Boolean Expressions and If Statement Lesson

• If Statement
• If Else Statement
• Else If Statement
• Switch Cases
• De Morgan's Law

If Statement

An if statement executes a block of code when a specified boolean expression is evaluated as true.

boolean condition = true;

if (condition == true) {
    System.out.println("This code executes");
}
// Prints: This code executes
 
if (condition == false) {
    System.out.println("This code does not execute");
}
// There is no output for the above statement

This code executes

If Else Statement

The else statement executes a block of code when the condition inside the if statement is false. The else statement is always the last condition.

boolean condition = false;
 
if (condition){
    System.out.println("condition is true");
}
else{
    System.out.println("condition is not true");
}
// Prints: condition1 is not true

condition is not true

Else If Statement

Else-if statements can be chained together to check multiple conditions. Once a condition is true, a code block will be executed and the conditional statement will be exited.

int classGrade = 76;
char grade;
 
if (classGrade >= 90) {
  grade = 'A';
} else if (classGrade >= 80) {
  grade = 'B';
} else if (classGrade >= 70) {
  grade = 'C';
} else if (classGrade >= 60) {
  grade = 'D';
} else {
  grade = 'F';
}
 
System.out.println(grade);

C

Switch Cases

Similar to else if statements but switch cases can have a number of possible execution paths. The value of the expression is compared with the values of each case, iff there is a match, the associated block of code is executed.

int month = 9;
        String monthName;
        switch (month) {
            case 1:  monthName = "January";
                     break;
            case 2:  monthName = "February";
                     break;
            case 3:  monthName = "March";
                     break;
            case 4:  monthName = "April";
                     break;
            case 5:  monthName = "May";
                     break;
            case 6:  monthName = "June";
                     break;
            case 7:  monthName = "July";
                     break;
            case 8:  monthName = "August";
                     break;
            case 9:  monthName = "September";
                     break;
            case 10: monthName = "October";
                     break;
            case 11: monthName = "November";
                     break;
            case 12: monthName = "December";
                     break;
            default: monthName = "Invalid month";
                     break;
        }
        System.out.println(monthName);

September

De Morgan's Law

Laws that define how we can negate an AND statement and how we can negate an OR statement. De Morgan’s Laws simply state: !(a && b) is equivalent to !a || !b and !(a || b) is equivalent to !a && !b. not (a and b) is the same as (not a) or (not b) and not (a or b) is the same as (not a) and (not b)

public class DeMorgansAndTest
{
   public static void main(String[] args)
   {
     int x = 13;
     int y = 151;
     System.out.println(!(x<15 && y > 150));
     System.out.println(!(x<15) || !(y > 150));
   }
}

DeMorgansAndTest.main(null)

false
false

public class DeMorgansAndTest
{
   public static void main(String[] args)
   {
     int x = 28;
     int y = 100;
     System.out.println(!(x<21 || y > 130));
     System.out.println(!(x<21) && !(y > 130));
   }
}

DeMorgansAndTest.main(null)

true
true