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Variables

Introduction to Variables

Let's say we need a program that connects a user with new jobs. We need the user's name, their salary, and they current employment status. All of these pieces of information are stored in our program.

We store information in variables, which are named locations in memory.

Naming a piece of information allows us to use that name later, accessing the information we stored. Variables also give context and meaning to the data we're storing. The value 42 could be someone's age, a weight in pounds, or the number of orders placed. With a name, we know the value of 42 is age, weightInPounds, or numOrdersPlaced.

In Java, we specify the type of information we're storing. Primitive datatypes are types of data built-in to the Java system. The three main primitive types we will cover in these notes are:

  • int - Stores whole numbers

  • double - Stores bigger whole numbers and decimal numbers

  • boolean - Stores true and false

  • char - Stores single characters using single quotes ('')

  • String - Stores multiple characters using double quotes ("")

We must declare a variable to reference it within our program. Declaring a variable requires that we specify the type and name:

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// datatype variableName
int age;
double salaryRequirement;
boolean isEmployed;
The names of the variables above are age, salaryRequirement, and isEmployed.

These variables don't have any associated value. To assign a value to a variable, we use the assignment operator =:

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age = 85;
Now, age has a value of 85. When code is used to represent a fixed value, like 85, it is referred to as a literal.

For example, to assign 2013 to a variable named yearNickGraduatedHighSchool of type int, we write: 

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int yearNickGraduatedHighSchool = 2013;
Using this logic towards the System.out.println() statements we used from Java Basics, instead of putting text within parenthesis, we could print a variable:

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public class Nick {
    public static void main(String[] args) {
    String name = "Nick";
    int yearBorn = 1995;
    System.out.println(name);
    System.out.println(yearBorn);
    }
}
This would have the output: 
Nick
1995

int

The first type of data we will store is the whole number. Whole numbers are very common in programming. You often see them stored as ages, maximum sizes, or the number of times some code has been run, among many other uses.

In Java, whole numbers are stored in the int primitive data type.

ints hold positive numbers, negative numbers, and zero. They do not store fractions or numbers with decimals in them.

The int data type allows values between -2,147,483,648 and 2,147,483,647, inclusive.

To declare a variable of type int, we use the int keyword before the variable name:

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// int variable declaration
int yearJavaWasCreated;
// assignment
yearJavaWasCreated = 1996;
// declaration and assignment
int numOfPrimitiveTypes = 8;

double

Whole numbers don't accomplish what we need for every program. What if we wanted to store the price of stomething? We need a decimal point. What if we need to store the world's population? That number would be larger than the int type can hold.

The double primitive data type can help. double can hold decimals as well as very small/large numbers. The maximum value is 1.797,693,134,862,315,7 E+308, which is approximately 17 followed by 307 zeros. The minimum value is 4.9 E-324, which is 324 decimal places!

To declare a variable of type double, we use the double keyword in the declaration:

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// doubles can have decimal places:
double price = 8.99;
// doubles can have values bigger than an int can hold:
double gdp = 12237700000;

boolean

Often our programs face questions that can only be answered with yes or no. These questions could be: is the oven on? Is the light green? Did I eat breakfast?

These questions are answered with a boolean, a type that references on of two values: true or false.

We declare boolean variables by using the keyword boolean before the variable name:

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boolean javaIsACompiledLanguage = true;
boolean javaIsACupOfCoffee = false;

char

How do we answer questions like: What grade did you get on the test? What letter does your name start with?

The char data type can hold any character, like a letter, space, or punctuation mark. It must be surrounded by single quotes, '.

For example:

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char grade = 'A';
char firstLetter = 'p';
char punctuation = '!';

String

So far, we have learned about primitive data types, which are the simplest types of data with no built-in behavior. Our programs will also use Strings, which are objects, instead of primitives. Objects have built-in behavior.

Strings hold sequnces of characters. We've already seen instances of a String, for example, when we printed out "Hello World!". There are two ways to create a String object, using a String literal or calling the String class to create a new String object.

A String literal is any sequence of characters enclosed in double-quotes (""). Like primitive-type variables, we declare a String variable by specifying the type first:

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String greeting = "Hello World!";
We could also create a new String object by calling the String class when declaring a String like so:

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String salutations = new String("Hello World!");
There are subtle differences in behavior depending on whether you create a String using a String literal or a new String object. For these notes, we will almost always be using String literals.

Static Checking

The Java programming language has static typing. Java programs will not compile if a variable is assigned a value of an incorrect type. This is a bug, specifically a type declaration bug.

Bugs are dangerous! They cause our code to crash, or produce incorrect results. Static typing helps because bugs are caught during programming rather than during the execution of code.

The program will not compile if the declared type of the variable does not match the type of the assigned value: 

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int greeting = "Hellow World!";

The String "Hello World!" cannot be held in a variable of type int.

For the example above, we see an error in the console at compilation: 

error: incompatible types: String cannot be converted to int
    int greeting = "Hello World";
When bugs are not caught at compilation, they interrupt execution of the code by causing runtime errors. The program will crash.

Java's static typing helps programmers avoid runtime errors, and thus have much safer code that is free from bugs.

Naming

Let's imagine that we are storing a user's name for their profile. Which of the following code is better named? 

String data = "Manfred";
or 
String nameOfUser = "Manfred";
While both of these will compile, the second example is easier to understand. Readers of the code will know the purpose of the value: Manfred.

Naming variables according to convention leads to clear, readable, and maintainable code. When someone else, or our future self, reads the code, there is no confusion about the purpose of the variable.

In Java, variable names are case-sensitive. myHeight is a different variable from myheight. Then length of a variable name is unlimited, but we should keep it concise while keeping the meaning clear.

A variable starts with a valid letter, or a $, or a _. No other symbol or numbers can begin a variable name. 1stPlace and *Gazer are not valid names.

Variable names of only one word are spelling in all lowercase letters. Variable names of more than one word have the first letter lowercase while the beginning letter of each subsequent word is capitalized. This style of capitalization is called camelCase. 

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// good style
boolean isHuman;

// bad styles
// no capitalization for new word
boolean ishuman;
// first word should be lowercase
boolean IsHuman;
//underscores don't separate words
boolean is_human;

Quick Review

Below is a program that contains all of the above concepts: 

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public class NickProfile {
    public static void main(String[] args) {   
    String name = "Nick";
    int age = 26;
    double desiredSalary = 110000.00;
    char gender = 'm';
    boolean lookingForJob = true;
    }
}
Tips

  • Addition and subtraction, using + and -

  • Multiplication and division, using * and /

  • The modulo operator for finding remainders, %

  • Compound assignment operators +=, -=, *=, /=, and %=.

  • The order of operations: parentheses -> multiplication -> division -> modulo -> addition -> subtraction

  • Greater than, >, and less than, <

  • Equal to, ==, and not equal to, !=

  • Greater than or equal to, >=, and less than or equal to, <=

  • equals() for comparing Strings and other objects

  • Using + to concatenate Strings

  • The final keyword which makes variables unchangeable

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