Java Class Attributes
Java 类属性
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:
例子
Create a class called "Main" with two attributes: x 和y:
public class Main {
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 Main class, with the name myObj. We use the x attribute on the object to print its value:
例子
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);
}
}
Modify Attributes
You can also modify attribute values:
例子
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:
例子
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:
例子
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);
}
}
这 final keyword is useful when you want a variable to always store the same value, like PI (3.14159...).
这 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:
例子
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
}
}
Multiple Attributes
You can specify as many attributes as you want:
例子
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.
