For-Each Loop in Java
The for-each loop in Java (also called the enhanced for loop) was introduced in Java 5 to simplify iteration over arrays and collections. It is cleaner and more readable than the traditional for loop and is commonly used when the exact index of an element is not required.
Example:
Below is a basic example of using the for-each loop to iterate through an array and print each element. A for-each loop directly accesses elements without needing index variables.
// Java Program to Iterate through an array
// Using for-each loop
import java.io.*;
class ForEach {
public static void main(String[] args) {
// Array declaration
int arr[] = { 1, 2, 3, 4, 5 };
// Using for-each loop to
// print each element
for (int e : arr) {
System.out.print(e + " ");
}
}
}
Output
1 2 3 4 5
Explanation: In this example, the for-each loop iterates over the array “arr" and prints each element. We use the variable element "e" to access each value in the array, making the loop more concise compared to using a traditional for loop with an index.
Syntax of For-each Loop
for (type var : array) {
statements using var;
}
Parameters:
type: The data type of the elements in the array or collection.var: The variable that holds the current element during each iteration.array: The array or collection being iterated over.
Finding Maximum in an Array using for-each Loop
Now, let’s go through another example that finds the maximum value in an integer array using a for-each loop.
// Java Program to find maximum in an array
// Using for-each loop
public class Maximum {
public static void main(String[] args) {
int[] mark = {125, 132, 95, 116, 110};
int max = findMax(mark);
System.out.println("" + max);
}
public static int findMax(int[] n) {
int maximum = n[0];
// Iterates through the array
for (int n1: n) {
if (n1 > maximum) {
maximum = n1;
}
}
return maximum;
}
}
Output
132
Limitations of the For-each Loop
While the for-each loop is convenient, there are some important limitations to consider:
1. Cannot Modify Array Elements Directly
for (int num : marks) {
// only changes num, not the array element
num = num*2;}
Explanation: The for-each loop gives a copy of each element, not a reference. So modifying the loop variable (num in this case) does not affect the actual array or collection.
2. No Access to Index
for (int num : numbers) {
if (num == target) {
// do not know the index of ‘num’ here
return ???; // Index is unavailable in for-each loop
}
}
Explanation: The for-each loop does not provide access to the index of the current element. If we need the index for any reason (e.g., in a search operation), a traditional loop would be more appropriate.
3. Single-direction Iteration Only
for (int i = numbers.length – 1; i >= 0; i–) {
System.out.println(numbers[i]); // Reverse iteration not possible with for-each
}
Explanation: The for-each loop only iterates over the elements in a forward direction. If we need to iterate in reverse, we have to use a traditional for loop with a manually managed index.
4. Complex Conditions are Difficult to Implement
for (int i = 0; i < numbers.length; i++) {
if (numbers[i] == arr[i]) {
// Complex conditional checks are easier in a traditional for loop
}
}
Explanation: If our logic requires checking multiple conditions or using the index in more complex ways, the for-each loop can be limiting. In such cases, a traditional for loop offers more flexibility.
5. Performance Overhead
List<Integer> list = new ArrayList<>();
for (int i = 0; i < 1000000; i++) list.add(i);
// Using for-each loop
long startTime = System.currentTimeMillis();
for (int i : list) { int a = i; }
long endTime = System.currentTimeMillis();
System.out.println(“For-each loop: ” + (endTime – startTime) + ” ms”);
// Traditional loop
startTime = System.currentTimeMillis();
for (int i = 0; i < list.size(); i++) { int a = list.get(i); }
endTime = System.currentTimeMillis();
System.out.println(“Traditional loop: ” + (endTime – startTime) + ” ms”);
Explanation: The for-each loop sometimes might be slower compared to traditional for loops because for iterating over collections it uses an iterator internally.
Example:
// Java program to compare the performance of
// different loop types for
// iterating over a large list
import java.io.*;
import java.util.*;
class Geeks
{
public static void main(String[] args)
{
List<Integer> list = new ArrayList<>();
long startTime;
long endTime;
// Adding 1 million integers to the list
for (int i = 0; i < 1000000; i++)
{
list.add(i);
}
// Type 1: Using a for-each loop
startTime = Calendar.getInstance().getTimeInMillis();
for (int i : list)
{
// Looping over each element
int a = i;
}
endTime = Calendar.getInstance().getTimeInMillis();
System.out.println("For each loop :: " + (endTime - startTime) + " ms");
// Type 2: Using list.size() in the loop condition (with precomputation)
startTime = Calendar.getInstance().getTimeInMillis();
int size = list.size();
for (int j = 0; j < size; j++)
{
// Accessing list elements using index
int a = list.get(j);
}
endTime = Calendar.getInstance().getTimeInMillis();
System.out.println("Using collection.size() (precomputed) :: " +
(endTime - startTime) + " ms");
// Type 3: Calculating the collection size before the loop (already optimized)
startTime = Calendar.getInstance().getTimeInMillis();
for (int j = 0; j < size; j++)
{
// Accessing list elements using index
int a = list.get(j);
}
endTime = Calendar.getInstance().getTimeInMillis();
System.out.println("By calculating collection.size() first :: " +
(endTime - startTime) + " ms");
// Type 4: Iterating the list in reverse order
startTime = Calendar.getInstance().getTimeInMillis();
for (int j = size - 1; j >= 0; j--) {
// Accessing list elements in reverse
int a = list.get(j);
}
endTime = Calendar.getInstance().getTimeInMillis();
System.out.println("Using [int j = size - 1; j >= 0; j--] (precomputed) :: " +
(endTime - startTime) + " ms");
}
}
Output
For each loop :: 42 ms Using collection.size() (precomputed) :: 13 ms By calculating collection.size() first :: 13 ms Using [int j = size - 1; j >= 0; j--] (precomputed) :: 23 ms
Explanation: Each time we run the above code it will give a different output because the execution and the performance times depend on different factors when JVM starts it takes time to optimize so it might appear that the loops work differently. as mentioned the for-each loop uses iterator internally which make a slight performance overhead compared to the traditional for loop.
Points to Remember:
- The loop starts with the keyword for like a normal for-loop.
- Instead of declaring and initializing a loop counter variable, we 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, we can use the loop variable we created rather than using an indexed array element.
- It is commonly used to iterate over an array or a Collections class (e.g., ArrayList)
