Java Strings and String Methods – Complete Guide with Examples

Learn everything about Java Strings, including creation, manipulation, and commonly used string methods with detailed examples for beginners and advanced learners.

Strings and String Methods in Java – Complete Detailed Tutorial

A String in Java is an object that represents a sequence of characters.

Strings are widely used in text processing, user input, file handling, and data manipulation.

1. Creating Strings in Java

1.1 Using String Literal


String str1 = "Hello World";
  1. Stored in String Pool
  2. Reuses memory if same string exists

1.2 Using new Keyword


String str2 = new String("Hello Java");
  1. Stored in Heap memory
  2. Creates a new object every time

2. String Immutability

  1. Strings are immutable, meaning cannot be changed once created
  2. Any modification creates a new string

Example:


String str = "Hello";
str.concat(" World");
System.out.println(str); // Hello
  1. Use str = str.concat(" World"); to update the string

3. Commonly Used String Methods

MethodDescriptionExample
length()Returns length of string"Hello".length() → 5
charAt(index)Returns character at index"Hello".charAt(1) → 'e'
concat(str)Concatenates string"Hi".concat(" Java") → "Hi Java"
equals(str)Compares strings"Hi".equals("Hi") → true
equalsIgnoreCase(str)Case-insensitive compare"hi".equalsIgnoreCase("HI") → true
toUpperCase()Converts to uppercase"hello".toUpperCase() → "HELLO"
toLowerCase()Converts to lowercase"HELLO".toLowerCase() → "hello"
trim()Removes leading/trailing spaces" Java ".trim() → "Java"
substring(start, end)Extracts substring"Hello".substring(1,4) → "ell"
replace(old, new)Replaces characters"Hello".replace('l','p') → "Heppo"
split(regex)Splits string into array"a,b,c".split(",") → ["a","b","c"]
contains(str)Checks if string contains substring"Hello".contains("ll") → true
startsWith(prefix)Checks start"Java".startsWith("Ja") → true
endsWith(suffix)Checks end"Java".endsWith("va") → true
indexOf(ch)Returns first index"Hello".indexOf('l') → 2
lastIndexOf(ch)Returns last index"Hello".lastIndexOf('l') → 3

4. String Comparison

  1. == operator: compares references (memory address)
  2. equals() method: compares content

String s1 = "Hello";
String s2 = "Hello";
String s3 = new String("Hello");

System.out.println(s1 == s2); // true
System.out.println(s1 == s3); // false
System.out.println(s1.equals(s3)); // true

5. String Concatenation

  1. Using + operator:

String s = "Hello" + " Java";
  1. Using concat() method:

String s = "Hello".concat(" Java");

6. String Conversion

  1. String → int

String str = "100";
int num = Integer.parseInt(str);
  1. int → String

int num = 100;
String str = String.valueOf(num);
  1. String → double

double d = Double.parseDouble("3.14");
  1. String → char array

char[] chars = "Hello".toCharArray();

7. StringBuilder and StringBuffer

  1. Strings are immutable
  2. For mutable strings, use StringBuilder or StringBuffer

Example:


StringBuilder sb = new StringBuilder("Hello");
sb.append(" World");
System.out.println(sb); // Hello World
  1. StringBuffer → thread-safe (synchronized)
  2. StringBuilder → faster (non-synchronized)

8. Example Program – Strings in Java


public class StringDemo {
public static void main(String[] args) {
String str = " Java Programming ";
System.out.println("Original String: '" + str + "'");
System.out.println("Length: " + str.length());
System.out.println("Trimmed: '" + str.trim() + "'");
System.out.println("Uppercase: " + str.toUpperCase());
System.out.println("Lowercase: " + str.toLowerCase());
System.out.println("Substring (5,16): " + str.substring(5,16));
System.out.println("Replace 'a' with '@': " + str.replace('a','@'));
System.out.println("Contains 'Program': " + str.contains("Program"));

// Splitting string
String[] words = str.trim().split(" ");
System.out.println("Words:");
for(String word : words) {
System.out.println(word);
}

// StringBuilder example
StringBuilder sb = new StringBuilder("Hello");
sb.append(" Java");
System.out.println("StringBuilder: " + sb);
}
}

Sample Output:


Original String: ' Java Programming '
Length: 20
Trimmed: 'Java Programming'
Uppercase: ' JAVA PROGRAMMING '
Lowercase: ' java programming '
Substring (5,16): 'Java Program'
Replace 'a' with '@': ' J@v@ Progr@mming '
Contains 'Program': true
Words:
Java
Programming
StringBuilder: Hello Java

9. Summary

  1. String → sequence of characters, immutable
  2. Use methods like length(), charAt(), substring(), replace(), split()
  3. Compare strings with equals(), not ==
  4. Use StringBuilder/StringBuffer for mutable strings
  5. Strings are essential for text processing in Java