Java Programming

Lesson Overview

A computer program works with information called data. Java programs store data in memory using variables.

In this lesson, you will learn how to create variables, store information inside variables, change their values, and display their contents.

1. What Is a Variable?

A variable is a named location in the computer's memory that stores information.

Think of a variable as a labeled box. The label is the variable's name, and the information inside the box is the variable's value.

For example:

int age = 16;

This statement creates a variable named age and stores the number 16 inside it.

2. Declaring a Variable

Before Java can use a variable, the variable must normally be declared.

The general format is:

dataType variableName;

Example:

int score;

This tells Java to create a variable named score that can store an integer.

3. Assigning a Value

The = symbol is called the assignment operator.

It places a value into a variable.

int score;

score = 95;

The variable score now contains the value 95.

4. Initializing a Variable

When a variable is given its first value at the same time that it is created, the variable is being initialized.

int score = 95;

This single statement both declares and initializes the variable.

5. Common Java Data Types

char grade = 'A';

String name = "Alex";

6. Displaying Variables

Variables can be displayed using System.out.println().

int age = 16;

System.out.println(age);

Output:

16

Combining Text and Variables

Use the + operator to combine text with a variable.

int age = 16;

System.out.println("My age is " + age);

Output:

My age is 16

7. Variable Values Can Change

The word variable means the value can vary or change.

int score = 80;

System.out.println(score);

score = 95;

System.out.println(score);

Output:

80
95

The variable is still named score, but its value changed from 80 to 95.

8. Variables and Math

Variables can be used in mathematical expressions.

int num1 = 10;
int num2 = 5;

int total = num1 + num2;

System.out.println(total);

Output:

15

Another Example

double price = 4.50;
int quantity = 3;

double total = price * quantity;

System.out.println("Total Cost: $" + total);

Output:

Total Cost: $13.5

9. Java Variable Naming Rules

A Java variable name:

• May contain letters.
• May contain numbers.
• May contain an underscore _.
• Cannot begin with a number.
• Cannot contain spaces.
• Cannot be a Java reserved word.
• Is case-sensitive.

10. camelCase

Java programmers normally use camelCase when naming variables.

Examples:

studentName
studentAge
testScore
favoriteGame
numberOfStudents
totalPrice

The first word begins with a lowercase letter. Each additional word begins with a capital letter.

11. Complete Java Example

public class VariableExample
{
    public static void main(String[] args)
    {
        String studentName = "Jordan";
        int age = 16;
        double gpa = 3.75;
        char grade = 'A';
        boolean passing = true;

        System.out.println("Student Name: " + studentName);
        System.out.println("Age: " + age);
        System.out.println("GPA: " + gpa);
        System.out.println("Grade: " + grade);
        System.out.println("Passing: " + passing);
    }
}

Programming Assignment

P2_Cusack_Variable.java

Starter Program

public class PX_lastname_Variable
{
    public static void main(String[] args)
    {
        // Create a String variable
        String firstName = "YourName";

        // Create another String variable
        String lastName = "YourLastName";

        // Create an integer variable
        int age = 0;

        // Create an integer for grade level
        int gradeLevel = 0;

        // Create a decimal variable
        double favoriteNumber = 0.0;

        // Create a character variable
        char favoriteLetter = 'A';

        // Create a boolean variable
        boolean likesProgramming = true;


        // Display the variables

        System.out.println("First Name: " + firstName);

        System.out.println("Last Name: " + lastName);

        System.out.println("Age: " + age);

        System.out.println("Grade Level: " + gradeLevel);

        System.out.println("Favorite Number: " + favoriteNumber);

        System.out.println("Favorite Letter: " + favoriteLetter);

        System.out.println("Likes Programming: " + likesProgramming);

    }
}

Example Program

public class P2_Cusack_Variable
{
    public static void main(String[] args)
    {
        String firstName = "Joe";
        String lastName = "Cusack";

        int age = 17;
        int gradeLevel = 11;

        double favoriteNumber = 7.5;

        char favoriteLetter = 'J';

        boolean likesProgramming = true;

        System.out.println("First Name: " + firstName);
        System.out.println("Last Name: " + lastName);
        System.out.println("Age: " + age);
        System.out.println("Grade Level: " + gradeLevel);
        System.out.println("Favorite Number: " + favoriteNumber);
        System.out.println("Favorite Letter: " + favoriteLetter);
        System.out.println("Likes Programming: " + likesProgramming);
    }
}

Example Output

First Name: Joe
Last Name: Cusack
Age: 17
Grade Level: 11
Favorite Number: 7.5
Favorite Letter: J
Likes Programming: true

Part 2 - Change a Variable

After displaying your information, change the value of at least one variable.

Example:

int score = 75;

System.out.println("Original Score: " + score);

score = 95;

System.out.println("New Score: " + score);

Output:

Original Score: 75
New Score: 95

Part 3 - Perform a Calculation

Create at least two numeric variables and use them in a calculation.

Example:

int test1 = 90;
int test2 = 80;

int totalPoints = test1 + test2;

System.out.println("Total Points: " + totalPoints);

Output:

Total Points: 170

double test1 = 85;
double test2 = 92;
double test3 = 88;

double average = (test1 + test2 + test3) / 3;

System.out.println("Average: " + average);

Common Mistakes

Mistake #1 - Forgetting quotation marks

Incorrect:

String name = Joe;

Correct:

String name = "Joe";

Mistake #2 - Using double quotes with char

Incorrect:

char grade = "A";

Correct:

char grade = 'A';

Mistake #3 - Putting a decimal into an int

Incorrect:

int price = 19.99;

Correct:

double price = 19.99;

Mistake #4 - Forgetting the semicolon

Incorrect:

int age = 16

Correct:

int age = 16;

Knowledge Check

1. What is a variable?
2. What Java data type stores whole numbers?
3. What Java data type stores decimal numbers?
4. What Java data type stores true or false?
5. What is the difference between char and String?
6. What does the = operator do?
7. What does it mean to initialize a variable?
8. Can the value stored in a variable change?

Assignment Checklist

Before submitting your program, make sure:

• Your program compiles without errors.
• Your filename follows the required naming convention.
• Your class name matches your filename.
• You created at least one String.
• You created at least one int.
• You created at least one double.
• You created at least one char.
• You created at least one boolean.
• You displayed each variable.
• You changed the value of at least one variable.
• You performed at least one calculation.
• Your code is properly indented.
• Your code contains comments.

What You Learned

In this lesson you learned that variables allow Java programs to store and manipulate information.

You practiced:

• Declaring variables
• Initializing variables
• Assigning values
• Using int
• Using double
• Using boolean
• Using char
• Using String
• Changing variable values
• Performing calculations
• Displaying variables with System.out.println()