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An index is the position of an item in an array. The first item is at position 0, the second at position 1, and so on.
int[] marks = {50, 55, 60, 70, 80};
System.out.println(marks[0]); // Output: 50
System.out.println(marks[4]); // Output: 80
An array is a collection of items of the same type. You start counting from 0 up to one less than the total number of items.
// Create an array with 5 elements
int[] marks = {10, 20, 30, 40, 50};
In Java, arrays store a list of items of the same type. You can create arrays in different ways.
int[] age = {20, 21, 30};
int[] marks = new int[3];
marks[0] = 50;
marks[1] = 70;
marks[2] = 93;
Arrays have a fixed size, but you can change the value of an item at any index.
int[] nums = {1, 2, 0, 4};
// Change the value at index 2
nums[2] = 3;
An ArrayList is a flexible array that can grow in size. You can add, remove, and change items easily.
// Import the ArrayList class
import java.util.ArrayList;
// Create an ArrayList called students
ArrayList<String> students = new ArrayList<String>();
To modify an ArrayList, you can add, remove, or change items by specifying their position or value.
class Animal {
// The sound method is overridden by subclasses
public void sound() {
System.out.println("Animal makes a sound");
}
}
class Cat extends Animal {
@Override
public void sound() {
System.out.println("Meow");
}
}
class Dog extends Animal {
@Override
public void sound() {
System.out.println("Woof");
}
}
class Pig extends Animal {
@Override
public void sound() {
System.out.println("Oink");
}
}
public class Main {
public static void main(String[] args) {
Animal cat = new Cat();
Animal dog = new Dog();
Animal pig = new Pig();
Animal[] animals = {cat, dog, pig};
for (Animal animal : animals) {
animal.sound();
}
}
}
The for-each loop is used to go through each item in an array or ArrayList easily, without needing to know the size or index.
// Array of numbers
int[] numbers = {1, 2, 3, 4, 5};
// Loop through each number
for (int num : numbers) {
System.out.println(num);
}
Nested loops are loops inside other loops. The inner loop completes all its iterations before the outer loop moves to the next iteration.
for (int outer = 0; outer < 3; outer++) {
System.out.println("Outer index: " + outer);
for (int inner = 0; inner < 4; inner++) {
System.out.println(" Inner index: " + inner);
}
}
2D arrays are arrays of arrays. They are declared with two sets of square brackets.
int[][] twoDIntArray;
String[][] twoDStringArray;
double[][] twoDDoubleArray;
To access an item in a 2D array, use two indices: the first for the row and the second for the column.
int[][] arr = {{1, 2, 3},
{4, 5, 6}};
int retrieved = arr[1][0]; // Output: 4
Initializer lists let you set the values of a 2D array when you create it. You can define values in curly brackets.
int[][] matrix = {{1, 2},
{3, 4}};
To change an element in a 2D array, you assign a new value to a specific row and column.
int[][] matrix = {{1, 2},
{3, 4}};
matrix[0][1] = 10; // Change value at first row, second column to 10
Row-major order means going through each row one at a time. Use nested loops to traverse a 2D array row by row.
for (int i = 0; i < matrix.length; i++) {
for (int j = 0; j < matrix[i].length; j++) {
System.out.println(matrix[i][j]);
}
}
Column-major order means going through each column one at a time. Use nested loops to traverse a 2D array column by column.
for (int i = 0; i < matrix[0].length; i++) {
for (int j = 0; j < matrix.length; j++) {
System.out.println(matrix[j][i]);
}
}
You can use enhanced for loops to easily traverse a 2D array. The outer loop goes through rows, and the inner loop goes through columns.
for (String[] rowOfStrings : twoDStringArray) {
for (String s : rowOfStrings) {
System.out.println(s);
}
}
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