface.staticMethod();
    }
   
    static void staticMethod(){
        System.out.println("another static..");
    }
   

   }

A static method is a method that applies to the class 'namespace', so to speak. So a static method foo of interface Interface is accessed by Interface.foo(). Note that the function call does not apply to any particular instance of the interface.

A default implementation bar on the other hand, is called by

Interface x = new ConcreteClass();
x.bar();

A static interface method cannot know about the this variable, but a default implementation can.

> 1. explain the difference of the two

Static interface methods are like static class methods(here they belong to Interface only). Where as the default interface methods provide default implementation of interface methods (which implementing classes may override)
But remember in case a class is implementing more than one interface with same default method signature then the implementing class needs to override the default method

You can find a simple example below (can DIY for different cases)

public class Test {
	public static void main(String[] args) {
		// Accessing the static member
		I1.hello();

		// Anonymous class Not overriding the default method
		I1 t = new I1() {
			@Override
			public void test() {
				System.out.println("Anonymous test");
			}
		};
		t.test();
		t.hello("uvw");

		// Referring to class instance with overridden default method
		I1 t1 = new Test2();
		t1.test();
		t1.hello("xyz");

	}
}

interface I1 {

	void test();
    //static method
	static void hello() {
		System.out.println("hello from Interface I1");
	}

	// default need not to be implemented by implementing class
	default void hello(String name) {
		System.out.println("Hello " + name);
	}
}

class Test2 implements I1 {
	@Override
	public void test() {
		System.out.println("testing 1234...");
	}

	@Override
	public void hello(String name) {
		System.out.println("bonjour" + name);
	}
}

> 2. when we would use this would be nice.

That depends on your problem statement. I would say Default methods are useful, if you need same implementation for a method in your specification in all the classes in that contract, Or it may be used like Adapter classes.

here is a good read: https://softwareengineering.stackexchange.com/questions/233053/why-were-default-and-static-methods-added-to-interfaces-in-java-8-when-we-alread

also below oracle doc explains default & static methods for evolving existing interfaces:

> Users who have classes that implement interfaces enhanced with new > default or static methods do not have to modify or recompile them to > accommodate the additional methods.

http://docs.oracle.com/javase/tutorial/java/IandI/nogrow.html

Here is my view:

static method in interface:

• You can call it directly (InterfacetA.staticMethod())

• Sub-class will not be able to override.

• Sub-class may have method with same name as staticMethod

default method in interface:

• You can not call it directly.

• Sub-class will be able to override it

Advantage:

• static Method: You don't need to create separate class for utility method.

• default Method: Provide the common functionality in default method.

This link has some useful insights, have listed few of them here.

default & static methods have bridged down the differences between interfaces and abstract classes.

Interface default methods:

• It helps in avoiding utility classes, such as all the Collections class method can be provided in the interfaces itself.
• It helps in extending interfaces without having the fear of breaking implementation classes.

Interface static methods:

• They are part of interface, we can’t use it for implementation class objects.
• It helps in providing security by not allowing implementation classes to override them.

Like to quote another useful reference.

As per Java14 JLS doc:

Default Method:

• It is an instance method declared in an interface with the default modifier

• It can be accessed only by the instance of the implementing class only

• Its body is always represented by a block, which provides a default implementation or behaviour for any implementing class without overriding the method

• It can never be static or private

Static Method:

• It can be invoked by interface without reference to a particular object, just like class static methods

• Static method can be private

• The implementing class can not access static method

Lets understand it with the help of below example code:

            public interface MyInterface {
        
            private void privateMethod() {
                System.out.println("Hi, this is privateMethod");
            }
        
            private static void staticPrivateMethod() {
                System.out.println("Hi, this is staticPrivateMethod");
            }
        
            static void staticMethod() {
                //privateMethod();    // Non-static method cannot be referenced from a static contex
                System.out.println("Hi, this is staticMethod");
                staticPrivateMethod();
            }
        
            default void defaultMethod() {
                System.out.println("Hi, this is defaultMethod");
            }
        
        }
    
    public class MyInterfaceImpl implements MyInterface{
        public static void main(String[] args) {
    
            MyInterface.staticMethod();
            // myInterface.staticMethod(); // Not allowed
    
            MyInterface myInterface = new MyInterfaceImpl();
            myInterface.defaultMethod();
            // MyInterface.defaultMethod(); // Not allowed
    
        }
    }

According to Oracle's Javadocs: http://docs.oracle.com/javase/tutorial/java/IandI/defaultmethods.html

> Default methods enable you to add new functionality to the interfaces of your libraries and ensure binary compatibility with code written for older versions of those interfaces. > > A static method is a method that is associated with the class in which it is defined rather than with any object. Every instance of the class shares its static methods.

Normally, static method in interface is used as Helper methods while default method are used as a default implementation for classes that implements that interface.

