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 "MyClass" with two attributes: x and y:
public class MyClass {
int x = 5;
int y = 3;
}
Another term for class attributes is fields.
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 MyClass 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 MyClass {
int x = 5;
public static void main(String[] args) {
MyClass myObj = new MyClass();
System.out.println(myObj.x);
}
}
Modify Attributes
You can also modify attribute values:
Example
Set the value of x to 40:
public class MyClass {
int x;
public static void main(String[] args) {
MyClass myObj = new MyClass();
myObj.x = 40;
System.out.println(myObj.x);
}
}
Or override existing values:
Example
Change the value of x to 25:
public class MyClass {
int x = 10;
public static void main(String[] args) {
MyClass myObj = new MyClass();
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 MyClass {
final int x = 10;
public static void main(String[] args) {
MyClass myObj = new MyClass();
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.
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 MyClass {
int x = 5;
public static void main(String[] args) {
MyClass myObj1 = new MyClass(); // Object 1
MyClass myObj2 = new MyClass(); // Object 2
myObj2.x = 25;
System.out.println(myObj1.x); // Outputs 5
System.out.println(myObj2.x); // Outputs 25
}
}
Multiple Attributes
You can specify as many attributes as you want:
Example
public class Person {
String fname = "John";
String lname = "Doe";
int age = 24;
public static void main(String[] args) {
Person myObj = new Person();
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.
