# 3.8. Unit 3 - Summary¶

In this chapter you learned about conditionals. Conditionals are used to execute code when a Boolean expression is true or false. A Boolean expression is one that is either true or false like `x > 0`.

## 3.8.1. Concept Summary¶

• Block of statements - One or more statements enclosed in an open curly brace ‘{’ and a closing curly brace ‘}’.

• Boolean expression - A mathematical or logical expression that is either true or false.

• compound Boolean expressions - A Boolean expression with two or more conditions joined by a logical and ‘&&’ or a logical or ‘||’.

• conditional - Used to execute code only if a Boolean expression is true.

• DeMorgan’s Laws - Rules about how to distribute a negation on a complex conditional.

• logical and - Used in compound boolean expressions that are true if both conditions are true.

• logical or - Used in compound boolean expressions that are true if one of the conditions is true.

• negation - turns a true statement false and a false statement true

• short circuit evaluation - The type of evaluation used for logical and (&&) and logical or (||) expressions. If the first condition is false in a compound boolean expression joined with a logical and, then the second condition won’t be evaluated. If the first condition is true in a compound boolean expression joined with a logical or then the second condition won’t be evaluate.

## 3.8.2. Java Keyword Summary¶

• if (Boolean expression) - used to start a conditional statement. This is followed by a statement or a block of statements that will be executed if the Boolean expression is true.

• else - used to execute a statement or block of statements if the Boolean expression on the if part was false.

• else if (Boolean expression) - used to have 3 or more possible outcomes such as if x is equal, x is greater than, or x is less than some value. It will only execute if the condition in the ‘if’ was false and the condition in the else if is true.

## 3.8.3. Vocabulary Practice¶

For more practice, see this Quizlet.

## 3.8.4. Common Mistakes¶

• Using = instead of == in `if`’s. Remember that = is used to assign values and == is used to test. Ifs always use ==.

• Putting a ; at the end of `if (test);`. Remember that the if statement ends after `if (test) statement;` or use curly brackets `if (test) { statements; }`.

• Using two `if`’s one after the other instead of an `if` and `else`.

• Trouble with compound boolean expressions which are two or more Boolean expressions joined by and (&&)`` or **or (||).

• Not understanding that || is an inclusive-or where one or both conditions must be true.

• Trouble with understanding or applying negation (`!`). See the section on DeMorgan’s Laws.

• Not understanding short circuit evaluation which is that if evaluation of the first Boolean expression is enough to determine the truth of a complex conditional the second expression will not be evaluated.

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