4: Booleans

The data type for a variable that can only store one fundamental piece of information is called a boolean. A boolean only has two different possible states: true or false, yes or no, 1 or 0, etc.

In Java the type name is boolean, and the possible values are expressed as true or false.

The primary use of booleans is for branching program logic. This lets us execute different code depending on conditions that can change as the code runs.

This dramatically increases the number of useful things we can do in our programs.

If/ Else

The statement used for program branching takes the form:

if (condition1) {
    // execute some code...
} else if (condition2) {
    // execute some different code...
} else {
    // more different code...
}

Each different condition is a boolean value (or evaluates to a boolean value). A flowchart of how an if statement would look something like this:

flowchart TD
    start[Program Entry]
    exit[Program Exit]
    condition1[Meets condition 1?]
    condition2[Meets condition 2?]
    execute1[Execute Some Code]
    execute2[Execute Some Different Code]
    execute3[Execute More Different Code]
    start-->condition1
    condition1-->|Yes|execute1
    condition1-->|No|condition2
    condition2-->|Yes|execute2
    condition2-->|No|execute3
    execute1-->exit
    execute2-->exit
    execute3-->exit

An if statement can include as many else if conditions as you like, including none. else is optional but can only have one instance.

Exercise 1

Write a simple if/else statement to print different messages depending on whether isFriend is true.

Show

public class Main { public static void main(String[] args) { boolean isFriend = true; // pretend that this value isn't known } }

Solution

public class Main { public static void main(String[] args) { boolean isFriend = true; // pretend that this value isn't known if (isFriend) { System.out.println("Hello! Nice to see you again!"); } else { System.out.println("Nice to meet you!"); } } }

Comparisons

So far we have no way of converting between "regular" values and booleans, meaning that our branching logic will be limited. Comparisons check for equality¹ between variables of the same type, returning true if they are equal and false if not.²

A comparison is written using the syntax a == b. This expression evaluates to (can be substituted for) a boolean.

Warning

The syntax == (comparison) looks very similar to = (assignment). There can be some cryptic error messages that arise from mixing these up.

Exercise 2

Write a piece of code that prints 6 if a variable is equal to 5, 4 if the variable is equal to 9, and 0 otherwise.

Show

public class Main { public static void main(String[] args) { int a = 5; // pretend this can change } }

Solution

public class Main { public static void main(String[] args) { int a = 5; // pretend this can change if (a == 5) { System.out.println(6); } else if (a == 9) { System.out.println(4); } else { System.out.println(0); } } }

Exercise 2.5

Rewrite your solution to Exercise 2, using only one print statement.

Hint

Reread section 1 and specifically variable declaration

Solution

public class Main { public static void main(String[] args) { int a = 5; // pretend this can change int out; if (a == 5) { out = 6; } else if (a == 9) { out = 4; } else { out = 0; } System.out.println(out); } }

While working on that exercise, you may have noticed that the compiler was not very happy with you if you failed to assign a value to out in all program branches. If you declare a variable, it must be defined for every program flow before you use it. Additionally, if you declare a variable inside a program branch, it can't be used outside of it.

Boolean Logic

	0	1
0	0	1
1	1	1

Other

"Equality" can be somewhat tricky to figure out automatically for more complex data types. In these cases, the type itself provides a specification for the equality check. ↩
Checks for equality of floating-point variables occasionally don't behave entirely correctly. This is because floating-point numbers can be susceptible to a very small amount of rounding error (this is what's happening if you ever see a computer, for example, returning 41.9999999999998 instead of 42). ↩