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Java修饰符有哪些？IT培训网小编来为大家解答。

修饰符

到目前为止，您已经public对我们几乎所有示例中出现的关键字非常熟悉：

public class Main

public关键字是访问修饰符，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

对于classes，您可以使用public或者default：

Modifier	Description
public	The class is accessible by any other class
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

对于属性、方法和构造函数，您可以使用以下之一：

Modifier	Description
public	The code is accessible for all classes
private	The code is only accessible within the declared class
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
protected	The code is accessible in the same package and subclasses. You will learn more about subclasses and superclasses in the Inheritance chapter

非访问修饰符

对于classes，您可以使用final或abstract：

Modifier	Description
final	The class cannot be inherited by other classes (You will learn more about inheritance in the Inheritance chapter)
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)

对于attributes 和 methods，您可以使用以下之一：

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"

最后

如果您不想覆盖现有属性值，请将属性声明为final：

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);
  }
}

静止的

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

static一个例子来演示和方法之间的差异public：

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
  }
}

抽象的

abstract方法属于一个类abstract，它没有主体。主体由子类提供：

// 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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