Unit 3: Boolean Expressions and if statements

Boolean expressions - logical tests

Boolean expressions evaluate to a Boolean value (true, false).

Comparing primitives and reference values using relational operators:

==, !=, <, >, <=, >=

Examples:

a=5;

b=3;

Boolean x=(a!=b); // x will initialize to false as 3 does not equal 5.

return password.equals("123"); //returns true if the value in the password variable equals "123".

Task: Practice writing Boolean expressions (logical tests) here and evaluating Boolean expressions here.

Control flow

One-way selection: if statements

if (logical test)

   do this stuff;

The code under "do this stuff" will only execute if the logical test evaluates to true.

Task: Practice the if statement syntax here.

Two-way selection: if-else statements

if (logical test)

    do command A;

else

    do command B;

If the logical test evaluates to true, command A will be executed, otherwise command B will be executed. Remember to use curly brackets in case you have more than one command under the if or else branch, e.g.:

if (logical test)

   {

    do command A;

    do command B;

   }

else 

   {

    do command C;

    do command D;

   }

Task: Check your understanding of if-else statements here, here, here, here, and here .

Multi-way selection: else if statements

if (logical test 1)

    do command A;

else if (logical test 2)

    do command B;

else if (logical test 3)

    do command C;

...

else 

    do command X;

If logical test 1 evaluates to true, then command A is executed; otherwise logical test 2 is performed - if it evaluates to true, then command B is executed, otherwise, command C is executed.

Task: Consolidate your understanding of one-way selection, two-way selection and multi-way selection by doing the following Runestone academy units: 3.2 (try the Magic 8 Ball challenge), 3.3, 3.4.

Compound boolean expressions

Complex conditionals are either nested if statements or using boolean operators (&&, ||, !) in logical tests.

Nested if statement example:

if (username=="user1")

   {

   if (password="123")

       return true;

   }

else 

   return false;

The same program implemented using boolean operators instead of a nested if statement:

if (username=="user1" && password=="123")

   return true;

else

   return false;

Task: read on Compound Boolean expressions on Runestone Academy and do the tasks on the page.

Short-circuited evaluation of compound boolean expressions:

• The "AND" operator &&: it is enough when the first condition evaluates to false for the whole expression to evaluate as false. The second condition does not get evaluated.
• The "OR" operator ||: it is enough when the first condition evaluates to true for the whole expression to evaluate as true. The second condition is not evaluated.

Equivalent boolean expressions

Main points:

• DeMorgan's Laws can be applied to Boolean expressions
• Truth tables can be used to prove Boolean identities
• Equivalent Boolean expressions will evaluate to the same value in all cases

Task: Read on DeMorgan's Laws and do the tasks on the page on Runestone Academy

Comparing objects using boolean expressions

• two object references (pointers) - not objects - are considered aliases if they both refer to the same object
• object references can be compared to see if they refer to the same object (i.e. if they are aliases) using the == and != operators.
• to find out if a reference points to an actual object, use on of the following Boolean operators: !=null or ==null
• to compare actual objects for equivalency, use the built-in equals() method if the class features such method.

Task: Read more on comparing String objects and do the task on the page on Runestone Academy

Task:

What will be the output of the following program snippet?

public class Main

{

    public static void Main()

    {

        String s1 = new String("Hi There");

        String s2 = new String("Hi There");

        String s3 = s1;

        s1="hello";

        System.out.println(s1 == s2);

        System.out.println(s1.equals(s2));

        System.out.println(s1 == s3);

        System.out.println(s2.equals(s3));

    }

}