Arrays and Strings in Java

Introduction

Arrays and strings are among the most frequently used data structures in Java. Arrays allow storing multiple values of the same type in a single variable, while strings are used to handle text data. Java provides powerful built-in classes such as String, StringBuilder, and StringBuffer to perform efficient text manipulation. Understanding these concepts is essential for real-world Java programming, problem solving, and interviews.

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1. Single-Dimensional Arrays

Description

A single-dimensional array is a collection of elements of the same data type stored in contiguous memory locations. Each element can be accessed using an index, starting from 0.

Key Points

• Fixed size once created
• Stores same type of data
• Index starts from 0
• Faster access using index

Syntax

dataType[] arrayName = new dataType[size];

Example

int[] marks = new int[5];

marks[0] = 85;
marks[1] = 90;
marks[2] = 78;
marks[3] = 88;
marks[4] = 92;

System.out.println(marks[2]);

Explanation

The array marks stores 5 integer values. The value at index 2 is printed.

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2. Multi-Dimensional Arrays

Description

A multi-dimensional array is an array of arrays. The most commonly used type is the two-dimensional array, which is represented in rows and columns like a table or matrix.

Key Points

• Useful for matrix operations
• Represent data in tabular form
• Accessed using row and column index

Syntax

dataType[][] arrayName = new dataType[rows][columns];

Example

int[][] matrix = {
    {1, 2, 3},
    {4, 5, 6}
};

System.out.println(matrix[1][2]);

Explanation

The value at row index 1 and column index 2 is printed, which is 6.

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3. Array Operations and Methods

Java provides the Arrays class in java.util package to perform common operations.

Common Array Operations

a) Traversing an Array

int[] numbers = {10, 20, 30, 40};

for (int num : numbers) {
    System.out.println(num);
}

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b) Sorting an Array

import java.util.Arrays;

int[] nums = {5, 2, 8, 1};
Arrays.sort(nums);

System.out.println(Arrays.toString(nums));

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c) Searching an Element

int index = Arrays.binarySearch(nums, 8);
System.out.println(index);

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d) Copying an Array

int[] copy = Arrays.copyOf(nums, nums.length);

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Important Array Methods

Method	Description
sort()	Sorts array
binarySearch()	Searches element
copyOf()	Copies array
equals()	Compares arrays
toString()	Prints array

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4. String Class in Java

Description

The String class is used to represent a sequence of characters. In Java, strings are immutable, meaning once created, their values cannot be changed.

Key Points

• Immutable in nature
• Stored in String Constant Pool
• Thread-safe
• Uses equals() for comparison

Example

String name = "Java";
name = name.concat(" Programming");

System.out.println(name);

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5. Common String Methods

Frequently Used Methods

String text = "Java Programming";

Method	Example	Result
length()	text.length()	Length
toUpperCase()	text.toUpperCase()	Uppercase
toLowerCase()	text.toLowerCase()	Lowercase
charAt()	text.charAt(2)	Character
substring()	text.substring(5)	Substring
contains()	text.contains("Java")	true/false
replace()	text.replace("Java", "Core Java")	Replaced

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6. StringBuilder Class

Description

StringBuilder is used to create mutable strings. It is faster than String because it does not create new objects during modification.

Key Points

• Mutable
• Not thread-safe
• Faster performance
• Used in single-threaded applications

Example

StringBuilder sb = new StringBuilder("Hello");
sb.append(" World");

System.out.println(sb);

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7. StringBuffer Class

Description

StringBuffer is similar to StringBuilder but is thread-safe.

Key Points

• Mutable
• Thread-safe
• Slightly slower than StringBuilder
• Used in multi-threaded applications

Example

StringBuffer sbf = new StringBuffer("Java");
sbf.append(" Course");

System.out.println(sbf);

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8. String vs StringBuilder vs StringBuffer

Feature	String	StringBuilder	StringBuffer
Mutability	Immutable	Mutable	Mutable
Thread Safety	Yes	No	Yes
Performance	Slow	Fast	Medium
Use Case	Fixed text	Single thread	Multi-thread

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9. String Manipulation

Description

String manipulation refers to operations performed on strings to modify, analyze, or extract data.

Common String Manipulation Examples

a) Reverse a String

String input = "Java";
StringBuilder rev = new StringBuilder(input);
System.out.println(rev.reverse());

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b) Count Characters

String s = "Programming";
System.out.println(s.length());

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c) Check Palindrome

String str = "madam";
String reversed = new StringBuilder(str).reverse().toString();

if (str.equals(reversed)) {
    System.out.println("Palindrome");
} else {
    System.out.println("Not Palindrome");
}

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d) Remove Spaces

String sentence = "Java is powerful";
System.out.println(sentence.replace(" ", ""));

Example:

java

public class ArraysAndStrings {
    public static void main(String[] args) {
        // Arrays
        int[] numbers = {10, 20, 30, 40, 50};
        System.out.println("First element: " + numbers[0]);
        
        // Loop through array
        for (int num : numbers) {
            System.out.print(num + " ");
        }
        System.out.println();
        
        // 2D Array
        int[][] matrix = {
            {1, 2, 3},
            {4, 5, 6},
            {7, 8, 9}
        };
        System.out.println("Element at [1][2]: " + matrix[1][2]);
        
        // Strings
        String text = "Java Programming";
        System.out.println("Length: " + text.length());
        System.out.println("Uppercase: " + text.toUpperCase());
        System.out.println("Substring: " + text.substring(0, 4));
        System.out.println("Contains 'Java': " + text.contains("Java"));
        
        // StringBuilder for efficient string manipulation
        StringBuilder sb = new StringBuilder("Hello");
        sb.append(" World");
        sb.insert(5, ",");
        System.out.println(sb.toString());
    }
}