Java

TOPIC 14 – IF STATEMENTS I

LESSON NOTE

BOOLEAN VARIABLE

A boolean variable is a variable that can have a value of either true or false. It only takes up one bit of memory.

Like all other variables, a boolean variable has to be declared and initialized before being used.

EXAMPLE – Declare a boolean variable named t and initialize it to true.

boolean t = true;

We will return to boolean variables later.

BOOLEAN EXPRESSIONS

A boolean expression is an expression that evaluates to a boolean value (true or false).

EXAMPLE – Here is a simple boolean expression:

t > 4

The above expression will be true if t is greater than 4 and false otherwise.

CONDITIONS

Boolean expressions used inside If statements are referred to as conditions. And if statements are often called conditional statements.

IF STRUCTURE

Here is the structure of a basic if-statement:

if (condition)

{

//statement block to be executed if condition is true

}

Whenever the condition is true, then the statement block inside the { and } is executed. However, whenever the condition is false, the entire statement block is skipped.

IF STRUCTURE FLOWCHART

Below is a flowchart that shows how an IF statement is executed.

EXAMPLE PROGRAM

Here is a program that asks the user for their grade. It outputs a message only if they are failing. Nothing is done if they are passing.

System.out.println("Enter your grade in this course.");
Scanner scr = new Scanner(System.in);

int mark = scr.nextInt();

if (mark < 50)

{

System.out.println("You are failing.");

}

CONDITIONAL OPERATORS

Here are Java’s conditional operators:

Conditional Operator	Meaning
> >= < <=	Greater than Greater than or equal to Less than Less than or equal to
== !=	Equal Not equal

EXAMPLES – Write the boolean expression that is true if

a) k is less than 17

k < 17

b) coco is greater or equal to zero

coco >= 0

c) t is greater than or equal to the square root of 34

t >= Math.sqrt(34)

d) c² is equal to a² + b²

c * c == a * a + b * b

e) w is an even number

w % 2 == 0

f) x is an odd number

x % 2 == 1

g) y is not equal to 4

y != 4

IF / ELSE STRUCTURE

It is often useful to also have a statement block that gets executed whenever the condition in the if statement is false. This can be done by adding an else to the if structure.

if (condition)

{

//the IF statement block (executed if condition is true)

}

else

{
//the ELSE statement block (executed if condition is false)
}

IF / ELSE STRUCTURE FLOWCHART

Here is the flowchart:

VARIABLE SCOPE

A variable that is declared inside a statement block only exists inside that block.

Variables that are declared before the block can be used inside and after the block.

COMPARING STRINGS

To compare Strings, we need to use the .equals method.

EXAMPLE

The condition below is true is String s is equal to “James”.

if (s.equals("James"))

{

…

}

The condition below is true if Strings word1 and word2 are the same.

if (word1.equals(word2))

{

…

}

Note that the above is the same as below

if (word2.equals(word1))

{

…

}