Fundamentals

Arrays & Strings

An array is a fixed-size collection of elements of the same data type.

java

1int[] numbers = {10, 20, 30, 40};
2 
3System.out.println(numbers[0]); // 10

A 2D array is an array of arrays, commonly used to represent a matrix.

java

1int[][] matrix = {
2    {1, 2, 3},
3    {4, 5, 6}
4};
5 
6System.out.println(matrix[0][1]); // 2

java

1for (int i = 0; i < matrix.length; i++) {
2    for (int j = 0; j < matrix[i].length; j++) {
3        System.out.print(matrix[i][j] + " ");
4    }
5}

String is used to represent a sequence of characters.

java

1String name = "Akash";

Strings are immutable, meaning their value cannot be changed after creation.

java

1String s = "Hello";
2s = s + " World";

The original "Hello" string is not modified; a new string is created.

Interview Tip: Use .equals() to compare String values instead of ==.

java

1String a = "Hello";
2String b = new String("Hello");
3 
4System.out.println(a.equals(b)); // true
5System.out.println(a == b);     // false

StringBuilder is used when a string needs to be modified frequently.

Unlike String, StringBuilder is mutable.

java

1StringBuilder sb = new StringBuilder("Hello");
2 
3sb.append(" World");
4 
5System.out.println(sb); // Hello World

Used to sort an array in ascending order.

java

1import java.util.Arrays;
2 
3int[] arr = {5, 2, 8, 1, 3};
4 
5Arrays.sort(arr);
6 
7System.out.println(Arrays.toString(arr));
8// [1, 2, 3, 5, 8]

Used to convert a 1D array into a readable String representation.

java

1import java.util.Arrays;
2 
3int[] arr = {10, 20, 30};
4 
5System.out.println(Arrays.toString(arr));
6// [10, 20, 30]

For a 2D array, use Arrays.deepToString():

java

1int[][] matrix = {
2    {1, 2},
3    {3, 4}
4};
5 
6System.out.println(Arrays.deepToString(matrix));
7// [[1, 2], [3, 4]]