面试题与答案

There are lots of new features which were added in Java 8. Here is the list of important features:

So we can divide structure of Lambda expression to three parts:

Lambda expression can have zero or more arguments. First part before '->' is called as argument list or parameters.

()->{System.out.println("Hello")}; //Without argument, will print hello 

(int a)->{System.out.println(a)} //; One argument, will print value of a

(int a,int b)-> {a+b};//two argument, will return sum of these two integers

A default method is a method with an implementation – which can be found in an interface.

We can use a default method to add a new functionality to an interface while maintaining backward compatibility with classes that are already implementing the interface:

public interface CarBehaviour {

    public void move();

    default void peep() {

        System.out.println("peep!");

    }

}

Usually, when a new abstract method is added to an interface, all implementing classes will break until they implement the new abstract method. In Java 8, this problem has been solved by the use of default method.

For example, Collection interface does not have forEach method declaration. Thus, adding such method would simply break the whole collections API.

Java 8 introduces default method so that Collection interface can have a default implementation of forEach method without requiring the classes implementing this interface to implement the same.

Optional is a container object used to contain not-null objects. Optional object is used to represent null with absent value. This class has various utility methods to facilitate code to handle values as ‘available’ or ‘not available’ instead of checking null values.

public final class Optional<T> extends Object

public class OptionalTester {

   public static void main(String args[]) {

      OptionalTester optionalTester = new OptionalTester();

      Integer value1 = null;

      Integer value2 = new Integer(10);

      //Optional.ofNullable - allows passed parameter to be null.

      Optional<Integer> a = Optional.ofNullable(value1);

      //Optional.of - throws NullPointerException if passed parameter is null

      Optional<Integer> b = Optional.of(value2);

      System.out.println(optionalTester.sum(a,b));

   }

   public Integer sum(Optional<Integer> a, Optional<Integer> b) {

      //Optional.isPresent - checks the value is present or not

      System.out.println("First parameter is present: " + a.isPresent());

      System.out.println("Second parameter is present: " + b.isPresent());

      //Optional.orElse - returns the value if present otherwise returns

      //the default value passed.

      Integer value1 = a.orElse(new Integer(0));

      //Optional.get - gets the value, value should be present

      Integer value2 = b.get();

      return value1 + value2;

   }

}

First parameter is present: false

Second parameter is present: true

10

You can simply use now() method of LocalDate to get today’s date.

LocalDate currentDate = LocalDate.now();

System.out.println(currentDate);

2017-09-09

You can use now() method of LocalTime to get current time.

LocalTime currentTime = LocalTime.now();

System.out.println(currentTime);

 

23:17:51.817