## Conditionals

BOOLEAN LOGIC

Boolean

A boolean value is either true or false. You can find a boolean in a few forms in Java:

• A boolean expression, such as x > 5

• A boolean variable, such as isAlive

• A method that returns a boolean value such as isGameOver()

George Boole, for whom boolean is named after.

Fun Fact: If you look closely at his face, you can see a boolean expression.

Logical Operators

You can use logical operators to modify or combine boolean expressions. There are three logical operators in Java:

• AND is represented with the && operator

• These statements are true if both expressions are true and false otherwise

• OR is represented with the || operator

• These statements are true if the one or both expressions are true, and false if neither statement is true

• NOT is represented with the ! operator

• This simply reverses a single boolean expression

Comparison Operators

You can use comparison operators to evaluate two numerical values, resulting in a boolean value. There are six comparison operators in Java:

• Greater Than >

• Greater Than or Equal To >=

• Less Than <

• Less Than or Equal To <=

• Is Equal To ==

• Is Not Equal To !=

Tip #1 - Equality vs. Assignment

It is important to note when checking if two primitive types are equal, you must use the equality (==) operator rather than the assignment operator (=). If you use the assignment operator, often you'll find your logic is messed up even when the code compiles. That is A Very Bad Thing.

Code

// Literally killing a dude

Tip #2 - Chaining Comparisons

In mathematics, you can say something like 5 < x < 8. But this doesn't work in Java. Let's think about it. Imagine if x is 7. How would a computer evaluate this expression?

5 < 7 < 8

true < 8

*compile error*

Instead, we need to use two comparisons joined by a logical operator.

Does Not Compile

5 < x < 8

Proper Code

x > 5 && x < 8

IF / ELSE STATEMENTS

If Statements

An if statement branches your code's behavior to do something special when a boolean expression evaluates to true.

Syntax

if(boolean expression)

{

statement(s)

}

Eample #1

if(isHungry && hasMoney)

{

System.out.print("I am going to buy tacos");

}

Example #2

if(!isHungry || myMoney < tacoPrice)

{

System.out.print("I am not buying tacos today");

}

Tip #3 - Points For Style

Consider the examples below. You'll notice that while you can compare a boolean to true or false directly, it can make the code look a little clunkier than is needed. To make matters worse, you risk accidentally writing = instead of ==. As a result, saying == true or == false is considered bad style.

if(isHungry == true)

{

// code

}

GOOD STYLE

if(isHungry)

{

// code

}

if(isHungry == false)

{

// code

}

GOOD STYLE

if(!isHungry)

{

// code

}

Else

An if / else pair will branch your code one way if a boolean expression is true, and another way if it is false. Remember that an else statement must always come last, and you can only have one per if statement.

Syntax

if(boolean expression)

{

statement(s)

}

else

{

statement(s)

}

Example

if(number % 2 == 0)

{

System.out.print("The number is even");

}

else

{

System.out.print("The number is odd");

}

Else If

Finally, you can include any number of else if statements after an if statement. You can either have them on their own, or end the block with an else.

Syntax

if(boolean expression)

{

statement(s)

}

else if(boolean expression)

{

statement(s)

}

else

{

statement(s)

}

Example

if(dir == ‘N’)

{

System.out.println(“Go north!”);

}

else if(dir == ‘E’)

{

System.out.println(“Go east!”);

}

else if(dir == ‘S’)

{

System.out.println(“Go south!”);

}

else

{

System.out.println(“Go west!);

}

SWITCH STATEMENTS

Using Switch

When you have a lot of cases based on a single variable, you may find that a switch statement works better than an if statement. This doesn't offer any new functionality, but is very clean and organized. Consider the example code below, using an integer that stores an AP Score.

Code

switch(apScore)

{

case 5:

System.out.println(“You’re breathtaking!”);

break;

case 4:

System.out.println(“Good job, almost there.”);

break;

case 3:

System.out.println(“You’re passing, but not by much...”);

break;

case 2:

System.out.println(“Ruh roh.”);

break;

default:

System.out.println(“You have brought dishonor upon your family.”);

break;

}

RESOURCES

Alex Lee - Booleans

Alex Lee - Logical Operators
AND, OR, and NOT

Alex Lee - If Else Statements

Alex Lee - Comparison Operators
> >= < <= == !=