静态的

static 关键字用于类，方法或字段，以使它们独立于类的任何实例工作。

• 静态字段对于类的所有实例都是通用的。他们不需要实例来访问它们。
• 静态方法可以在没有它们所在类的实例的情况下运行。但是，它们只能访问该类的静态字段。
• 静态类可以在其他类中声明。它们不需要实例化它们所在类的实例。

public class TestStatic
{
    static int staticVariable;

    static {
        // This block of code is run when the class first loads
        staticVariable = 11;
    }

    int nonStaticVariable = 5;

    static void doSomething() {
        // We can access static variables from static methods
        staticVariable = 10;
    }

    void add() {
        // We can access both static and non-static variables from non-static methods
        nonStaticVariable += staticVariable;
    }

    static class StaticInnerClass {
        int number;
        public StaticInnerClass(int _number) {
            number = _number;
        }

        void doSomething() {
            // We can access number and staticVariable, but not nonStaticVariable
            number += staticVariable;
        }

        int getNumber() {
            return number;
        }
    }
}

// Static fields and methods
TestStatic object1 = new TestStatic();

System.out.println(object1.staticVariable); // 11
System.out.println(TestStatic.staticVariable); // 11

TestStatic.doSomething();

TestStatic object2 = new TestStatic();

System.out.println(object1.staticVariable); // 10
System.out.println(object2.staticVariable); // 10
System.out.println(TestStatic.staticVariable); // 10

object1.add();

System.out.println(object1.nonStaticVariable); // 15
System.out.println(object2.nonStaticVariable); // 10

// Static inner classes
StaticInnerClass object3 = new TestStatic.StaticInnerClass(100);
StaticInnerClass object4 = new TestStatic.StaticInnerClass(200);

System.out.println(object3.getNumber()); // 100
System.out.println(object4.getNumber()); // 200

object3.doSomething();

System.out.println(object3.getNumber()); // 110
System.out.println(object4.getNumber()); // 200