Module 2: Boolean Variables and Arrays

What is a Boolean variable?

• An int variable takes integer values like: 12, 256, or -9.

• A double variable takes on values like 3.141, -14.0, or 2.718.

• A boolean variable takes on one of only two values: true or false.

An example:

• boolean, true and false are reserved words.

  

• Note: true and false are not in quotes.

Boolean operators:

• Recall: an operator is something that "acts" on the values of variables (like multiply takes two values and does the familiar multiplication of those values).

• Thus, a Boolean operator will "do things" with Boolean values like true and false.

• Two binary operators: && and ||

• One unary operator: !

• An example:

  

• To understand the && operator:

  

  Let's see what c could be based on all possible combinations of a and b values:

  a	b	c = a && b
  false	false	false
  false	true	false
  true	true	true
  true	false	false

• In short, the result of an AND operation is true only when both operands are true.

• Similarly, let's look at the || operator:

  

  Let's see what c could be based on all possible combinations of a and b values:

  a	b	c = a || b
  false	false	false
  false	true	true
  true	true	true
  true	false	true

• In short, the result of an OR operation is false only when both operands are false.

• The ! operator:

  a	!a
  true	false
  false	true

• In short, the NOT operator flips the value.

• Note: the above table describes some of the "rules" for logic.

Let's do a few examples.

First, consider

public class BooleanExample2 {

    public static void main (String[] argv)
    {
	boolean a = true;
	boolean b = false;
	boolean c = true;
	boolean d = false;

	boolean w = a && b;
	System.out.println (w);

	boolean x = a && c;
	System.out.println (x);

	boolean y = a || b;
	System.out.println (y);
	
	boolean z = b || d;
	System.out.println (z);

	boolean u = !a;
	System.out.println (u);
    }

}
     

2.1 Exercise: Type up (in BooleanExamle2.java) and see what the above prints out.

Now let's explain:

• First, look at:

  	boolean a = true;
  	boolean b = false;
  	boolean c = true;
  	boolean d = false;
       

  • Here, there are four boolean variables, each of which is assigned a (boolean) value.
  • Think of the variables as "boxes" in the usual way but as boxes that can hold only boolean values (true or false).

• Next, look at

  	boolean w = a && b;
  	System.out.println (w);
       

  • We know that a has the value true in it, while b has the value false.
  • Thus, the && operator is applied to the values true and false.
  • You can picture this as: true && false.
  • What is the result?
  • Similar to applying the "rules of multiplication" to two numbers, we apply the rules of AND (that is, &&) to true and false.

    

  • The result is: false.
  • Thus, the value false is assigned to the variable w.
  • This is what's printed.

• Similarly, with

  	boolean y = a || b;
  	System.out.println (y);
       

  the result is true

  

• The unary operation ! is a bit different:

  	boolean u = ! a;
  	System.out.println (u);
       

  • Here, the value in a is copied and "flipped" to its opposite.
  • Thus, because the value in a is true the result is false

    

  • This is what gets assigned to the variable u.

2.2 Video: