Streams in Java

The conventional Java SE 8 version came with many new peculiarities, out of which the most striking of which are assuredly lambda expressions and the method references. Streams and Streams API are also some of the most outstanding characteristics in Java SE 8. Many projects are enhanced to Java 8 to use the friendly lambda expressions syntax and the Java streams.

Java implements a newly added package in Java SE 8 known as java.util.stream. The package contains many different interfaces, classes, and enum that allows the user to perform functional-style operations on the elements. The entire idea of the Java streams is to facilitate functional-style processes on streams of elements.

A Java stream cannot be said to be a data structure. However, we can say that Java Stream is an abstraction. However, Java Streams is not a collection or set where we can save elements and store data. The primary relevant difference between a structure and a stream is that a stream doesn’t store the data and components. A stream is a reflection of a non-changeable compilation of processes that are applied in any classification of the data.

In simple words, a Java Stream is a package that contains various classes and interfaces that are capable of doing the iteration of its own elements internally. Conversely, when we are working with iteration characteristics of the Java Collections, then we have to perform the iteration of the elements personally. For example, we can use a Java Iterator or the Java for each loop in order to work with a Java Iterable.

Characteristics of Java Stream

Java Stream describes a series of objects from an origin that supports aggregate processes. There are many characteristics of Java Streams. These characteristics are given as follows:

• No storing of elements: The Java streams simply carry elements from a source such as an I/O channel, an array, a data structure, or a Collections framework through a pipeline of computational operations.
• A sequence of elements: A Java stream implements a collection of elements of a particular kind in a subsequent manner. A stream only prepares and computes elements on demand. It never saves the data components.
• Functional in nature: The Java streams are also functional in nature. By functional nature, we can say that all the processes executed on a Java stream do not change their origin. For example, when we filter a Stream that is collected from a source, it creates a new distinct Stream that does not contain the elements that were filtered. However, it will remove those elements from the source collection.
• Aggregate operations: Java Stream also provides support for aggregate processes and intermediate operations like map, limit, reduce, match, filter, find, and many more.
• Automatic Iterations: All the iterations and traversal are done internally in Java Stream processes and methods over the different source elements. Whereas in the Collections framework, there is a need for explicit iteration.
• Lazy nature: Java Streams are very lazy. A code is only evaluated in the Java stream when it is required. This lazy nature is beneficial as it saves a lot of time.
• Pipelining: The operations (or Stream operations) that return stream to itself so that we can pipeline their result are known as intermediate operations. The principal function of intermediate operations is to take the input, process the accepted input, and return particular output to the destination. The method which is usually present at the end of the pipelining process that marks the end of the Java stream, is the collect() method.
• Source of Java Streams: The Java stream takes Collections, Arrays, or I/O sources as the input references.

Creating Java Streams

In Java SE 8, there are many different methods or ways to create a Java stream. These methods are given below -

Stream.of() - The Java stream.of() method is used to generate a stream of the fixed quantity of elements.

Example of Stream.of()

StreamOfExample.java

import java.util.stream.*;

public class StreamOfExample

{

     public static void main(String[] args)

     {

         Stream<String> strm = Stream.of("ab","cd","ef","gh","ij","kl","mn");

         strm.forEach(str -> System.out.println(str));

     }

}

Output:

ab

cd

ef

gh

ij

kl

mn

Stream.of(array): The Java stream.of(array) method is practiced to produce a stream from the array. The elements of the stream created are taken from the array elements.

Example of Stream.of(array)

StreamOfArrayExample.java

import java.util.stream.*;

public class StreamOfArrayExample

{

     public static void main(String[] args)

     {

         Integer arr[]=new Integer[5];

         for(int i=0;i<5;i++){

             arr[i]=i+1;

         }

         Stream<Integer> stream = Stream.of(arr);

         stream.forEach(n -> System.out.println(n));

     }

}

Output:

1

2

3

4

5

List.stream(): The Java List.stream() method is used to generate a stream from the list. The elements of the stream created are taken from the list elements.

