What is a Variable?

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• Variables are used to hold values in your Java program

• Before you can put a value into a variable you must declare, or create it. How?

  • First describe the "type" of information it will hold, for example:

    • String or int

  • Next give it a "meaningful" name or identifier, for example:

    • nameOfSchool or numberOfStudents

    String nameOfSchool;
    Int numberOfStudents;
    

• Rules for naming variables

  • Start with a lowercase letter
  • Use camel case
  • Can include numbers and underscores, but not spaces or special characters
  • Should be meaningful, i.e. self-documenting
  • You declare a variable once, but you can use it many times in the code

Using a Variable

After you have declared a variable, you need to give it a value before it can be useful to your program. The first time you put a value in a variable, is called "variable initialization". See example below:

nameOfSchool = "Queen Mary";
numberOfStudents = 1000;

Note: The type of information you put into the variable must match the type you gave the variable when you declared it.

You can also initialize a variable in the same line of code that you declared it as follows:

String nameOfSchool = "Queen Mary";
Int numberOfStudents = 1000;

If you try to use a variable that has no value, you might get an error like this:

Exception in thread "main" java.lang.NullPointerException