Java 简明教程

Java - Class Attributes

Java Class Attributes

Java class attributes 是绑定在类中的 variables，也就是说，用于定义类的变量是类属性。

Java class attributes are the variables that are bound in a class i.e., the variables which are used to define a class are class attributes.

类属性在程序执行期间定义类的状态。类属性在类方法中可以默认访问。

A class attribute defines the state of the class during program execution. A class attribute is accessible within class methods by default.

例如，有一个类“Student”，它有一些数据成员（变量），比如 roll_no、age 和 name。这些数据成员被视为类属性。

For example, there is a class "Student" with some data members (variables) like roll_no, age, and name. These data members are considered class attributes.

Creating (Declaring) Java Class Attributes

要创建（声明）一个类属性，请使用 access modifier 后跟 data type 和属性名称。这与声明一个变量类似。

To create (declare) a class attribute, use the access modifier followed by the data type and attribute name. It’s similar to declaring a variable.

Syntax

使用下面的语法来声明类属性：

Use the below syntax to declare a class attribute:

access_modifier type attribute_name;

Example: Declaring Java Class Attributes

public class Dog {
   String breed;
   int age;
   String color;

   void barking() {
   }

   void hungry() {
   }

   void sleeping() {
   }
}

在上面的类中，我们有 breed、age 和 color 等字段，它们也称为类属性。

In above class, we’ve fields like breed, age, and color which are also known as class attributes.

Accessing Java Class Attributes

要访问类属性，您需要首先创建一个 object，然后对对象名称使用点 (.) 运算符。类属性也可以直接在类方法中调用。

To access the class attribute, you need to create an object first and then use the dot (.) operator with the object name. Class attributes can be also called within the class methods directly.

Syntax

使用以下语法访问类属性：

Use the below syntax to access a class attribute:

object_name.attribute_name;

Example: Accessing Java Class Attributes

试想一下这个例子，演示如何访问类属性。

Consider this example, demonstrating how to access the class attributes.

class Dog {
  // Declaring and initializing the attributes
  String breed = "German Shepherd";
  int age = 2;
  String color = "Black";
}

public class Main {
  public static void main(String[] args) {
    // Creating an object of the class Dog
    Dog obj = new Dog();

    // Accessing class attributes & printing the values
    System.out.println(obj.breed);
    System.out.println(obj.age);
    System.out.println(obj.color);
  }
}

German Shepherd
2
Black

Modifying Java Class Attributes

若要修改类属性，访问属性并使用赋值 (=) 运算符分配新值。

To modify a class attribute, access the attribute and assign a new value using the assignment (=) operator.

Syntax

使用以下语法修改类属性：

Use the below syntax to modify a class attribute:

object_name.attribute_name  = new_value;

Example: Modifying Java Class Attributes

试想一下这个例子，演示如何修改类属性。

Consider this example, demonstrating how to modify the class attributes.

class Dog {
  // Declaring and initializing the attributes
  String breed = "German Shepherd";
  int age = 2;
  String color = "Black";
}

public class Main {
  public static void main(String[] args) {
    // Creating an object of the class Dog
    Dog obj = new Dog();

    // Accessing class attributes & printing the values
    System.out.println("Before modifying:");
    System.out.println(obj.breed);
    System.out.println(obj.age);
    System.out.println(obj.color);

    // Modifying class attributes
    obj.breed = "Golden Retriever";
    obj.age = 3;
    obj.color = "Golden";

    // Printing
    System.out.println("\nAfter modifying:");
    System.out.println(obj.breed);
    System.out.println(obj.age);
    System.out.println(obj.color);
  }
}

Before modifying:
German Shepherd
2
Black

After modifying:
Golden Retriever
3
Golden

Making Java Class Attributes Read Only

还可以在声明属性时，使用访问修饰符后使用 final 关键字将类属性设为只读。

You can also make the class attributes read-only by using the final keyword after the access modifier while declaring an attribute.

Syntax

使用以下语法将类属性设为只读：

Use the below syntax to make class attribute read-only:

access_modifier final data_type attribute_name;

Example: Making Java Class Attributes Read Only

在下面的例子中，name 属性使用 final 关键字设为只读。现在，此属性不可修改，并且如果我们尝试修改此属性，JVM 将发出抱怨。

In the below example, the name attribute is set to read-only using the final keyword. Now this attribute can not be modified and JVM will complain if we try to modify this attribute.

package com.tutorialspoint;

class Dog {
   final String name = "Tommy";
}

public class Tester {
   public static void main(String[] args) {
      Dog dog = new Dog();
      dog.name = "Tommy";  // Error while modifying name
      System.out.println(dog.name);
   }
}

编译并运行 Tester。这将产生以下结果：

Compile and run Tester. This will produce the following result −

Exception in thread "main" java.lang.Error: Unresolved compilation problem:
	The final field Dog.name cannot be assigned

	at com.tutorialspoint.Tester.main(Tester.java:10)