Java 中的静态类 [英] Static Classes In Java

问题描述

java中有没有类似static class的东西?

Is there anything like static class in java?

这样的类是什么意思.静态类的所有方法都需要static吗?

What is the meaning of such a class. Do all the methods of the static class need to be static too?

是否需要反过来，如果一个类包含所有静态方法，该类是否也应该是静态的?

Is it required the other way round, that if a class contains all the static methods, shall the class be static too?

静态类有什么用?

推荐答案

Java 具有静态嵌套 类，但听起来您正在寻找顶级静态类.Java 无法将顶级类设为静态，但您可以像这样模拟静态类:

Java has static nested classes but it sounds like you're looking for a top-level static class. Java has no way of making a top-level class static but you can simulate a static class like this:

• 声明你的类 final - 防止类的扩展，因为扩展静态类没有意义
• 使构造函数private - 防止客户端代码实例化，因为实例化静态类没有意义
• 使所有类的成员和函数成为static - 由于该类无法实例化，因此无法调用实例方法或访问实例字段
• 请注意，编译器不会阻止您声明实例(非静态)成员.只有在您尝试调用实例成员时才会出现此问题

• Declare your class final - Prevents extension of the class since extending a static class makes no sense
• Make the constructor private - Prevents instantiation by client code as it makes no sense to instantiate a static class
• Make all the members and functions of the class static - Since the class cannot be instantiated no instance methods can be called or instance fields accessed
• Note that the compiler will not prevent you from declaring an instance (non-static) member. The issue will only show up if you attempt to call the instance member

以上建议的简单示例:

public class TestMyStaticClass {
     public static void main(String []args){
        MyStaticClass.setMyStaticMember(5);
        System.out.println("Static value: " + MyStaticClass.getMyStaticMember());
        System.out.println("Value squared: " + MyStaticClass.squareMyStaticMember());
        // MyStaticClass x = new MyStaticClass(); // results in compile time error
     }
}

// A top-level Java class mimicking static class behavior
public final class MyStaticClass {
    private MyStaticClass () { // private constructor
        myStaticMember = 1;
    }
    private static int myStaticMember;
    public static void setMyStaticMember(int val) {
        myStaticMember = val;
    }
    public static int getMyStaticMember() {
        return myStaticMember;
    }
    public static int squareMyStaticMember() {
        return myStaticMember * myStaticMember;
    }
}

静态类有什么好处?静态类的一个很好的用途是定义一次性的、实用程序和/或库类，在这些类中，实例化没有意义.一个很好的例子是 Math 类，它包含一些数学常数，例如 PI 和 E，并且只提供数学计算.在这种情况下需要实例化将是不必要和混乱的.请参阅数学类和源代码.请注意，它是 final 并且其所有成员都是 static.如果 Java 允许将顶级类声明为 static，那么 Math 类确实是静态的.

What good are static classes? A good use of a static class is in defining one-off, utility and/or library classes where instantiation would not make sense. A great example is the Math class that contains some mathematical constants such as PI and E and simply provides mathematical calculations. Requiring instantiation in such a case would be unnecessary and confusing. See the Math class and source code. Notice that it is final and all of its members are static. If Java allowed top-level classes to be declared static then the Math class would indeed be static.

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