Booleans and Conditional Statements

Erin R. Hoffman, February 2022

Prerequisites

Before this lesson, students should already be able to:

Create variables in Java, particularly integers and doubles
Use expressions to manipulate and combine variables
Follow the appropriate order of operations in Java expressions, including math operations

Learning Objectives

By the end of this lesson, students will be able to:

Define the term control flow and explain why it is a useful tool in programming
Describe the two values of the boolean data type in Java
Apply operators to variables in order to form boolean expressions
Utilize boolean expressions in conditional statements that determine whether or not a given code block will be executed
Practice translating English-language word problems into conditional statements and code blocks

The Motivation: Control Flow

When do we need control flow?

Workflow icons created by Uniconlabs - Flaticon

A better question might be...when do we not need control flow?

The Boolean Data Type

Recall: What Is a Data Type?

Some data types you have seen already include:

Integers

// Declaring and assigning variable
int month = 12;

// Printing variable value
System.out.println(month);

// Performing math operation on variable
System.out.println(month - 2);

// Performing multiple math operations, with parentheses for order of operations
System.out.println((month - 2) % 2);

Doubles

// Declaring and assigning variable
double length = 1.5;

// Printing variable value
System.out.println(length);

// Performing math operation on variable
System.out.println(length * 7);

// Performing multiple math operations, with parentheses for order of operations
System.out.println(10 / (length + 0.5));

Boolean is another data type

boolean isSunny = true;

isSunny

boolean isRaining = false;

isRaining

Clarification (click to expand)

Confusingly, there is also a wrapper object called Boolean (note the capital "B"). We won't be using it, but you can read the documentation here if you're curious

Boolean Data Type Recap

A data type is something like int or double, which defines the "kind of thing" being stored in a variable
You have just been introduced to a data type called boolean
It can only have two values: true or false

Ok but...why?

Boolean Operators and Expressions

Let's set up some example variables:

int lower = 0;
int upper = 100;

Equality

Is 5 the minimum?

5 == lower

Is the maximum 100?

upper == 100

Are the minimum and maximum the same?

lower == upper

Not

Is 5 not the minimum?

!(5 == lower)

5 != lower

Greater Than, Less Than

The user has input a value of 50. Is that more than the minimum?

50 > lower

Is it less than the maximum?

50 < upper

Now the user has input 100. Is that less than the maximum?

100 < upper

Greater Than Or Equal To, Less Than Or Equal To

What if we want to define the valid range as 0 to 100 inclusive? Is 100 a valid input?

100 >= lower

100 <= upper

And and Or

And

Can we check whether an input is valid in a single line?

(lower <= 100) && (100 <= upper)

(lower <= 500) && (500 <= upper)

(lower <= -5) && (-5 <= lower)

Or

What if, instead of an inclusive range, inputs are valid if they're either below the lower bound or above the upper bound?

(100 <= lower) || (upper <= 100)

(500 <= lower) || (upper <= 500)

(-5 <= lower) || (upper <= -5)

Boolean Operators and Expressions Recap

Boolean expressions are expressions that result in a boolean, although they can (and typically do) compare non-boolean values
Boolean operators are used to make comparisons in boolean expressions. The Java boolean operators are:
- == (is equal to)
- != (is not equal to)
- > (is greater than)
- < (is less than)
- >= (is greater than or equal to)
- <= (is less than or equal to)
- && (and)
- || (or)
- ! (not)

Conditional Statements

Now we can finally start to implement control flow!

Process icons created by Uniconlabs - Flaticon

`if`

User input is invalid if it's smaller than the minimum.

int userInput = -5;

if (userInput < lower) System.out.println("Input too small");

`else if`

User input is also invalid if it's larger than the maximum.

if (userInput < lower) System.out.println("Input too small");
else if (userInput > upper) System.out.println("Input too big");

userInput = 500;

if (userInput < lower) System.out.println("Input too small");
else if (userInput > upper) System.out.println("Input too big");

`else`

Otherwise, user input is valid.

if (userInput < lower) System.out.println("Input too small");
else if (userInput > upper) System.out.println("Input too big");
else System.out.println("Valid input!");

userInput = 75;

if (userInput < lower) System.out.println("Input too small");
else if (userInput > upper) System.out.println("Input too big");
else System.out.println("Valid input!");

Code Blocks: The Dreaded Curly Braces

if (userInput < lower) {
    System.out.println("Input too small");
} else if (userInput > upper) {
    System.out.println("Input too big");
} else {
    System.out.println("Valid input!");
}

Conditional Statements Recap

A conditional statement = a boolean expression + if, else if, or else keyword
Conditional statements determine whether a given piece of code will be executed
Typically this code is contained in a code block surrounded by curly braces

Practice

A tricky but exemplary conditional statement word problem is FizzBuzz, which comes from a children's game.

It goes like this:

Given a number, print it out
Except if the number is divisible by 3, print out "Fizz" instead
And if the number is divisible by 5, print out "Buzz" instead
Finally, if the number is divisible by both 3 and 5, print out "FizzBuzz" instead

// Given a number, print it out
// Except if the number is divisible by 3, print out "Fizz" instead
// And if the number is divisible by 5, print out "Buzz" instead
// Finally, if the number is divisible by both 3 and 5, print out "FizzBuzz" instead

Recap

Reviewing Learning Objectives

Now you should be able to:

Define the term control flow and explain why it is a useful tool in programming
Describe the two values of the boolean data type in Java
Apply operators to variables in order to form boolean expressions
Utilize boolean expressions in conditional statements that determine whether or not a given code block will be executed
Practice translating English-language word problems into conditional statements and code blocks

That's all, folks! Up next, another type of control flow: loops.

hoffm386 / java-if-else