স্ট্রিং (Strings)

public class Main {
    public static void main(String[] args) {
        String greeting = "Hello, World!";

        // Length — how many characters?
        System.out.println("Length: " + greeting.length());  // 13

        // charAt — grab one character by position
        System.out.println("First char: " + greeting.charAt(0));  // H
        System.out.println("Last char: " + greeting.charAt(greeting.length() - 1));  // !

        // substring — cut out a piece
        System.out.println("Sub: " + greeting.substring(0, 5));  // Hello
        System.out.println("From 7: " + greeting.substring(7));  // World!

        // Case changes
        System.out.println("Upper: " + greeting.toUpperCase());  // HELLO, WORLD!
        System.out.println("Lower: " + greeting.toLowerCase());  // hello, world!

        // Finding & checking
        System.out.println("Contains 'World': " + greeting.contains("World"));
        System.out.println("Starts with 'Hello': " + greeting.startsWith("Hello"));
        System.out.println("Index of 'W': " + greeting.indexOf('W'));
    }
}

Length: 13
First char: H
Last char: !
Sub: Hello
From 7: World!
Upper: HELLO, WORLD!
Lower: hello, world!
Contains 'World': true
Starts with 'Hello': true
Index of 'W': 7

public class Main {
    public static void main(String[] args) {
        // equals() vs ==
        String a = "hello";
        String b = "hello";
        String c = new String("hello");

        System.out.println("a == b: " + (a == b));         // true (same pool object)
        System.out.println("a == c: " + (a == c));         // false! (different objects)
        System.out.println("a.equals(c): " + a.equals(c)); // true (same content)

        // String.format — like a template with blanks to fill in
        String name = "Laboni";
        int score = 95;
        String report = String.format("%s scored %d out of 100 (%.1f%%)",
                                       name, score, (double) score);
        System.out.println(report);

        // Useful string operations
        String messy = "   too many spaces   ";
        System.out.println("Trimmed: '" + messy.trim() + "'");

        String csv = "আম,জাম,কাঁঠাল";
        String[] fruits = csv.split(",");
        System.out.println("Second fruit: " + fruits[1]);
    }
}

a == b: true
a == c: false
a.equals(c): true
Luna scored 95 out of 100 (95.0%)
Trimmed: 'too many spaces'
Second fruit: জাম

public class Main {
    public static void main(String[] args) {
        // Bad: creating tons of throwaway String objects
        // String result = "";
        // for (...) result += something; // slow!

        // Good: StringBuilder is like a whiteboard you can erase and rewrite
        StringBuilder sb = new StringBuilder();
        sb.append("I");
        sb.append(" love");
        sb.append(" Java");
        sb.append("!");
        System.out.println(sb.toString());

        // You can also insert and delete
        sb.insert(6, " learning");
        System.out.println(sb.toString());

        sb.delete(0, 2); // remove "I "
        System.out.println(sb.toString());

        // Reverse!
        StringBuilder word = new StringBuilder("desserts");
        System.out.println("Reversed: " + word.reverse());
    }
}

I love Java!
I love learning Java!
love learning Java!
Reversed: stressed