Java Strings

String Immutability

In Java, String objects are immutable — once created, their value cannot be changed. Every operation that appears to modify a string actually creates a new String object. Strings are stored in the String Pool (a special area of the heap) for memory efficiency.

Tip: Use == to compare references, and .equals() to compare content. Always use .equals() for string comparison.

Common String Methods

public class StringMethods {
    public static void main(String[] args) {
        String s = "  Hello, Java World!  ";

        System.out.println(s.length());              // 22
        System.out.println(s.trim());                // "Hello, Java World!"
        System.out.println(s.trim().toLowerCase());  // "hello, java world!"
        System.out.println(s.trim().toUpperCase());  // "HELLO, JAVA WORLD!"

        String t = s.trim();
        System.out.println(t.charAt(7));             // J
        System.out.println(t.indexOf("Java"));       // 7
        System.out.println(t.contains("World"));     // true
        System.out.println(t.substring(7, 11));      // Java
        System.out.println(t.replace("Java", "Core Java")); // Hello, Core Java World!
        System.out.println(t.startsWith("Hello"));   // true
        System.out.println(t.endsWith("!"));         // true

        // split
        String csv = "one,two,three,four";
        String[] parts = csv.split(",");
        for (String p : parts) System.out.print(p + " ");  // one two three four
        System.out.println();

        // equals vs ==
        String a = new String("test");
        String b = new String("test");
        System.out.println(a == b);       // false (different objects)
        System.out.println(a.equals(b));  // true  (same content)
    }
}

StringBuilder vs StringBuffer

When you need to build or modify strings frequently, use StringBuilder (single-threaded) or StringBuffer (thread-safe). Both are mutable, unlike String.

Feature	String	StringBuilder	StringBuffer
Mutable	No	Yes	Yes
Thread-safe	Yes (immutable)	No	Yes (synchronized)
Performance	Slow for concatenation	Fast	Slower than StringBuilder
Use when	Fixed text	Single-threaded string building	Multi-threaded string building

StringBuilder & String.format()

public class StringBuilderDemo {
    public static void main(String[] args) {
        // StringBuilder — efficient mutable string
        StringBuilder sb = new StringBuilder();
        sb.append("Hello");
        sb.append(", ");
        sb.append("World");
        sb.append("!");
        System.out.println(sb.toString());       // Hello, World!
        System.out.println(sb.length());         // 13
        sb.insert(5, " Java");
        System.out.println(sb.toString());       // Hello Java, World!
        sb.delete(5, 10);
        System.out.println(sb.toString());       // Hello, World!
        sb.reverse();
        System.out.println(sb.toString());       // !dlroW ,olleH

        // String.format() — printf-style formatting
        String name = "Alice";
        int age = 30;
        double gpa = 3.85;
        String formatted = String.format("Name: %-10s | Age: %3d | GPA: %.2f", name, age, gpa);
        System.out.println(formatted);
        // Name: Alice      | Age:  30 | GPA: 3.85
    }
}

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Java Strings

String Immutability

StringBuilder vs StringBuffer

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