Java 9: Stream API Improvements

In Java 9, following methods are added to Stream interface of stream package. These methods are default and static.
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The takeWhile() Method

This method is included in Stream interface and used to collect the elements into a stream. It takes all the elements till the condition specified as predicate.

For example, we want to collect a few elements from a stream of elements; for this purpose, we can use takeWhile() method.

The following is the syntax of the method.

default Stream takeWhile(Predicate predicate)

Example

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

public class Main{
    public static void main(String[] args){
        List list = Stream.of(20,30,40,62,85,21)  
                .takeWhile(i -> (i<85)).collect(Collectors.toList());
        System.out.println(list); // collecting into a list

        // traverse elements
        Stream stream = Stream.of(20,30,40,62,85,21);
        stream.takeWhile(num -> num < 85).forEach(num -> System.out.println(num));
    }
}

Output

The dropWhile() Method

This method is just opposite of takeWhile(). It drops the result when the specified condition occurs and takes rest of elements.

If stream is ordered then it returns a stream that contains remaining elements after dropping the elements. If the stream is unordered, it returns a stream that includes the remaining elements after dropping a subset of elements. The following is the syntax of the method.

default Stream dropWhile(Predicate predicate)

Example

package myjavaproject;

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

public class Main{
    public static void main(String[] args){
        List list = Stream.of(20,30,40,62,85,21)  
                .dropWhile(i -> (i<85)).collect(Collectors.toList());
        System.out.println(list); // collecting into a list

        // traverse elements
        Stream stream = Stream.of(20,30,40,62,85,21);
        stream.dropWhile(num -> num < 85).forEach(num -> System.out.println(num));
    }
}

Output

The ofNullable() Method

This method is used for null checking and returns either a sequential Stream of a single element or an empty Stream. The syntax of the method is given below.

static  Stream ofNullable(T t)

Example

import java.util.stream.Stream;

public class Main{
    public static void main(String[] args){
        Stream stream = Stream.ofNullable(12);
        stream.forEach(System.out::print);

        stream = Stream.ofNullable(null);
        stream.forEach(System.out::print); // empty stream
    }
}

Output

The iterate() Method

This method is added into Java 9 that helps to iterate over a sequence of stream and takes three arguments. The first argument is called seed and used to initialize a stream, the second argument is called predicate and used to specify the condition while third argument is used to generate next elements.

The syntax of the method is given below.

static  Stream iterate(T seed, Predicate hasNext, UnaryOperator next)

Example

import java.util.stream.Stream;

public class Main{
    public static void main(String[] args){
        Stream stream = Stream.iterate(1, a->a<5, a->a+1);
        stream.forEach(System.out::println);
    }
}

Output

This is similar to writing a for loop as below:

for(int i=1;i<5;i++)
{
   System.out.println(i)
}

That's all about Java 9 Stream API improvements.