Example of List.stream()

ListStreamExample.java

import java.util.stream.*;

public class ListStreamExample

{

     public static void main(String[] args)

     {

         ArrayList<String> names = new ArrayList<>();

        names.add("Mohit");

        names.add("Prateek");

        names.add("Arun");

        names.add("Nishkarsh");

        names.add("Daksh");

        names.add("Nirbhay");

        names.add("Ayush");

        names.add("Anshul");

         Stream<String> stream = names.stream();

         stream.forEach(str -> System.out.println(str));

     }

}

Output:

Mohit

Prateek

Arun

Nishkarsh

Daksh

Nirbhay

Ayush

Anshul

Stream.generate(): The Java Stream.generate() method is used to create a stream from the generated elements. In the example given below, we have used the Random() method to generate a stream of random integers.

Example of Stream.generate()

StreamGenerateExample.java

import java.util.stream.*;

public class StreamGenerateExample

{

     public static void main(String[] args)

     {

         Stream<Integer> stream = Stream

         .generate(() -> (new Random()).nextInt(50));

         stream.limit(10)

         .forEach(System.out::println);

     }

}

(Output may vary in every execution as we are using the Random() method, that generates random integers at every execution).

Output:

18

11

31

38

5

28

19

19

18

36

In the above example, we are creating a stream of 10 random integers by restricting the element count of the stream to 10 with the help of the limit() method.

Common operations of Java Streams

Java Stream holds two different types of operations. These operations are:

• Intermediate operations: Intermediate operations are the operations that return a stream so that the user can chain various intermediate operations without using semicolons, as we do in other programming languages like Scala.
• Terminal operations: The terminal operations are the operations that are mainly void and null, and if not null, these operations return a non-stream as a result.

Different Intermediate Operations:

As we have stated earlier that the intermediate operations return the stream itself so we can chain multiple method calls in a row.

Different intermediate operations in Java are:

• Stream.filter(): The Java Stream filter() method is one of the major operations under intermediate operations. The filter() method accepts a Predicate to separate all components of the stream. This operation is under intermediate operations because it enables the users to call another stream operation.  For example, Stream.forEach() method) on the result. The predicate should declare true if the component is to be introduced in the final Stream, and the Predicate will reflect false if the component should not be included in the final stream.

Example of Stream.filter()

StreamFilterExample.java:

            import java.util.*;

public class StreamFilterExample

{

                        public static void main(String[] args)

                        {

                                ArrayList<String> names = new ArrayList<>();

        names.add("Mohit");

        names.add("Prateek");

        names.add("Arun");

        names.add("Nishkarsh");

        names.add("Daksh");

        names.add("Nirbhay");

        names.add("Ayush");

        names.add("Anshul");

        names.stream().filter((s) -> s.startsWith("A")).forEach(System.out::println);

                        }

}

Output:

Arun

Ayush

Anshul

• Stream.map(): The intermediate operation Stream.map(), with the help of the given function, transforms each component in the stream into some other object. In simple words, the Stream.map() operation converts the element of a stream into something else. It takes the given function and applies that function to every element of the stream, which at the end returns a stream of the values that are produced by the parameter function. It also supports the user to create a calculation on the data inside a Java stream.

For example, if a user has a list of strings, the user can transform each string from the given list to uppercase, lowercase, or to a substring of the original string and something entirely else.

Example of Stream.map()

StreamMapExample.java

            import java.util.*;

public class StreamMapExample

{

     public static void main(String[] args)

     {

        ArrayList<String> names = new ArrayList<>();

        names.add("Mohit");

        names.add("Prateek");

        names.add("Arun");

        names.add("Nishkarsh");

        names.add("Daksh");

        names.add("Nirbhay");

        names.add("Ayush");

        names.add("Anshul");

       names.stream().map(String::toUpperCase)

        .forEach(System.out::println);

     }

}

Output:

MOHIT

PRATEEK

ARUN

NISHKARSH

DAKSH

NIRBHAY

AYUSH

ANSHUL

• Stream.sorted(): The Java Stream sorted() method is also an intermediate operation or method. This sorted() operation returns a classified (or we can say the sorted) view of the stream. The components in the stream are sorted in a normal manner unless the user passes or uses a custom Comparator if they want to sort the stream in a different manner.

