For-each loop in Java

Course Curriculum

For-each loop in Java

For-each loop 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;
}
is equivalent to:

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

// Java program to illustrate
// for-each loop
class For_Each
{
public static void main(String[] arg)
{
{
int[] marks = { 145, 122, 85, 196, 130 };

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 196

Limitations of for-each loop

decision-making

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:

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

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

class P_AI {
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");
}
}

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