JAVA面试题: Java 8 新特性

Quick Overview of Java 8 Features

Some of the important Java 8 features are;

1. forEach() method in Iterable interface

1. default and static methods in Interfaces

1. Functional Interfaces and Lambda Expressions

1. Java Stream API for Bulk Data Operations on Collections

1. Java Time API

1. Collection API improvements

1. Concurrency API improvements

1. Java IO improvements

1. 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 the 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. The forEach method takes java.util.function.Consumer object as an argument, so it helps in having our business logic at a separate location that we can reuse. Let’s see forEach usage with a 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.

2. 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 a method body. From Java 8, interfaces are enhanced to have a 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 an 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 the same signature, then default methods become irrelevant. The Object is the base class, so if we have equals(), hashCode() default methods in the interface, it will become irrelevant. That’s why for better clarity, interfaces are not allowed to have Object default methods.

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

3. Functional Interfaces and Lambda Expressions

If you notice the above interface code, you will notice @FunctionalInterface annotation. Functional interfaces are a new concept introduced in Java 8. An interface with exactly one abstract method becomes a Functional Interface. We don’t need to use @FunctionalInterface annotation to mark an interface as a Functional Interface.

@FunctionalInterface annotation is a facility to avoid the accidental addition of abstract methods in the functional interfaces. You can think of it like @Override annotation and it’s best practice to use it. java.lang.Runnable with a single abstract method run() is a great example of a functional interface.

One of the major benefits of the functional interface is the possibility to use lambda expressions to instantiate them. We can instantiate an interface with an anonymous class but the code looks bulky.

Runnable r = new Runnable(){
            @Override
            public void run() {
                System.out.println("My Runnable");
            }};

Since functional interfaces have only one method, lambda expressions can easily provide the method implementation. We just need to provide method arguments and business logic. For example, we can write above implementation using lambda expression as:

Runnable r1 = () -> {
            System.out.println("My Runnable");
        };

If you have single statement in method implementation, we don’t need curly braces also. For example above Interface1 anonymous class can be instantiated using lambda as follows:

Interface1 i1 = (s) -> System.out.println(s);
        
i1.method1("abc");

So lambda expressions are a means to create anonymous classes of functional interfaces easily. There are no runtime benefits of using lambda expressions, so I will use it cautiously because I don’t mind writing a few extra lines of code.

A new package java.util.function has been added with bunch of functional interfaces to provide target types for lambda expressions and method references. Lambda expressions are a huge topic, I will write a separate article on that in the future.

You can read complete tutorial at Java 8 Lambda Expressions Tutorial.

4. Java Stream API for Bulk Data Operations on Collections

A new java.util.stream has been added in Java 8 to perform filter/map/reduce like operations with the collection. Stream API will allow sequential as well as parallel execution. This is one of the best features for me because I work a lot with Collections and usually with Big Data, we need to filter out them based on some conditions.

Collection interface has been extended with stream() and parallelStream() default methods to get the Stream for sequential and parallel execution. Let’s see their usage with a simple example.

package com.journaldev.java8.stream;

import java.util.ArrayList;
import java.util.List;
import java.util.stream.Stream;

public class StreamExample {

    public static void main(String[] args) {
        
        List<Integer> myList = new ArrayList<>();
        for(int i=0; i<100; i++) myList.add(i);
        
        //sequential stream
        Stream<Integer> sequentialStream = myList.stream();
        
        //parallel stream
        Stream<Integer> parallelStream = myList.parallelStream();
        
        //using lambda with Stream API, filter example
        Stream<Integer> highNums = parallelStream.filter(p -> p > 90);
        //using lambda in forEach
        highNums.forEach(p -> System.out.println("High Nums parallel="+p));
        
        Stream<Integer> highNumsSeq = sequentialStream.filter(p -> p > 90);
        highNumsSeq.forEach(p -> System.out.println("High Nums sequential="+p));

    }

}

If you will run above example code, you will get output like this:

High Nums parallel=91
High Nums parallel=96
High Nums parallel=93
High Nums parallel=98
High Nums parallel=94
High Nums parallel=95
High Nums parallel=97
High Nums parallel=92
High Nums parallel=99
High Nums sequential=91
High Nums sequential=92
High Nums sequential=93
High Nums sequential=94
High Nums sequential=95
High Nums sequential=96
High Nums sequential=97
High Nums sequential=98
High Nums sequential=99

Notice that parallel processing values are not in order, so parallel processing will be very helpful while working with huge collections.

Covering everything about Stream API is not possible in this post, you can read everything about Stream API at Java 8 Stream API Example Tutorial.