Java Lesson 2: Variables and Data Types

Hello everybody,

It’s Michael here, and today’s post will be on variables and data types in Java. Variables are pieces of memory in Java that store information necessary for your program to run. Take a look  at this statement:

• int gamesWon = 2

This assignment statement (which is what you call the line of code where you create a variable and give it a value) displays the variable’s name (known in Java jargon as an identifier), value, and data type (int). Data types, such as int, indicate what kind of value the variable is. In the above example, the data type int indicates that the value is an integer.

• When naming your variables, remember that you can only use letters, the numbers 0-9, and the underscore character. Also, the first character of your variable name can’t be a number.
  • The variable names 4twenty, go-Browns, and Linkin....Park won’t work, and the complier will complain if you try to use any of them.
• One thing I didn’t mention in the previous Java post is that Java is case-sensitive, so that variables like southPark, Southpark, and SouthPark would be treated as 3 different variables rather than the same variable. Having 3 same-named but differently-capitalized variables is perfectly acceptable in Java, though it will get confusing for you the programmer.
• A peculiar Java syntax convention indicates that, with multi-word variable names like gamesWon, the first word (in this case games) should start with a lowercase letter and any subsequent words should start with an uppercase letter. It’s just good practice though; after all, the Java compiler won’t display error messages if you use GamesWon or gamesWon instead of gamesWon.

There are two types of variables in Java-primitive types and class types. Class type variables belong to a certain class (which can either be a built-in class such as String or Array or a class you create in your program). On the other hand, primitive type variables don’t belong to a certain class but are rather the eight basic types of variables in Java, which consist of:

• byte
• short
• int
• long
• float
• double
• char
• boolean

The first four variable types mentioned (byte, short, int, long) are all of type int. The only difference between them is the integer ranges they represent. Byte has the smallest range, spanning from only -128 to 127, while short goes from approximately -32,000 to 32,000, int from roughly negative 2 billion to positive 2 billion, and long from negative 9 quintillion to positive 9 quintillion.

Float and double are floating-point numbers, which is a more technical way of saying that float and double are decimals. Numbers like 3.5, -9.99, and 1.042 would qualify as either float or double. Like the four integer variable types mentioned in the previous paragraph, the only difference between float and double are the ranges they represent, with double having the wider range of decimals.

The last two variable types, char and boolean, don’t store numbers. Rather, char stores single characters, which can consist of any of the letters of the alphabet, the numbers 0-9, and symbols like the pound sign (or hashtag for the millennial crowd) or percent sign. Char can only be 1 character, so the variables g^ksf and dipppk can’t be of type char since they have multiple characters (String would be a more appropriate type). And yes, even though I said char doesn’t store numbers, a variable like char = 0 wouldn’t be regarded as a number. Boolean stores the values true and false and is used when dealing with logic statements (more on those in a future post). For instance the statement (41 > 55) would qualify as a logic statement by Java and thus would be boolean. If you’re wondering, this statement would return false (which can be figured out easily because 41 is less than 55).

Now, if you’d like to see how variables work, here’s a little demonstration:

Inside the main method, I declared an int variable-currentYear-and gave it a value-2019 (well, because that’s the current year). Remember to put the assignment statement inside the main method, because the compiler will complain if you try to do otherwise. I then asked the program to print out the currentYear, which it did.

One thing to know with System.out.println and variable names is that you can simply put the name of your variable in the parentheses and the value will print out.

For those that would like to see a video demonstration of how variables work, click this link-Variables Demo Java.

Thanks for reading,

Michael