Using Boolean Expressions

As you saw in the algorithm section, an algorithm is comprised of sequential steps, conditionals, and iterative processes. Sequential steps are those done in order. Conditionals allow the program to behave in different ways based on decisions. Iterative processes repeat series of steps.

In this tutorial we will focus on Boolean expressions, which are critical to conditionals and iterative processes.

Boolean Expressions

Recall that a boolean variable is assigned either true or false. A Boolean expression is any expression that can be evaluated to a boolean value, true or false. They are comprised of relationship operators and logic operators.

Relationship Operators

The relationship operators compare values and evaluate to either true or false. They are as follows:

Operator	Name	Description	Example
`>`	greater than	evaluates to `true` if the value on the left is greater than the value on the right and `false` otherwise.	`3 > 5` evaluates to `false` `5 > 3` evaluates to `true` `5 > 5` evaluates to `false` since the values are equal
`<`	less than	evaluates to `true` if the value on the left is less than the value on the right and `false` otherwise.	`3 < 5` evaluates to `true` `5 < 3` evaluates to `false` `5 < 5` evaluates to `false` since the values are equal
`>=`	greater than or equal to	evaluates to `true` if the value on the left is greater than or equal to the value on the right and `false` otherwise.	`3 >= 5` evaluates to `false` `5 >= 3` evaluates to `true` `5 >= 5` evaluates to `true` since the values are equal
`<=`	less than or equal to	evaluates to `true` if the value on the left is less than or equal to the value on the right and `false` otherwise.	`3 <= 5` evaluates to `true` `5 <= 3` evaluates to `false` `5 <= 5` evaluates to `true` since the values are equal
`==`	equal to	evaluates to `true` if the values are equal to each other and `false` otherwise.	`3 == 5` evaluates to `false` `5 == 5` evaluates to `true` since the values are equal
`!=`	not equal to	evaluates to `true` if the values are not equal to each other and `false` otherwise.	`3 != 5` evaluates to `true` `5 != 5` evaluates to `false`

Try these out in the Java Playground. Take a few minutes to change the values and see the new results.

The values being compared could be variables, expressions, or values. In these examples, we have integrated variables and expressions. Take a few minutes to change the values and see the new results.

Logic Operators

Logic operators are used to combine boolean expressions. The most common ones are listed in the table below.

Operator	Name	Description	Example
`!`	not	evaluates to `true` if the boolean expression is `false` and `false` if the boolean expression is `true`.	`!(3 > 5)` evaluates to `true` since `3 > 5` evaluates to `false`. `!(5 > 3)` evaluates to `false` since `5 > 3` evaluates to `true`.
`&&`	and	evaluates to `true` if both boolean expressions evalute to `true` and `false` if one or both boolean expressions evaluate to `false`.	`true && true` evaluates to `true`. `true && false` evaluates to `false`. `false && true` evaluates to `false`. `false && false` evaluates to `false`.
`\|\|`	or	evaluates to `true` if at least one of the boolean expressions evalute to `true` and `false` if both boolean expressions evaluate to `false`.	`true \|\| true` evaluates to `true`. `true \|\| false` evaluates to `true`. `false \|\| true` evaluates to `true`. `false && false` evaluates to `false`.

Try these out in the Java Playground. Take a few minutes to change the values and see the new results.

!(3 < 5) is equivalent to 3 >= 5. When the not (!) operator is used on an expression with a relationship operator, the equivalent expression is uses the opposite relational operator. The opposite of less than is greater than or equal to. The opposite of greater than is less than or equal to.

DeMorgan's Law

DeMorgan's Law applies in situations when a not (!) operator is being used in combination with either the and (&&) or the or (||) operators.

Rule	Description	Example
`!(value1 && value2)`	evaluates to `!value1 \|\| !value2` where `value1` and `value2` are boolean expressions.	`!((5 > 7) && (3 !=0))` is equivalent to `(5 <= 7) \|\| (3 == 0)`
`!(value1 \|\| value2)`	evaluates to `!value1 && !value2` where `value1` and `value2` are boolean expressions.	`!((val + 5 <= 0) \|\| (5 == 5))` is equivalent to `(val + 5 > 0) && (5 != 5)`

Short-Circuit Evaluation

If the value of a boolean expression that involves a logic operator can be determined based on only the first boolean expression, the second boolean expression is not evaluated.

And (&&) Operator Short-Circuit

If the first expression evaluates to false, the expression with the && operator evaluates to false.

This example is a popular use of short-circuit for &&. If the value of numGrades is 0, we would get a divide by 0 error when attempting totalGrades / numGrades. Checking that numGrades is not 0 before performing division can prevent this error. Try it out! Run the code as is, and then change the value of numGrades to 0.

Comparing Objects

To compare objects, you will want to use a method. The equals method, which is avaiable for all classes, can be used. Not all classes have a specific implementation of the equals method, so you will want to look at the API to see if there is an implementation for the class.

String Class Equals Method

boolean equals(Object anObject)

This method returns

true if the value of this String is the same as the String value of anObject. NOTE: All classes extend the Object class, meaning a String is an Object.
false if the value of this String is not the same as theString value of anObject.

LocalDate Class Equals Method

boolean equals(Object obj)