4.1. Classes for Java

In previous programming studies, we have come across classes and objects. Classes and objects in Java are similar to classes and objects in other languages.

4.1.1. A Minimal Class and Object

Classes may contain fields and methods. Fields contain our data for the class and methods define actions a class can take. We say that fields and methods are members of a class.

Example

Let’s create a class called HelloWorld with one field, message, and one method, sayHello(). message will be a string and have a value of "Hello World". sayHello() will not return a specific value and instead print out the value of message.

public class HelloWorld {

   public String message = "Hello World";

   public void sayHello() {
      System.out.println(message);
   }

}

The only field in the HelloWorld class is the string message, while the only method is sayHello, which prints the value of the message field and doesn’t return anything.

Note

There is no main method, which is required to run a Java program. Without it, we have to do some additional work to get our program to run!

To execute sayHello, we’ll need to create an instance of the class HelloWorld. We refer to an object created from a particular class as an instance of that class.

Here’s how this might look with our HelloWorld class:

Example

public class HelloWorldRunner {

   public static void main(String[] args) {
         HelloWorld hello = new HelloWorld();
         hello.sayHello();
   }
}

In order to call the sayHello method of HelloWorld, we must first have an instance of HelloWorld, which we create using the syntax new HelloWorld(). As with built-in classes, classes that we create define their own types. So the object hello is a variable of type HelloWorld.

We introduced this HelloWorld class as a means of illustrating the simplest representation of some basic concepts in Java. The goal of the next few lessons is to build up the machinery to create a wide variety of interesting classes that can be used to create complex programs and elegantly solve difficult problems.

4.1.2. The this Keyword

In HelloWorld above, we could have written sayHello this way, with the same net effect:

public void sayHello() {
    System.out.println(this.message);
}

In this context, inside of the class, we can refer to fields (and methods) that belong to the class using the special object, this. Whenever you use this, it always refers to the object that the given code is currently within. In other words, this will always be an instance of the given class. Since it is not legal to create code outside of a class in Java, this nearly always makes sense to use (there’s one exception, that we’ll encounter soon).

You are allowed to create local variables (variables declared within a method) with the same name as a field of the given class. In this case, in order to refer to the field, we must use this.

Example

Let’s look at how this works with our HelloWorld class:

public class HelloWorld {

   public String message = "Hello World";

   public void sayHello() {

      String message = "Goodbye World";

      // The line below prints "Goodbye World"
      System.out.println(message);

      // The line below prints "Hello World"
      System.out.println(this.message);
   }
}

Warning

When a local variable has the same name as a field, we say that the local variable shadows the field. Errors caused by shadowing can be tricky to spot, so it’s best to avoid doing this in your code.

Note

If you want to learn more about this subject, check out the Oracle Documentation on using the this keyword.

4.1.3. Check Your Understanding

Question

The following code block contains several bugs. Mark all of the lines that contain a bug in the code.

public class Greeting {

   public String name = "Jess"

   public void sayHello() {
      System.out.println("Hello " + here.name + "!");

}

1. line 7

2. line 3

3. line 6

4. line 1