A String in Java represents a sequence of characters. Strings are objects, and the String class provides many methods for string manipulation.

Creating Strings

Using String Literals

String message = "Hello, World!";

Using the new Keyword

String greeting = new String("Hello");

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Common String Methods

length()

The length() method returns the number of characters in a string.

String text = "Java Programming";
System.out.println("Length: " + text.length());  // Output: 16

concat()

The concat() method joins two strings.

String first = "Hello, ";
String second = "World!";

String result = first.concat(second);
System.out.println(result);  // Output: Hello, World!

equals()

The equals() method compares two strings for equality.

String str1 = "Java";
String str2 = "java";

boolean isEqual = str1.equals(str2);  // Case-sensitive
System.out.println(isEqual);  // Output: false

To perform a case-insensitive comparison, use equalsIgnoreCase().

boolean isEqualIgnoreCase = str1.equalsIgnoreCase(str2);
System.out.println(isEqualIgnoreCase);  // Output: true

indexOf()

The indexOf() method returns the index of the first occurrence of a character or substring.

String text = "Programming in Java";

System.out.println(text.indexOf('a'));       // Output: 8
System.out.println(text.indexOf("Java"));    // Output: 16

charAt()

The charAt() method returns the character at a specified index.

String word = "Hello";
System.out.println(word.charAt(1));  // Output: e

substring()

The substring() method returns a part of the string.

Extracting from a starting index

String text = "Java Programming";
System.out.println(text.substring(5));  // Output: Programming

Extracting a range

System.out.println(text.substring(5, 8));  // Output: Pro

toUpperCase() and toLowerCase()

These methods convert strings to uppercase or lowercase.

String text = "Hello, Java!";

System.out.println(text.toUpperCase());  // Output: HELLO, JAVA!
System.out.println(text.toLowerCase());  // Output: hello, java!

Combining Methods

String original = "  Java Programming  ";
String trimmed = original.trim();  // Removes leading and trailing spaces

System.out.println("Length: " + trimmed.length());           // Output: 16
System.out.println(trimmed.concat(" is fun!"));              // Output: Java Programming is fun!
System.out.println(trimmed.equals("Java Programming"));      // Output: true
System.out.println("Index of 'P': " + trimmed.indexOf('P')); // Output: 5
System.out.println("Char at 5: " + trimmed.charAt(5));       // Output: P
System.out.println("Substring: " + trimmed.substring(5, 11)); // Output: Progra

System.out.println(trimmed.toUpperCase());                   // Output: JAVA PROGRAMMING
System.out.println(trimmed.toLowerCase());                   // Output: java programming

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Summary

• length() returns the number of characters in a string.

• concat() concatenates two strings.

• equals() compares two strings for equality (case-sensitive).

• indexOf() returns the index of the first occurrence of a character.

• charAt() returns the character at a specific index.

• substring() extracts a substring from the string.

• toUpperCase() converts all characters to uppercase.

• toLowerCase() converts all characters to lowercase.