Variables

Learning Goals

• Explain what a variable is in Java.
• Show how to initialize a variable.
• Discuss the naming convention when naming variables.

Introduction

Consider our program we have been working with thus far:

public class Main {
    public static void main(String[] args) {
        System.out.println("Hello World");
    }
}

As we have already discovered, this program will print out the phrase, "Hello World". We can change what this program prints out by changing the "Hello World" to something else, like this:

public class Main {
    public static void main(String[] args) {
        System.out.println("My name is Katie.");
    }
}

Now if we run this program in IntelliJ as we had before, instead of printing out "Hello World", it will print "My name is Katie".

My name is Katie.

But what if your name isn't Katie? We could change the entire phrase to be "My name is ". But there are a lot of names in this world. And we may not always want to change this phrase every single time we encounter a new person.

What is a Variable?

A variable is a container in which we can store a value in. For example, maybe we want to store the value of the name "Katie". To do that, we can write the following:

public class Main {
    public static void main(String[] args) {
        String name = "Katie";
    }
}

The first part of that statement is the type of variable, which indicates to Java the kind of value we are expecting to store in this variable. The second part is the name we want to be able to refer to the variable as. Once we declare a variable, we can give it a value, and we can refer to it at a later stage.

Now if we want to change the name, we can easily just change the name variable rather than the whole phrase, like this:

public class Main {
    public static void main(String[] args) {
        String name = "Katie";
        System.out.println("My name is:");
        System.out.println(name);
    }
}

Notice we can pass the name variable right into the println()! If we run this program now, it'll print the following:

My name is:
Katie

Try changing the name variable to your name now! Then run the program and see if it prints out your name!

Terminology

Let's go over a couple of terms you may hear: declaring, initializing, and assigning.

Declaring: A variable declaration is when we introduce a variable into a program. We declare a variable like this:

// The variable, name, is declared
String name;

Notice when we declare a variable, we must put the data type in front of the variable name, as this is how we define a variable.

Initializing: A variable initialization is when we give or assign an initial value to a variable. We can initialize a variable like this:

// The variable, name1, is declared AND initialized to "Katie"
String name1 = "Katie";

// The variable, name2, is declared
String name2;

// The variable, name2, is now initialized to "Emily"
name2 = "Emily";

When we declare and initialize a variable at the same time, we must put the data type in front of the variable name to first declare the variable.

If we initialize after, we do not need to put the data type in front of the variable name as it has already been declared.

Assigning: A variable assignment or re-assignment occurs when we provide a value to a variable. We can assign a variable like this:

// The variable, name, is declared
String name;

/* The variable is now being initialized AND assigned to "Katie"
   Note: Initialization is referred to the first time a variable is assigned a value */     
name = "Katie";

// The variable is now being assigned (or re-assigned) to "Emily"
name = "Emily";

It is important to note that in order to use the variable, it must be initialized to a value prior to trying to use it in a statement. Let's look at what happens when we declare a variable, but don't initialize it and try to use it.

IntelliJ will first warn us of an error:

If we ignore the error and still try to run the program, the compiler will then throw the following error:

java: variable name might not have been initialized

We cannot use a variable that has not been initialized because we have not assigned it an initial value yet.

Comprehension Check

Consider the following code:

public class Main {
    public static void main(String[] args) {
        String name;
        name = "Katie";
        System.out.println(name);
    }
}

What do you think the program will output?

Answer:

Katie

Even though the variable, name, is declared on a different line from where it is initialized, it will still assign the name variable to "Katie".

In this case, it would make more sense for us to just initialize the variable to the value, "Katie", but this is just another possibility for demonstration purposes.

Let's modify the code a little bit. Consider the modified program:

public class Main {
    public static void main(String[] args) {
        String name = "Katie";
        System.out.println(name);
        
        // Change the variable's value
        name = "Emily";
        System.out.println(name);
    }
}

What do you think the program will output now?

Answer:

Katie

Emily

We re-assign the variable name to "Emily" halfway through the program. Therefore, when we do that, if we print out the namevariable again, it will now print the re-assigned value.

Variable Naming Convention

By convention, in Java the name of the variable usually starts with a lowercase character.

• If the name of the variable has multiple words in it, the first word will start with a lowercase character and every word after will start with an uppercase character. For example: bikeColor.
  • This is often referred to as "camel case".
• There are some exceptions to this style guide, but we will cover them as they come up.