Looping Through Java Arrays - Complete Guide
Introduction to Array Looping
Looping through arrays is a core operation in Java. It allows you to read, update, and process each element in an array systematically.
Java provides multiple looping constructs—traditional for loops, enhanced for loops, while loops, and do-while loops—each suited for different scenarios.
For Loop with Index
The traditional for loop gives full control over the index, making it useful when you need both the element and its position, or when you want to iterate in a custom order.
Example
public class ForLoopWithIndex {
public static void main(String[] args) {
int[] numbers = {10, 20, 30, 40, 50};
// Basic for loop
System.out.println("Basic for loop:");
for (int i = 0; i < numbers.length; i++) {
System.out.println("Index " + i + ": " + numbers[i]);
}
// Skipping elements
System.out.println("\nEvery second element:");
for (int i = 0; i < numbers.length; i += 2) {
System.out.println("Index " + i + ": " + numbers[i]);
}
// Reverse order
System.out.println("\nReverse order:");
for (int i = numbers.length - 1; i >= 0; i--) {
System.out.println("Index " + i + ": " + numbers[i]);
}
// Partial iteration
System.out.println("\nFirst 3 elements:");
for (int i = 0; i < 3 && i < numbers.length; i++) {
System.out.println("Index " + i + ": " + numbers[i]);
}
// Modifying elements
System.out.println("\nDoubling values:");
for (int i = 0; i < numbers.length; i++) {
numbers[i] = numbers[i] * 2;
System.out.println("Index " + i + ": " + numbers[i]);
}
// Nested for loop for 2D array
int[][] matrix = {{1, 2, 3}, {4, 5, 6}, {7, 8, 9}};
System.out.println("\n2D array iteration:");
for (int i = 0; i < matrix.length; i++) {
for (int j = 0; j < matrix[i].length; j++) {
System.out.print(matrix[i][j] + " ");
}
System.out.println();
}
}
}Output
Basic for loop: Index 0: 10 Index 1: 20 Index 2: 30 Index 3: 40 Index 4: 50 Every second element: Index 0: 10 Index 2: 30 Index 4: 50 Reverse order: Index 4: 50 Index 3: 40 Index 2: 30 Index 1: 20 Index 0: 10 First 3 elements: Index 0: 10 Index 1: 20 Index 2: 30 Doubling values: Index 0: 20 Index 1: 40 Index 2: 60 Index 3: 80 Index 4: 100 2D array iteration: 1 2 3 4 5 6 7 8 9
Enhanced For Loop (For-Each)
The enhanced for loop simplifies iteration when only element values are needed. It improves readability but does not give direct access to indices or allow modification of array elements in place.
Example
public class EnhancedForLoop {
public static void main(String[] args) {
int[] numbers = {10, 20, 30, 40, 50};
String[] fruits = {"Apple", "Banana", "Orange", "Mango"};
// Simple for-each loop
System.out.println("Numbers:");
for (int number : numbers) {
System.out.println(number);
}
System.out.println("\nFruits:");
for (String fruit : fruits) {
System.out.println(fruit);
}
// Modification attempt (does not change array)
System.out.println("\nTrying to modify elements (affects local variable only):");
for (int number : numbers) {
number = number * 2;
System.out.println(number);
}
System.out.println("Original array unchanged: " + java.util.Arrays.toString(numbers));
// Aggregation
int sum = 0;
for (int number : numbers) {
sum += number;
}
System.out.println("Sum: " + sum);
// Finding maximum
int max = numbers[0];
for (int number : numbers) {
if (number > max) max = number;
}
System.out.println("Max: " + max);
// Condition counting
int count = 0;
for (int number : numbers) {
if (number > 25) count++;
}
System.out.println("Numbers greater than 25: " + count);
// With object arrays
Person[] people = {
new Person("Alice", 25),
new Person("Bob", 30),
new Person("Charlie", 35)
};
System.out.println("\nPeople:");
for (Person person : people) {
System.out.println(person.getName() + " - " + person.getAge());
}
}
}
class Person {
private String name;
private int age;
public Person(String name, int age) {
this.name = name;
this.age = age;
}
public String getName() { return name; }
public int getAge() { return age; }
}Output
Numbers: 10 20 30 40 50 Fruits: Apple Banana Orange Mango Trying to modify elements (affects local variable only): 20 40 60 80 100 Original array unchanged: [10, 20, 30, 40, 50] Sum: 150 Max: 50 Numbers greater than 25: 3 People: Alice - 25 Bob - 30 Charlie - 35
While and Do-While Loops with Arrays
While and do-while loops provide more control over iteration, useful for conditional traversal, searches, or processing until a specific condition is met.
Example
public class WhileDoWhileWithArrays {
public static void main(String[] args) {
int[] numbers = {10, 20, 30, 40, 50};
// While loop
System.out.println("While loop:");
int i = 0;
while (i < numbers.length) {
System.out.println("Index " + i + ": " + numbers[i]);
i++;
}
// Do-while loop
System.out.println("\nDo-while loop:");
int j = 0;
do {
System.out.println("Index " + j + ": " + numbers[j]);
j++;
} while (j < numbers.length);
// Condition-based termination
int[] data = {5, 15, 25, 35, 45, 55, 65};
System.out.println("\nWhile loop until value > 50:");
int index = 0;
while (index < data.length && data[index] <= 50) {
System.out.println("Value: " + data[index]);
index++;
}
// Searching for an element
int[] values = {10, 20, 30, 40, 50, 60};
int searchValue = 40;
int pos = 0;
boolean found = false;
System.out.println("\nSearching for " + searchValue + ":");
while (pos < values.length && !found) {
if (values[pos] == searchValue) {
found = true;
System.out.println("Found at index " + pos);
}
pos++;
}
if (!found) System.out.println("Value not found");
// Filtering values
int[] mixedNumbers = {15, 7, 22, 9, 31, 12, 45};
System.out.println("\nFiltering even numbers:");
int k = 0;
int count = 0;
while (k < mixedNumbers.length) {
if (mixedNumbers[k] % 2 == 0) {
System.out.println("Even number: " + mixedNumbers[k]);
count++;
}
k++;
}
System.out.println("Found " + count + " even numbers");
}
}Output
While loop: Index 0: 10 Index 1: 20 Index 2: 30 Index 3: 40 Index 4: 50 Do-while loop: Index 0: 10 Index 1: 20 Index 2: 30 Index 3: 40 Index 4: 50 While loop until value > 50: Value: 5 Value: 15 Value: 25 Value: 35 Value: 45 Searching for 40: Found at index 3 Filtering even numbers: Even number: 22 Even number: 12 Found 2 even numbers
Best Practices for Array Looping
- ✅ Use enhanced for loops when only values are needed
- ✅ Use traditional for loops for index-based operations or modifications
- ✅ Always check array bounds to avoid ArrayIndexOutOfBoundsException
- ✅ Use while/do-while when iteration depends on conditions, not fixed indices
- ✅ Apply break and continue carefully to control loop flow
- ✅ Prefer enhanced for loops for readability when appropriate
- ✅ Use System.arraycopy() or utility methods for copying instead of manual loops
- ✅ Consider Java 8+ Stream API for more complex operations like filtering and mapping