java中单态模式或单例模式(Singleton)有什么意义?

.单态模式或单例模式（Singleton）

单态模式有什么用呢？想一下Adobe Photoshop ，处理两张图，会启动两个photoshop吗？多耗费内存呀！ ( Consider Adobe or oracle, process two images with two adobes?)，所以单态模式在公司编程是非常重要的。有很多场合都要求，对象只能存在一个，多了的话就太耗费资源。（马克-to-win）
马克- to-win：马克 java社区：防盗版实名手机尾号： 73203。



class Photoshop {
/* 通过调试发现写成 static Photoshop photoshop或static Photoshop
photoshop=null;是一样的,开始时都为null,马克-to-win，另外在调试时 可以写上观察Photoshop.photoshop的值，它是独立于任何对象之外的，从程序开始运行的main方法时， 这个值就有了， 只不过为null罢了，
*/
    static Photoshop photoshop;//这个值独立于任何对象存在，实例化任何对象之前，这个成员就有可能有值。

    static Photoshop getInstanceQixy() {
        if (photoshop == null) {
            photoshop = new Photoshop();
            System.out.println("成功创建");
        } else {
            System.out.println("已经创建了该类的实例，不能再创建!");
        }

        return photoshop;
    }

    void drawImage() {
        System.out.println("draw image using photoshop");
    }
}

public class Test {
    public static void main(String[] args) {
        Photoshop photoshopI1 = Photoshop.getInstanceQixy();
        Photoshop photoshopI2 = Photoshop.getInstanceQixy();
        System.out.println(photoshopI1 == photoshopI2);
        System.out.println(photoshopI1 == Photoshop.photoshop);                      photoshopI1.drawImage();
        Photoshop photoshopI3 = new Photoshop();
        System.out.println(photoshopI1 == photoshopI3);
    }
}

　

result is:




成功创建
已经创建了该类的实例，不能再创建!
true
true
draw image using photoshop
false

反思一下： 如果写成：static Photoshop photoshop=new Photoshop();程序执行到main时，实例化就被执行了，而现在会在Photoshop.getInstanceQixy();时才对象实例化。执行的时机不一样。有时可能不用初始化的这么早。

class Photoshop {

    static Photoshop photoshop=new Photoshop();

    static Photoshop getInstanceQixy() {
        return photoshop;
    }

    void drawImage() {
        System.out.println("draw image using photoshop");
    }
}

public class Test {
    public static void main(String[] args) {
        Photoshop photoshopI1 = Photoshop.getInstanceQixy();
        Photoshop photoshopI2 = Photoshop.getInstanceQixy();
        System.out.println(photoshopI1 == photoshopI2);
        photoshopI1.drawImage();
        Photoshop photoshopI3 = new Photoshop();
        System.out.println(photoshopI1 == photoshopI3);
    }
}


Assignment: 　make a photoshop instance using Singleton, call its method of drawImage(), additional requiredment:after the first time to activate the photoshop, from the second time on, when you activate it, it will print "you have already activate one photoshop."