Booleans and selection: if/else and logic

Booleans and selection

• A boolean is a value that is either true or false.
• Programs make choices by testing a boolean and running different code.
• In this lesson you learn if / else if / else and how to build conditions.

Relational operators

• These compare two values and give a boolean:
  • < less than, > greater than
  • <= less or equal, >= greater or equal
  • == equal, != not equal
• Note: = puts a value into a variable, but == tests if two values are equal.

public class Main {
    public static void main(String[] args) {
        int score = 75;
        System.out.println(score > 50);   // true
        System.out.println(score == 100); // false
        System.out.println(score != 75);  // false
    }
}

if / else if / else

• if runs a block only when its condition is true.
• else if checks another condition when the ones before were false.
• else runs when none of the conditions were true.

public class Main {
    public static void main(String[] args) {
        int n = -4;
        if (n > 0) {
            System.out.println("positive");
        } else if (n < 0) {
            System.out.println("negative");
        } else {
            System.out.println("zero");
        }
    }
}

Logical operators

• && means and: true only when both sides are true.
• || means or: true when at least one side is true.
• ! means not: it flips true to false and false to true.

public class Main {
    public static void main(String[] args) {
        int age = 16;
        boolean teen = age >= 13 && age <= 19;
        System.out.println(teen);        // true
        System.out.println(!teen);       // false
        System.out.println(age < 13 || age > 19); // false
    }
}

Short-circuit

• Java reads && and || from left to right and stops early when it can.
• With &&, if the left side is false, the answer is already false, so the right side is skipped.
• With ||, if the left side is true, the answer is already true, so the right side is skipped.
• This lets you check something is safe before you use it.

public class Main {
    public static void main(String[] args) {
        int x = 0;
        // Left side is false, so x / x is never run (no error).
        boolean ok = (x != 0) && (10 / x > 1);
        System.out.println(ok);   // false
    }
}

The == vs .equals() String trap

• For int and boolean, == works fine.
• For String, == checks if they are the same object in memory, not the same letters.
• Two Strings with the same letters can still be different objects, so == may be false.
• Always compare Strings with .equals(...).

public class Main {
    public static void main(String[] args) {
        String a = "cat";
        String b = new String("cat");
        System.out.println(a == b);       // false (different objects)
        System.out.println(a.equals(b));  // true  (same letters)
    }
}

Common mistakes

• A condition must be a boolean; if (x = 5) does not compile.
• Each switch case needs a break;.

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.