A Simple Introduction To Java - Conditionals

Michael Otu - Mar 8 '22 - - Dev Community

In this session, we will discuss:

Assume the IT Department of the Highway Patrol Team contacted you to write a program that alerts them when a vehicle speeds at 100km/h on the highway. They have given you all the information and models you would need to write the program. Now, how would you know if a vehicle is speeding at or exceeding the speed limit set by the HPT?

A decision has to be made. I don't think it will be great and legal to flag a vehicle at 70km/h, would it? So before the alert is sent, we have to know if the vehicle's speed exceeds the said limit. Decision-making is the whole idea of if statements in programming.

If Statement

An if statement allows us to perform certain actions based on the truth value of some process (condition). These truth values are true and false (Refer to boolean types).

An if statement is of the form:

if (condition) {
    //
}
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Let's create a class, IfBoolean.

In this class, we will pass true into the if statement and print "This is true". On the next line after closing curly braces, instead of true, we'd pass false into an if statement and we'd print "This is false". This activity is for us to know that the if block is only executed when the condition evaluates to true.

public class IfBoolean {

    public static void main(String[] args) {
        if (true) {
            System.out.println("This is true");
        }

        if (false) {
            System.out.println("This is true");
        }
    }
}
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Only "This is true" was printed on the screen. If you were to be using vscode with the java extension, Extension Pack for Java (vscjava.vscode-java-pack), you would see that the second if statement would be called a Dead code. This means it is not useful since the body of that if statement will never be reached.

Comparison Operators

Conditions passed to the if statement is always evaluated to a boolean. We don't have to be passing the value to it all the time instead, we can compare the value of interest to some fixed or constant value. This will evaluate to a boolean.

Comparison operators are binary operators.

Operator Name Examples
< Less than 1 < 2 => true
> Greater than 1 > 2 => false
== Equal to 1 == 2 => false
<= Less than or equal to 1 <= 2 => true
>= Greater than or equal to 1 >= 2 => false
!= Not equal to 1 != 2 => true

==, the double equal to sign is used for comparing two values if they are the same. The opposite of == is !=, read as not equal to. =, the single equal to sign is called the assignment operator. It is used in the declaration and initialization of variables.

Let's create a class, IfCompare

We will compare the user's speed limit to the Highway Patrol Team's value they gave us which was 100km/h.

public class IfCompare {

    public static void main(String[] args) {
        int HIGHWAY_SPEED_LIMIT = 100;

        int vehicle_speed = 75;

        if (vehicle_speed >= HIGHWAY_SPEED_LIMIT) {
            System.out.println("Alert: HPT, we have a speedster");
        }
    }
}
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You should try your hands on the other comparison operators.

If-Else Statement

Let's reconsider the IfCompare class. What do you think we should have done when the vehicle's speed was within the speed limit? Well, nothing. If the vehicle is within the speed limit can not call the HPT on them. Maybe we could buy them pizza at the next drive-through or compliment them. To do compliment them we can pass the else body after the if statement's body to do just that.

Create a class, IfElseParity

We will write a program that takes a user input and assigns it to an int variable, userInput. Check if the user input is even then print "<userInput> is Even" else print, "<userInput> is Odd". where <userInput> is the userInput. (Make use of the % operator)

import java.util.Scanner;

public class IfElseParity {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);

        System.out.print("Enter a number: ");
        int userInput = scanner.nextInt();

        scanner.close();

        if (userInput % 2 == 0) {
            System.out.println(userInput + " is Even");
        } else {
            System.out.println(userInput + " is Odd");
        }
    }
}
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Nested If Statements and If-Else-If Statement

We know about the if-else statement. We can have if statements nested in each other. It will look like this:

if (condition1) {
    // if condition1 holds
    if (condition2) {
        // if condition2 holds do something
    } else {
        // do something if condition2 doesn't hold
    }
} else {
    // do something if condition1 doesn't hold
}
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There are instances where the nested if statement is useful and there at moments they don't shine. They obscure the logic and make it hard to read, for (some) humans. We can use the if..else-if..else approach. This will look more like this:

if (condition1) {
    // do task 1
} else if (condition2) {
    // do task 2
} else {
    // do some other task
}
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Let's solve a modified version of the FizzBuzz problem.

The FizzBuzz program: Given a list (array) of numbers, check if an element is divisible by three and print Fizz, check if the number is divisible by five and print Buzz. If the number is both divisible by three and five (line 15, 30), print FizzBuzz else print the number.

