Much like math class, we use variables to store values. Now in math class we are only ever storing numbers (integers, decimals, fractions, etc.). This would work fine for storing someone's age (an integer). But to store that persons name we need letters of the alphabet.
Data Types
Integer
Long
Double
Boolean
Character
String
Variables
In Java, variables must be declared before they can be used.
Example:
Store 5 in variable x
Store Bob in variable name
Store true in variable result
Store 5.25 in variable score
Store @ in variable symbol
Variable Names
You can't just give any name you want to a variable! Here are the rules to follow:
Variable names must start with either an upper or lowercase letter
A variable name may contain numbers or an underscore (ex: number_1)
You cannot use one of the reserved keywords (see D2L for a list)
Variable names should be camelCase (firstName, dayOfWeek, etc.)
Printing Variables
You can print variable x using below print statement.
System.out.println("x");IF-ELSE Statements
IF Statements
Until now our programs have always done the same thing each time they were run. But to be truly useful programs need to change based on user input. To accomplish this in programming we use IF Statements. Code inside an IF Statement will run only if certain conditions are met:
Example:
if (x == 5){
System.out.println("This only prints if x is 5");
System.out.println("This also only prints if x is 5");
}
System.out.println("This prints no matter what x is"); Else Statement
An Else Statement acts as a catch-all. If none of the previous conditions were met, whatever code is in the Else statement will run.
x = 7;
if (x == 5){
System.out.println ("if x is not 5, this will not print");
} else{
System.out.println ("This code runs because the previous condition was not met");
}Else If Statements
In Java you can use else if to check additional conditions if the previous ones have failed. You can have as many else if statements in an IF statement as you want. Note: As soon as one condition is met, Java skips to the end of the IF statement without checking any of the others.
Example:
x = 6;
if (x == 5){
System.out.println ("x is not 5, this will not print");
} else if (x==6){
System.out.println ("x does = 6, this line prints!");
} else{
System.out.println ("This code never runs because the previous condition was met");
} Comparison Operators
Comparison operators are used to check the size of values compared to each other. They will return a Boolean value of true or false.
Operator Description
== Equal To
!= Not Equal To
> Greater Than
< Less Than
>= Greater Than or Equal To
<= Less Than or Equal To
x.equals(y) Equal To for Strings x and y
Logical Operators
There are three logical operators: and, or and not
Operator Name Description
&& AND Returns True if a AND b are true
|| OR Returns True if at least one of a OR b is true
^ XOR Returns True if exactly one of a OR b is true
! NOT Returns True if a is false
Practice Examples
Ex:1
x = 10 y = 5 z = -5
if (y > 5) {
System.out.print("A");
} else {
System.out.print("B");
}Output: B
Ex:2
if (x > y) {
System.out.print("A");
} else if(x > z) {
System.out.print("B");
}Output: A
Ex:3
if (x == 5) {
System.out.print("A");
} else if(x < 10) {
System.out.print("B");
} else if(x > 10){
System.out.print("C");
}Output:
Ex:4
if (2*z >= x) {
System.out.print("A");
} else if(y == z) {
System.out.print("B");
} else {
System.out.print("C");
}
Output: C
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