Example of Stream.sorted()

StreamSortedExample.java

            import java.util.*;

public class StreamSortedExample

{

     public static void main(String[] args)

     {

        List<String> names = new ArrayList<>();

        names.add("Mohit");

        names.add("Prateek");

        names.add("Arun");

        names.add("Nishkarsh");

        names.add("Daksh");

        names.add("Nirbhay");

        names.add("Ayush");

        names.add("Anshul");

       names.stream().sorted()

        .forEach(System.out::println);

     }

}

Output:

Anshul

Arun

Ayush

Daksh

Mohit

Nirbhay

Nishkarsh

Prateek

Different Terminal Operations

As we stated earlier that Terminal operations are the operations that return a result of a particular type after doing processing on all the elements of the stream. As soon as the terminal operation is invoked of the given stream, the iteration of the given Stream and any of the chained streams will get started. After completing the iteration on the stream, the result of the applied terminal operation is returned.

Different Intermediate operations in Java are as follows:

• Stream.forEach(): The Java Stream forEach() method or operation is one of the terminal operations in which starts the internal iteration of the components in the stream.  

In simple words, the Java Stream forEach() method helps in repeating over all of the components of a stream and execute some process on each of them. The forEach() method returns nothing, or we can say void. The process to be performed on the elements of the stream is transferred as the Java lambda expression.

Example of Stream.forEach()

StreamForEachExample.java

import java.util.*;

public class StreamForEachExample

{

     public static void main(String[] args)

     {

        ArrayList<String> names = new ArrayList<>();

        names.add("Mohit");

        names.add("Prateek");

        names.add("Arun");

        names.add("Nishkarsh");

        names.add("Daksh");

        names.add("Nirbhay");

        names.add("Ayush");

        names.add("Anshul");

       names.forEach(System.out::println);

     }

}

Output:

Mohit

Prateek

Arun

Nishkarsh

Daksh

Nirbhay

Ayush

Anshul

• Stream.collect(): The Java Stream.collect() method is practiced to take elements from a stream and save all of them in a group. It is a method to get away from the environment of streams and receive a solid combination of values, like an array list or a list. The Java Stream collect() method is a terminal operation that starts the internal iteration of elements and collects the elements in the stream in a collection or object of some kind.

Example of Stream.collect()

StreamCollectExample.java

            import java.util.*;

            Import java.util.stream.Collectors;

public class StreamCollectExample

{

     public static void main(String[] args)

     {

        List<String> names = new ArrayList<>();

        names.add("Mohit");

        names.add("Prateek");

        names.add("Arun");

        names.add("Nishkarsh");

        names.add("Daksh");

        names.add("Nirbhay");

        names.add("Ayush");

        names.add("Anshul");

       List<String> namesInUppercase = names.stream()

                            .map(String::toUpperCase)

                            .collect(Collectors.toList());

     System.out.print(namesInUppercase);

     }

}

Output:

[MOHIT, PRATEEK, ARUN, NISHKARSH, DAKSH, NIRBHAY, AYUSH, ANSHUL]

• Stream.match() :  The Java Stream includes various matching operations that can be used to indicate whether a provided predicate resembles the elements of the stream or not. There are many different matching operations in Java Stream. The result returned by all the matching operations is of type boolean.

Different matching operations under Stream terminal operations are:

• Stream anyMatch():  The anyMatch() method of Java stream is also one of the terminal operations that inputs a single Predicate or condition as a parameter and starts the internal iteration of the Stream. It implements the Predicate parameter to every component of the Stream. If the given Predicate reflects true for any of the components of the stream, the method yields true. If no elements match the Predicate, the method returns false.

Example of Stream anyMatch() method

StreamAnyMatchExample.java

import java.util.*;


public class StreamAnyMatchExample
{
     public static void main(String[] args)
     {
        List<String> names = new ArrayList<>();


        names.add("Mohit");
        names.add("Prateek");
        names.add("Arun");
        names.add("Nishkarsh");
        names.add("Daksh");
        names.add("Nirbhay");
        names.add("Ayush");
        names.add("Anshul");
       boolean result = names.stream()
        .anyMatch((str) -> str.endsWith("h"));
 
       System.out.println(result);
     }
}

Output:

true

• Stream allMatch(): The allMatch() method of Java stream is also one of the terminal operations that inputs a single Predicate or condition as a parameter and starts the internal iteration of the Stream. It implements the Predicate parameter to every component of the Stream. If the given Predicate reflects true for all of the components of the stream, the method yields true.  If not all the elements match the Predicate,the method returns false.

