1. forEach() method in Iterable interface
  2. default and static methods in Interfaces

forEach() method in Iterable interface

Whenever we need to traverse through a Collection, we need to create an Iterator whose whole purpose is to iterate over and then we have business logic in a loop for each of the elements in the Collection. We might get ConcurrentModificationException if iterator is not used properly.

Java 8 has introduced forEach method in java.lang.Iterable interface so that while writing code we focus on business logic only. forEach method takes java.util.function.Consumer object as argument, so it helps in having our business logic at a separate location that we can reuse. Let’s see forEach usage with simple example.

package com.journaldev.java8.foreach;

import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;
import java.util.function.Consumer;
import java.lang.Integer;

public class Java8ForEachExample {

	public static void main(String[] args) {

		//creating sample Collection
		List<Integer> myList = new ArrayList<Integer>();
		for(int i=0; i<10; i++) myList.add(i);

		//traversing using Iterator
		Iterator<Integer> it = myList.iterator();
		while(it.hasNext()){
			Integer i = it.next();
			System.out.println("Iterator Value::"+i);
		}

		//traversing through forEach method of Iterable with anonymous class
		myList.forEach(new Consumer<Integer>() {

			public void accept(Integer t) {
				System.out.println("forEach anonymous class Value::"+t);
			}

		});

		//traversing with Consumer interface implementation
		MyConsumer action = new MyConsumer();
		myList.forEach(action);

	}

}

//Consumer implementation that can be reused
class MyConsumer implements Consumer<Integer>{

	public void accept(Integer t) {
		System.out.println("Consumer impl Value::"+t);
	}


}

The number of lines might increase but forEach method helps in having the logic for iteration and business logic at separate place resulting in higher separation of concern and cleaner code.

default and static methods in Interfaces

If you read forEach method details carefully, you will notice that it’s defined in Iterable interface but we know that interfaces can’t have method body. From Java 8, interfaces are enhanced to have method with implementation. We can use default and static keyword to create interfaces with method implementation. forEach method implementation in Iterable interface is:

default void forEach(Consumer<? super T> action) {
    Objects.requireNonNull(action);
    for (T t : this) {
        action.accept(t);
    }
}

We know that Java doesn’t provide multiple inheritance in Classes because it leads to Diamond Problem. So how it will be handled with interfaces now, since interfaces are now similar to abstract classes. The solution is that compiler will throw exception in this scenario and we will have to provide implementation logic in the class implementing the interfaces.

package com.journaldev.java8.defaultmethod;

@FunctionalInterface
public interface Interface1 {

	void method1(String str);

	default void log(String str){
		System.out.println("I1 logging::"+str);
	}

	static void print(String str){
		System.out.println("Printing "+str);
	}

    // trying to override Object method gives compile time error as
    // "A default method cannot override a method from java.lang.Object"

    //	default String toString(){
    //		return "i1";
    //	}
}
package com.journaldev.java8.defaultmethod;

@FunctionalInterface
public interface Interface2 {

	void method2();

	default void log(String str){
		System.out.println("I2 logging::"+str);
	}

}

Notice that both the interfaces have a common method log() with implementation logic.

package com.journaldev.java8.defaultmethod;

public class MyClass implements Interface1, Interface2 {

	@Override
	public void method2() {
	}

	@Override
	public void method1(String str) {
	}

	//MyClass won't compile without having it's own log() implementation
	@Override
	public void log(String str){
		System.out.println("MyClass logging::"+str);
		Interface1.print("abc");
	}

}

As you can see that Interface1 has static method implementation that is used in MyClass.log() method implementation. Java 8 uses default and static methods heavily in Collection API and default methods are added so that our code remains backward compatible.

If any class in the hierarchy has a method with same signature, then default methods become irrelevant. Since any class implementing an interface already has Object as superclass, if we have equals(), hashCode() default methods in interface, it will become irrelevant. Thats why for better clarity, interfaces are not allowed to have Object class default methods.

For complete details of interface changes in Java 8, please read Java 8 interface changes.