Using objects: String, Math, and wrappers

Using objects

• An object is a value that knows how to do things. You call its methods with a dot ..
• A String is an object. Math, Integer, and Double give you ready-made tools.
• In this lesson you learn to make objects and use their methods.

Making objects with new

• Some objects are made with the keyword new and a constructor.
• A String is special: you can write it directly with quotes, like "hi".
• You can also make a separate String object with new String("..."), but quotes are the normal way.

public class Main {
    public static void main(String[] args) {
        String greeting = "Hello";
        String copy = new String("Hello");
        System.out.println(greeting);
        System.out.println(copy);
    }
}

String methods

• s.length() — how many characters are in s.
• s.charAt(i) — the character at position i (the first position is 0).
• s.substring(a, b) — the part from a up to but not including b.
• s.indexOf("x") — the first position of "x", or -1 if not found.

public class Main {
    public static void main(String[] args) {
        String word = "computer";
        System.out.println(word.length());        // 8
        System.out.println(word.charAt(0));        // c
        System.out.println(word.substring(0, 4));  // comp
        System.out.println(word.indexOf("put"));   // 3
    }
}

Comparing Strings

• Use s.equals(other) to check if two Strings have the same letters.
• Do not use == to compare Strings — it can give the wrong answer. We learn why in lesson 3.
• s.compareTo(other) returns 0 when equal, a negative number if s comes first in order, and a positive number if it comes later.

public class Main {
    public static void main(String[] args) {
        String a = "cat";
        String b = "cat";
        String c = "dog";
        System.out.println(a.equals(b));   // true
        System.out.println(a.equals(c));   // false
        System.out.println(a.compareTo(c)); // negative: "cat" is before "dog"
    }
}

Math methods

• Math holds math tools. You call them on Math itself, like Math.abs(-5).
• Math.abs(x) — the size of x with no minus sign.
• Math.pow(b, e) — b to the power e (returns a double).
• Math.sqrt(x) — the square root of x (returns a double).
• Math.random() — a random double from 0.0 up to (not including) 1.0.

public class Main {
    public static void main(String[] args) {
        System.out.println(Math.abs(-7));       // 7
        System.out.println(Math.pow(2, 3));     // 8.0
        System.out.println(Math.sqrt(9));       // 3.0
        // Math.random() changes each run, so we do not print it here.
    }
}

Wrappers and autoboxing

• int and double are simple types. Their object versions are Integer and Double.
• Java can switch between them for you. This is called autoboxing.
• Integer.parseInt("42") turns text into an int. Double.parseDouble("3.5") turns text into a double.

public class Main {
    public static void main(String[] args) {
        Integer boxed = 10;        // int -> Integer (autoboxing)
        int back = boxed;          // Integer -> int (unboxing)
        int n = Integer.parseInt("42");
        double d = Double.parseDouble("3.5");
        System.out.println(back + n);   // 52
        System.out.println(d);          // 3.5
    }
}

Common mistakes

• Compare Strings (and other objects) with .equals(), not ==.
• Math.random() returns a value from 0 up to (but not including) 1.

Now you try

• Each task pre-fills the class skeleton — write your code inside main, or complete the method shown.
• Press Run to compile and run, then Check answer.
• Your code compiles and runs on the server, so even the first run is fast.