static 单例设计模式

利用 static 让类只能 "new" 一次?

public class Earth {
    private static Earth earthInstance = new Earth();

    private Earth() {
    }

    public static Earth getEarthInstance() {
        return earthInstance;
    }

    public void hello() {
        System.out.println("Hello");
    }
}

run:

public class Test {
    public static void main(String[] args) {
        // Earth earth = new Earth(); --->（x）
        Earth earthInstance = Earth.getEarthInstance();
        earthInstance.hello();	// Hello
    }
}