Example:

interface IDemo {

    //this method can be called directly from anywhere this interface is visible
    static int convertStrToInt(String numStr) {
       return Integer.parseInt(numStr);
    }


    //getNum will be implemented in a class
    int getNum();       

    default String numAsStr() {
       //this.getNum will call the class's implementation
       return Integer.toString(this.getNum());
    }   
	
}

Interface default methods:

It helps in avoiding utility classes, such as all the Collections class method can be provided in the interfaces itself.

It helps in extending interfaces without having the fear of breaking implementation classes.

Interface static methods:

They are part of interface, we can’t use it for implementation class objects.

It helps in providing security by not allowing implementation classes to override them.

Now how static method providing security. Let's see an example.

interface MyInterface {
	/*
	 * This is a default method so we need not to implement this method in the implementation classes
	 */
	default void newMethod() {
		System.out.println("Newly added default method in Interface");
	}

	/*
	 * This is a static method. Static method in interface is similar to default method except that we cannot override them in the implementation classes. Similar to default methods, we need to implement these methods in implementation classes so we can safely add them to the existing interfaces.
	 */
	static void anotherNewMethod() {
		System.out.println("Newly added static method in Interface");
	}

	/*
	 * Already existing public and abstract method We must need to implement this method in implementation classes.
	 */
	void existingMethod(String str);
}

public class Example implements MyInterface {
	// implementing abstract method
	public void existingMethod(String str) {
		System.out.println("String is: " + str);
	}

	public void newMethod() {
		System.out.println("Newly added default method in Class");
	}

	static void anotherNewMethod() {
		System.out.println("Newly added static method in Class");
	}

	public static void main(String[] args) {
		Example obj = new Example();

		// calling the default method of class
		obj.newMethod();
		// calling the static method of class

		obj.anotherNewMethod();
		
		// calling the static method of interface
		MyInterface.anotherNewMethod();
		
		// calling the abstract method of interface
		obj.existingMethod("Java 8 is easy to learn");

	}
}

Here obj.newMethod(); printing class implementation logic, means we can change the logic of that method inside implementation class.

But obj.anotherNewMethod(); printing class implementation logic ,but not changed interface implementation. So if any encryption-decryption logic written inside that method you can't change.

we cannot execute Interfacesample2.menthod3(); because it is not static method. In order to execute method3() we need an instance of Interfacesample2 interface.

Please find the following practical example:

public class Java8Tester {
   public static void main(String args[]){
	  // Interfacesample2.menthod3(); Cannot make a static reference to the non-static method menthod3 from the type Interfacesample2
	   
	  new Interfacesample2(){ }.menthod3();// so in order to call default method we need an instance of interface
	   
	   Interfacesample2.method(); // it
   }
}

interface Interfacesample2 {
    public static void method() {
        System.out.println("hello world");
    }

    public default void menthod3() {
        System.out.println("default print");
    }
}

Starting Java 8 interface can also have static method. Like static method of a class, static method of an interface can be called using Interface name.

Example

public interface Calculator {
    int add(int a, int b);
    int subtract(int a, int b);
    
    default int multiply(int a, int b) {
         throw new RuntimeException("Operation not supported. Upgrade to UltimateCalculator");
    }
    
    static void display(String value) {
        System.out.println(value);
    }
}

Difference between static and default method of interface is default method supports inheritance but static method does not. Default method can be overridden in inheriting interface.

Here is good read about interface default method and static method. Interface Default Method in Java 8

All good answers here. I would like to add another practical usage of the static function in the interface. The tip is coming from the book - Effective Java, 3rd Edition by Joshua Bloch in Chapter2: Creating and Destroying Object.

Static functions can be used for static factory methods. 

Static factory method are methods which return an object. They work like constructor. In specific cases, static factory method provides more readable code than using constructor.

Quoting from the book - Effective Java, 3rd Edition by Joshua Bloch

> Prior to Java 8, interfaces couldn’t have static methods. By > convention, static factory methods for an interface named Type were > put in a noninstantiable companion class (Item 4) named Types.

Author gives an example of Collections where such static factory method is implemented. Checking on the code, Josh Bloch can be seen as first author of Collections class. Although Collections is a class and not interface. But the concept still applies.

> For example, the Java Collections Framework has forty-five utility > implementations of its interfaces, providing unmodifiable collections, > synchronized collections, and the like. Nearly all of these > implementations are exported via static factory methods in one > noninstantiable class (java.util.Collections). The classes of the > returned objects are all nonpublic.

Further he explains that API is not only smaller, it helps with the code readability and API ease..

> It is not just the bulk of the API that is reduced but the conceptual > weight: the number and difficulty of the concepts that programmers > must master in order to use the API. The programmer knows that the > returned object has precisely the API specified by its interface, so > there is no need to read additional class documentation for the > implementation class.

Here is one of the static method from java.util.Collections class:

public static <T> Collection<T> unmodifiableCollection(Collection<? extends T> c) {
    return new UnmodifiableCollection<>(c);
}