Java Arrays - Complete Guide
Introduction to Arrays
In Java, an array is a container object that holds a fixed number of values of a single type. Each element in an array is accessed using its index, starting from 0.
Arrays provide a convenient way to group related data, especially when the number of elements is known in advance. They are commonly used for data storage, iteration, and as the basis for more complex data structures.
Creating and Initializing Arrays
There are several ways to declare, create, and initialize arrays in Java. Choosing the correct approach depends on whether you already know the values or just the required size.
Example
public class ArrayCreation {
public static void main(String[] args) {
// Method 1: Declaration and then creation
int[] numbers1;
numbers1 = new int[5]; // Array of 5 integers, default values = 0
// Method 2: Declaration and creation in one line
int[] numbers2 = new int[5];
// Method 3: Initialize directly with values
int[] numbers3 = {1, 2, 3, 4, 5};
// Method 4: Using new with values
int[] numbers4 = new int[]{1, 2, 3, 4, 5};
// Arrays of other data types
double[] prices = new double[3];
boolean[] flags = new boolean[3];
char[] letters = new char[2];
String[] names = {"Alice", "Bob", "Charlie"};
// Default values
System.out.println("Default int value: " + numbers1[0]); // 0
System.out.println("Default double value: " + prices[0]); // 0.0
System.out.println("Default boolean value: " + flags[0]); // false
System.out.println("Default char value: " + (int)letters[0]); // 0 (null character)
System.out.println("Default String value: " + names[0]); // "Alice" (explicit)
// Array length
System.out.println("Length of numbers3: " + numbers3.length);
System.out.println("Length of names: " + names.length);
// Accessing elements
System.out.println("First element: " + numbers3[0]);
System.out.println("Last element: " + numbers3[numbers3.length - 1]);
// Modifying an element
numbers3[0] = 10;
System.out.println("Modified first element: " + numbers3[0]);
}
}Output
Default int value: 0 Default double value: 0.0 Default boolean value: false Default char value: 0 Default String value: Alice Length of numbers3: 5 Length of names: 3 First element: 1 Last element: 5 Modified first element: 10
Array Properties and Characteristics
Arrays in Java are fixed in size, indexed from 0, and enforce type consistency. They also perform bounds checking at runtime, preventing access to invalid indexes.
Example
public class ArrayProperties {
public static void main(String[] args) {
int[] numbers = {1, 2, 3, 4, 5};
// Valid access
System.out.println("Valid access: " + numbers[2]); // 3
// Invalid index
try {
System.out.println(numbers[10]);
} catch (ArrayIndexOutOfBoundsException e) {
System.out.println("Error: " + e.getMessage());
}
// Negative index
try {
System.out.println(numbers[-1]);
} catch (ArrayIndexOutOfBoundsException e) {
System.out.println("Error: " + e.getMessage());
}
// Array reference assignment
int[] arr1 = {1, 2, 3};
int[] arr2 = arr1;
arr2[0] = 100;
System.out.println("arr1[0]: " + arr1[0]); // 100
// Copying arrays
int[] original = {1, 2, 3, 4, 5};
int[] copy = new int[original.length];
for (int i = 0; i < original.length; i++) {
copy[i] = original[i];
}
copy[0] = 999;
System.out.println("Original[0]: " + original[0]);
System.out.println("Copy[0]: " + copy[0]);
// Using System.arraycopy
int[] copy2 = new int[original.length];
System.arraycopy(original, 0, copy2, 0, original.length);
// Using Arrays.copyOf
int[] copy3 = java.util.Arrays.copyOf(original, original.length);
}
}Output
Valid access: 3 Error: Index 10 out of bounds for length 5 Error: Index -1 out of bounds for length 5 arr1[0]: 100 Original[0]: 1 Copy[0]: 999
Common Array Operations
Some of the most common operations on arrays include searching, finding maximum and minimum values, calculating aggregates such as sum or average, reversing, and sorting.
Example
public class ArrayOperations {
public static void main(String[] args) {
int[] numbers = {5, 2, 8, 1, 9, 3};
// Maximum
int max = numbers[0];
for (int num : numbers) {
if (num > max) max = num;
}
System.out.println("Max: " + max);
// Minimum
int min = numbers[0];
for (int num : numbers) {
if (num < min) min = num;
}
System.out.println("Min: " + min);
// Sum and average
int sum = 0;
for (int num : numbers) sum += num;
double avg = (double) sum / numbers.length;
System.out.println("Sum: " + sum);
System.out.println("Average: " + avg);
// Searching
int searchValue = 3;
int index = -1;
for (int i = 0; i < numbers.length; i++) {
if (numbers[i] == searchValue) {
index = i;
break;
}
}
System.out.println(index >= 0 ? "Found at index " + index : "Not found");
// Reversing
for (int i = 0; i < numbers.length / 2; i++) {
int temp = numbers[i];
numbers[i] = numbers[numbers.length - 1 - i];
numbers[numbers.length - 1 - i] = temp;
}
System.out.println("Reversed: " + java.util.Arrays.toString(numbers));
// Sorting
java.util.Arrays.sort(numbers);
System.out.println("Sorted: " + java.util.Arrays.toString(numbers));
}
}Output
Max: 9 Min: 1 Sum: 28 Average: 4.666666666666667 Found at index 5 Reversed: [3, 9, 1, 8, 2, 5] Sorted: [1, 2, 3, 5, 8, 9]
Best Practices for Working with Arrays
- ✅ Always check array bounds to avoid ArrayIndexOutOfBoundsException
- ✅ Use enhanced for loops when index is not needed
- ✅ Prefer ArrayList when you need a resizable collection
- ✅ Use Arrays.toString() for readable array output
- ✅ Use Arrays.sort() for sorting
- ✅ Use System.arraycopy() or Arrays.copyOf() for copying arrays
- ✅ Initialize arrays with clear default values when possible
- ✅ Encapsulate common array operations in helper methods