For-each loop in Java

Prerequisite: Decision making in Java
For-each is another array traversing technique like for loop, while loop, do-while loop introduced in Java5. 

• It starts with the keyword for like a normal for-loop.
• Instead of declaring and initializing a loop counter variable, you declare a variable that is the same type as the base type of the array, followed by a colon, which is then followed by the array name.
• In the loop body, you can use the loop variable you created rather than using an indexed array element. 
  
• It’s commonly used to iterate over an array or a Collections class (eg, ArrayList)

Syntax: 

for (type var : array) 
{ 
    statements using var;
}

Simple program with for each loop:

Java

/*package whatever //do not write package name here */

import java.io.*;

class Easy

{

    public static void main(String[] args)

    {

        // array declaration

        int ar[] = { 10, 50, 60, 80, 90 };

        for (int element : ar)

            System.out.print(element + " ");
    }
}

Output

10 50 60 80 90 

The above syntax is equivalent to:

for (int i=0; i<arr.length; i++) 
{ 
    type var = arr[i];
    statements using var;
}

Java

// Java program to illustrate
// for-each loop
class For_Each
{
    public static void main(String[] arg)
    {
        {
            int[] marks = { 125, 132, 95, 116, 110 };

            int highest_marks = maximum(marks);
            System.out.println("The highest score is " + highest_marks);
        }
    }
    public static int maximum(int[] numbers)
    {
        int maxSoFar = numbers[0];

        // for each loop
        for (int num : numbers)
        {
            if (num > maxSoFar)
            {
                maxSoFar = num;
            }
        }
    return maxSoFar;
    }
}

Output

The highest score is 132

Limitations of for-each loop
       decision-making

1. For-each loops are not appropriate when you want to modify the array:
   

for (int num : marks) 
{
    // only changes num, not the array element
    num = num*2; 
}

       2. For-each loops do not keep track of index. So we can not obtain array index using For-Each loop 

for (int num : numbers) 
{ 
    if (num == target) 
    {
        return ???;   // do not know the index of num
    }
}

        3.  For-each only iterates forward over the array in single steps 

// cannot be converted to a for-each loop
for (int i=numbers.length-1; i>0; i--) 
{
      System.out.println(numbers[i]);
}

        4. For-each cannot process two decision making statements at once 

// cannot be easily converted to a for-each loop 
for (int i=0; i<numbers.length; i++) 
{
    if (numbers[i] == arr[i]) 
    { ...
    } 
}

        5. For-each also has some performance overhead over simple iteration: 

Java

/*package whatever //do not write package name here */

import java.io.*;
import java.util.*;

class GFG {
    public static void main (String[] args) {
        List<Integer> list = new ArrayList<>();
        long startTime;
        long endTime;
        for (int i = 0; i < 1000000; i++) {
            list.add(i);
        }
        // Type 1
        startTime = Calendar.getInstance().getTimeInMillis();
        for (int i : list) {
            int a = i;
        }
        endTime = Calendar.getInstance().getTimeInMillis();
        System.out.println("For each loop :: " + (endTime - startTime) + " ms");

        // Type 2
        startTime = Calendar.getInstance().getTimeInMillis();
        for (int j = 0; j < list.size(); j++) {
            int a = list.get(j);
        }
        endTime = Calendar.getInstance().getTimeInMillis();
        System.out.println("Using collection.size() :: " + (endTime - startTime) + " ms");

        // Type 3
        startTime = Calendar.getInstance().getTimeInMillis();
        int size = list.size();
        for (int j = 0; j < size; j++) {
            int a = list.get(j);
        }
        endTime = Calendar.getInstance().getTimeInMillis();
        System.out.println("By calculating collection.size() first :: " + (endTime - startTime) + " ms");

        // Type 4
        startTime = Calendar.getInstance().getTimeInMillis();
        for(int j = list.size()-1; j >= 0; j--) {
            int a = list.get(j);
        }
        endTime = Calendar.getInstance().getTimeInMillis();
        System.out.println("Using [int j = list.size(); j > size ; j--] :: " + (endTime - startTime) + " ms");
    }
}

// This code is contributed by Ayush Choudhary @gfg(code_ayush)

Output

For each loop :: 45 ms
Using collection.size() :: 11 ms
By calculating collection.size() first :: 13 ms
Using [int j = list.size(); j > size ; j--] :: 15 ms