foreach() loop vs Stream foreach() vs Parallel Stream foreach()

foreach() loop

• Lambda operator is not used: foreach loop in Java doesn’t use any lambda operations and thus operations can be applied on any value outside of the list that we are using for iteration in the foreach loop. The foreach loop is concerned over iterating the collection or array by storing each element of the list on a local variable and doing any functionality that we want.

Java

public class GFG {

    public static void main(String[] args)
    {
        String[] arr = { "1", "2", "3" };
        int count = 0;

        String[] arr1 = { "Geeks", "For", "Geeks" };

        for (String str : arr) {
            System.out.print(arr1[count++]);
        }
    }
}

Output:

GeeksForGeeks

• Here operations on count are possible even being a variable outside the loop because it is in the scope of the foreach loop.
  
• It can be used for all collections and arrays: It can be used to iterate all collections and arrays in Java.

Java

public class GFG {

    public static void main(String[] args)
        throws Exception
    {

        String[] arr = { "1", "2", "3" };
        int count = 0;

        String[] arr1 = { "Geeks", "For", "Geeks" };

        for (String str : arr) {
            System.out.print(arr1[count++]);
        }
    }
}

Output:

GeeksForGeeks

• The return statements work within the loop: The function can return the value at any point of time within the loop. For example, if we want to print only the first 2 values of any collection or array and then we want to return any value, it can be done in foreach loop in Java. The code below is for printing the 2nd element of an array.

Java

public class GFG {

    public static String frechlop(String[] geek)
    {
        int count = 0;
        for (String var : geek) {
            if (count == 1)
                return var;
            count++;
        }
        return "";
    }

    public static void main(String[] args)
        throws Exception
    {

        String[] arr1 = { "Geeks", "For", "Geeks" };
        String secelt = frechlop(arr1);
        System.out.println(secelt);
    }
}

Output:

For

stream().forEach()

• Lambda operator is used: In stream().forEach(), lambdas are used and thus operations on variables outside the loop are not allowed. Only the operations on concerned collections are possible. In this, we can perform operations on Collections by single line code that makes it easy and simple to code.

Java

import Java.util.*;
public class GFG {

    public static void main(String[] args) throws Exception
    {

        List<String> arr1 = new ArrayList<String>();
        int count = 0;
        arr1.add("Geeks");
        arr1.add("For");
        arr1.add("Geeks");
        arr1.stream().forEach(s -> {

            // this will cause an error
            count++;

            // print all elements
            System.out.print(s);
        });
    }
}

• Note: The operation “count++” will cause an error because lambda operations do not allow any external variable operation within itself. 
  
• Only used to iterate collections: The stream().forEach() is only used for accessing the collections like set and list. It can also be used for accessing arrays.

Java

import java.util.*;
public class GFG {

    public static void main(String[] args)
        throws Exception
    {

        String[] arr1 = { "Geeks", "for", "Geeks" };

        // The next line will throw error
        // as there is no such method as stream()
        arr1
            .stream()
            .forEach(s -> {
                System.out.print(s);
            });
    }
}

• Return statements don’t work within this loop but the function calls are very easy to call:
  The return statement within the loop doesn’t work. The same functionality of getting 2nd element cannot be done in the same forEach() method.

Java

import Java.util.*;

public class GFG {

    static String second(List<String> list)
    {
        list
            .stream()
            .forEach(
                s -> {
                    // The next line will throw error
                    // as no return allowed

                    // if(s=="For")return s;

                });
        return "null";
    }

    public static void main(String[] args)
        throws Exception
    {

        List<String> arr1 = new ArrayList<String>();
        arr1.add("Geeks");
        arr1.add("For");
        arr1.add("Geeks");
        String f = second(arr1);
        System.out.println(f);
    }
}

Output:

null

parallel foreach()

• Works on multithreading concept: The only difference between stream().forEach() and parallel foreach() is the multithreading feature given in the parallel forEach().This is way more faster that foreach() and stream.forEach(). Like stream().forEach() it also uses lambda symbol to perform functions. 
  The example providing its multithreading nature which is given as follows.
  It’s clearly implied that the output will never be in the same order for printing the strings due to parallelStream’s multithreaded nature.

Java

import Java.util.*;

public class GFG {

    public static void main(String[] args)
        throws Exception
    {

        List<String> arr1
            = new ArrayList<String>();
        arr1.add("Geeks");
        arr1.add("For");
        arr1.add("Geeks");

        arr1
            .parallelStream()
            .forEach(
                s -> {
                    System.out.print(s);
                });
    }
}

Output:

ForGeeksGeeks

Tabular difference

foreach() loop	stream().foreach() loop	parallelStream().foreach() loop
Lambda operators is not used	Lambda operator is used	Lambda operator is used
Can be used to access arrays and collections	Can access collections only	Can access collections only
The return or control statements work within the loop	The return or control statements don’t work within the loop	The return or control statements don’t work within the loop
No multithreading thus slow data is in sequence	No multithreading thus slow data is in sequence	It is multithreaded thus very fast and sequence is different