For our modified version, we just take the input and apply the concept rather. We will use the nested and if..else-if..else approach.

Create a class, FizzBuzzNested.

import java.util.Scanner;

public class FizzBuzzNested {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);

        System.out.print("Enter a number: ");
        int userInput = scanner.nextInt();

        scanner.close();

        if (userInput % 3 == 0) {
            if (userInput % 5 == 0) {
                System.out.println("FizzBuzz");
            } else {
                System.out.println("Fizz");
            }
        } else {
            if (userInput % 5 == 0) {
                System.out.println("Buzz");
            } else {
                System.out.println(userInput);
            }
        }
    }
}
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Try this with several inputs like: 2, 3, 5, 9, 10, 15

Now for the other approach, let's create a class, FizzBuzzIfElseIfElse

import java.util.Scanner;

public class FizzBuzzIfElseIfElse {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);

        System.out.print("Enter a number: ");
        int userInput = scanner.nextInt();

        scanner.close();

        // check if it is divisible by 3 and 5
        if (userInput % 3 == 0) {
            if (userInput % 5 == 0) {
                System.out.println("FizzBuzz");
            } else {
                System.out.println("Fizz");
            }
        } else if (userInput % 5 == 0) {
            System.out.println("Buzz");
        } else {
            System.out.println(userInput);
        }

    }
}
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This looks a little different but it is better on our eye. We did run into some issues. Maybe you asked yourself, "how do I check if userInput is divisible by both three and five?" Let's discuss that.

Logical Operators

So far we have done Arithmetic and Comparison (Relational) Operators. The Logical Operators are used to combine comparison expressions to form a compound expression.

These are AND represented as && and OR represented as ||. These are also binary operators.

For AND, both sides must evaluate to true before the whole expression becomes true. If one of the two expressions is false, the whole will be false.

Operand A Operand B Result
true true true
true false false
false true false
false false false

In a code:

if (condition1 && condition2) {
    // do something
}
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For OR, both sides must be false before the whole expression becomes false. If at least, one, of the two expressions, is true, the whole becomes true.

Operand A Operand B Result
true true true
true false true
false true true
false false false

In a code:

if (condition1 || condition2) {
    // do something
}
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It is time to tackle the issue we had in the FizzBuzzIfElseIfElse class.

Create a class, FizzBuzzLogical.

We will make use of the logical operators instead of nesting.

import java.util.Scanner;

public class FizzBuzzLogical {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);

        System.out.print("Enter a number: ");
        int userInput = scanner.nextInt();

        scanner.close();

        if (userInput % 3 == 0 && userInput % 5 == 0) {
            System.out.println("FizzBuzz");
        } else if (userInput % 3 == 0) {
            System.out.println("Fizz");
        } else if (userInput % 5 == 0) {
            System.out.println("Buzz");
        } else {
            System.out.println(userInput);
        }

    }
}
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Conclusion

We discussed if and if..else statements. The body of the if statement executes when the condition evaluates to true else the else block is executed. We can nest an if statement but using the if..else-if..else construct is better. It makes the code readable. We can use the Logical && and || to form compound expressions. This can reduce the number of different blocks of code we have.

Projects

  • Write a program that prompts the user to input a number. The program should then output the number and a message saying whether the number is positive, negative, or zero.
  • Write a program that prompts the user to input three numbers. Sorts and prints the three numbers in ascending order.

  • One way to determine how healthy a person is, is by measuring the body fat of the person. The formulas to determine the body fat for females and males are as follows:

    • Body fat formula for women:

      A1 = (Body weight x 0.732) + 8.987
A2 = Wrist measurement (at fullest point) / 3.140
A3 = Waist measurement (at navel) x 0.157
A4 = Hip measurement (at fullest point) x 0.249
A5 = Forearm measurement (at fullest point) x 0.434
B  = A1 + A2 – A3 – A4 + A5
Body fat = body weight – B
Body fat percentage = body fat x 100 / body weight

    • Body fat formula for men:

      A1 = (Body weight x 1.082) + 94.42
A2 = Waist measurement x 4.15
B = A1 – A2
Body fat = body weight – B
Body fat percentage = body fat x 100 / body weight

    Write a program to calculate the body fat of a person.

Source

  • Sololearn
  • DS Malik

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