Example of Stream allMatch() method

StreamAllMatchExample.java

import java.util.*;


public class StreamAllMatchExample
{
     public static void main(String[] args)
     {
        List<String> names = new ArrayList<>();


        names.add("Mohit");
        names.add("Prateek");
        names.add("Arun");
        names.add("Nishkarsh");
        names.add("Daksh");
        names.add("Nirbhay");
        names.add("Ayush");
        names.add("Anshul");
       boolean result = names.stream()
        .allMatch((str) -> str.endsWith("h"));
 
       System.out.println(result);


     }
}

Output:

false

• Stream noneMatch(): The noneMatch() method of Java stream is also one of the terminal operations that inputs a single Predicate or condition as a parameter and starts the internal iteration of the Stream. It implements the Predicate parameter to every component of the Stream. It returns true if no elements are matched by the Predicate and returns false if one or more elements are matched with the predicate.

Example of Stream noneMatch() method

StreamNoneMatchExample.java

import java.util.*;


public class StreamNoneMatchExample
{
     public static void main(String[] args)
     {
        List<String> names = new ArrayList<>();


        names.add("Mohit");
        names.add("Prateek");
        names.add("Arun");
        names.add("Nishkarsh");
        names.add("Daksh");
        names.add("Nirbhay");
        names.add("Ayush");
        names.add("Anshul");


       boolean result = names.stream()
        .noneMatch((str) -> str.endsWith("h"));
 
       System.out.println(result);


     }
}

Output:

false

• Stream.count(): The Java Stream count() method is also a terminal operation that reflects the number of components present in the stream. The number of elements present in the stream is produced and returned in the form of a long return type. In simple words, the Java Stream count() method starts the internal iteration of the elements in the Stream and counts the elements present in it.

StreamCountExample.java

import java.util.*;

public class StreamCountExample

{

     public static void main(String[] args)

     {

        List<String> names = new ArrayList<>();

        names.add("Mohit");

        names.add("Prateek");

        names.add("Arun");

        names.add("Nishkarsh");

        names.add("Daksh");

        names.add("Nirbhay");

        names.add("Ayush");

        names.add("Anshul");

       long count = names.stream().count();

       System.out.println(count);

     }

}

Output:

8

• Stream.reduce(): The Java Stream reduce() method is also a terminal operation that helps in reducing all the components of the stream to only a single component. It performs a reduction on the components of the stream with the provided function. The result of the reduce() method is an Optional (Optional is a process of restoring a nullable T reference with a non-null value), which holds the reduced value of the stream.

StreamReduceExample.java

import java.util.*;

public class StreamReduceExample

{

     public static void main(String[] args)

     {

        List<String> names = new ArrayList<>();

        names.add("Mohit");

        names.add("Prateek");

        names.add("Arun");

        names.add("Nishkarsh");

        names.add("Daksh");

        names.add("Nirbhay");

        names.add("Ayush");

        names.add("Anshul");

       Optional<String> reducedString = names.stream()

        .reduce((str1,str2) -> str1 + "*_*" + str2);

       reducedString.ifPresent(System.out::println);

     }

}

Output:

Mohit *_* Prateek *_* Arun *_* Nishkarsh *_* Daksh *_* Nirbhay *_* Ayush *_* Anshul

Important Points to remember about Java Streams

• A Java stream is not a data structure. However, it receives input from the Collections, Arrays, or I/O channels.
• Java Streams never alter the primary data structure. In Java Streams, the result is given as per the pipelined methods.
• Java Stream contains two types of operations. These are terminal operations and Intermediate operations.
• Every intermediate operation is lazily performed, and it returns a stream as a result, hence multiple intermediate operations can be pipelined. At the same time, all the terminal operations indicate the termination of the stream and return the result.

Conclusion

The Java Streams are very efficient and simple to understand methods. Java Streams contains a collection of tools that are helpful in processing the series of elements. When Streams are used correctly, it allows us to reduce a considerable amount of boilerplate code, create more readable programs, and advance an app’s productivity.