Java Class Attributes

Java Class Attributes

In the previous chapter, we used the term "variable" for x in the example (as shown below). It is actually an attribute of the class. Or you could say that class attributes are variables within a class:

Example

Create a class called "Main" with two attributes: x and y:

public class Main {
  int x = 5;
  int y = 3;
}

Another term for class attributes is fields.
 

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Accessing Attributes

You can access attributes by creating an object of the class, and by using the dot syntax (.):

The following example will create an object of the Main class, with the name myObj. We use the x attribute on the object to print its value:

Example

Create an object called "myObj" and print the value of x:

public class Main {
  int x = 5;

  public static void main(String[] args) {
    Main myObj = new Main();
    System.out.println(myObj.x);
  }
}

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Modify Attributes

You can also modify attribute values:

Example

Set the value of x to 40:

public class Main {
  int x;

  public static void main(String[] args) {
    Main myObj = new Main();
    myObj.x = 40;
    System.out.println(myObj.x);
  }
}

Or override existing values:

Example

Change the value of x to 25:

public class Main {
  int x = 10;

  public static void main(String[] args) {
    Main myObj = new Main();
    myObj.x = 25; // x is now 25
    System.out.println(myObj.x);
  }
}

If you don't want the ability to override existing values, declare the attribute as final:

Example

public class Main {
  final int x = 10;

  public static void main(String[] args) {
    Main myObj = new Main();
    myObj.x = 25; // will generate an error: cannot assign a value to a final variable
    System.out.println(myObj.x);
  }
}

The final keyword is useful when you want a variable to always store the same value, like PI (3.14159...).

The final keyword is called a "modifier". You will learn more about these in the Java Modifiers Chapter.

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Multiple Objects

If you create multiple objects of one class, you can change the attribute values in one object, without affecting the attribute values in the other:

Example

Change the value of x to 25 in myObj2, and leave x in myObj1 unchanged:

public class Main {
  int x = 5;

  public static void main(String[] args) {
    Main myObj1 = new Main();  // Object 1
    Main myObj2 = new Main();  // Object 2
    myObj2.x = 25;
    System.out.println(myObj1.x);  // Outputs 5
    System.out.println(myObj2.x);  // Outputs 25
  }
}

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Multiple Attributes

You can specify as many attributes as you want:

Example

public class Main {
  String fname = "John";
  String lname = "Doe";
  int age = 24;

  public static void main(String[] args) {
    Main myObj = new Main();
    System.out.println("Name: " + myObj.fname + " " + myObj.lname);
    System.out.println("Age: " + myObj.age);
  }
}

The next chapter will teach you how to create class methods and how to access them with objects.
 

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