JAVA 修饰符

修饰符

到现在为止，你已经非常熟悉了public几乎出现在我们所有示例中的关键字：

public class Main

这个public关键字是一个访问修饰符，这意味着它用于设置类、属性、方法和构造函数的访问级别。

我们将修饰符分为两组：

访问修饰符- 控制访问级别
非访问修饰符- 不控制访问级别，但提供其他功能

访问修饰符

为了类，您可以使用public或者默认:

Modifier	Description	尝试一下
`public`	The class is accessible by any other class	尝试一下 »
default	The class is only accessible by classes in the same package. This is used when you don't specify a modifier. You will learn more about packages in the Packages chapter	尝试一下 »

为了属性、方法和构造函数，您可以使用以下其中一项：

Modifier	Description	尝试一下
`public`	The code is accessible for all classes	尝试一下 »
`private`	The code is only accessible within the declared class	尝试一下 »
default	The code is only accessible in the same package. This is used when you don't specify a modifier. You will learn more about packages in the Packages chapter	尝试一下 »
`protected`	The code is accessible in the same package and subclasses. You will learn more about subclasses and superclasses in the Inheritance chapter	尝试一下 »

非访问修饰符

为了类，您可以使用final或者abstract:

Modifier	Description	尝试一下
`final`	The class cannot be inherited by other classes (You will learn more about inheritance in the Inheritance chapter)	尝试一下 »
`abstract`	The class cannot be used to create objects (To access an abstract class, it must be inherited from another class. You will learn more about inheritance and abstraction in the Inheritance and Abstraction chapters)	尝试一下 »

为了属性和方法，您可以使用以下其中一项：

Modifier	Description
`final`	Attributes and methods cannot be overridden/modified
`static`	Attributes and methods belongs to the class, rather than an object
`abstract`	Can only be used in an abstract class, and can only be used on methods. The method does not have a body, for example abstract void run();. The body is provided by the subclass (inherited from). You will learn more about inheritance and abstraction in the Inheritance and Abstraction chapters
`transient`	Attributes and methods are skipped when serializing the object containing them
`synchronized`	Methods can only be accessed by one thread at a time
`volatile`	The value of an attribute is not cached thread-locally, and is always read from the "main memory"

最终的

如果您不希望能够覆盖现有属性值，请将属性声明为final:

示例

public class Main {
  final int x = 10;
  final double PI = 3.14;

  public static void main(String[] args) {
    Main myObj = new Main();
    myObj.x = 50; // will generate an error: cannot assign a value to a final variable
    myObj.PI = 25; // will generate an error: cannot assign a value to a final variable
    System.out.println(myObj.x);
  }
}

亲自试一试 »

静止的

static方法意味着可以在不创建类的对象的情况下访问它，这与public:

示例

一个例子来展示之间的差异static和public方法：

public class Main {
  // Static method
  static void myStaticMethod() {
    System.out.println("Static methods can be called without creating objects");
  }

  // Public method
  public void myPublicMethod() {
    System.out.println("Public methods must be called by creating objects");
  }

  // Main method
  public static void main(String[ ] args) {
    myStaticMethod(); // Call the static method
    // myPublicMethod(); This would output an error

    Main myObj = new Main(); // Create an object of Main
    myObj.myPublicMethod(); // Call the public method
  }
}

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抽象的

一个abstract方法属于abstract类，并且它没有主体。主体由子类提供：

示例

// Code from filename: Main.java
// abstract class
abstract class Main {
  public String fname = "John";
  public int age = 24;
  public abstract void study(); // abstract method
}

// Subclass (inherit from Main)
class Student extends Main {
  public int graduationYear = 2018;
  public void study() { // the body of the abstract method is provided here
    System.out.println("Studying all day long");
  }
}
// End code from filename: Main.java

// Code from filename: Second.java
class Second {
  public static void main(String[] args) {
    // create an object of the Student class (which inherits attributes and methods from Main)
    Student myObj = new Student();

    System.out.println("Name: " + myObj.fname);
    System.out.println("Age: " + myObj.age);
    System.out.println("Graduation Year: " + myObj.graduationYear);
    myObj.study(); // call abstract method
  }
}

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