java中單態(tài)模式或單例模式(Singleton)有什么意義?

.單態(tài)模式或單例模式（Singleton）

單態(tài)模式有什么用呢？想一下Adobe Photoshop ，處理兩張圖，會(huì)啟動(dòng)兩個(gè)photoshop嗎？多耗費(fèi)內(nèi)存呀！ ( Consider Adobe or oracle, process two images with two adobes?)，所以單態(tài)模式在公司編程是非常重要的。有很多場(chǎng)合都要求，對(duì)象只能存在一個(gè)，多了的話就太耗費(fèi)資源。（馬克-to-win）
馬克- to-win：馬克 java社區(qū)：防盜版實(shí)名手機(jī)尾號(hào)： 73203。



class Photoshop {
/* 通過(guò)調(diào)試發(fā)現(xiàn)寫成 static Photoshop photoshop或static Photoshop
photoshop=null;是一樣的,開(kāi)始時(shí)都為null,馬克-to-win，另外在調(diào)試時(shí) 可以寫上觀察Photoshop.photoshop的值，它是獨(dú)立于任何對(duì)象之外的，從程序開(kāi)始運(yùn)行的main方法時(shí)， 這個(gè)值就有了， 只不過(guò)為null罷了，
*/
    static Photoshop photoshop;//這個(gè)值獨(dú)立于任何對(duì)象存在，實(shí)例化任何對(duì)象之前，這個(gè)成員就有可能有值。

    static Photoshop getInstanceQixy() {
        if (photoshop == null) {
            photoshop = new Photoshop();
            System.out.println("成功創(chuàng)建");
        } else {
            System.out.println("已經(jīng)創(chuàng)建了該類的實(shí)例，不能再創(chuàng)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:




成功創(chuàng)建
已經(jīng)創(chuàng)建了該類的實(shí)例，不能再創(chuàng)建!
true
true
draw image using photoshop
false

反思一下： 如果寫成：static Photoshop photoshop=new Photoshop();程序執(zhí)行到main時(shí)，實(shí)例化就被執(zhí)行了，而現(xiàn)在會(huì)在Photoshop.getInstanceQixy();時(shí)才對(duì)象實(shí)例化。執(zhí)行的時(shí)機(jī)不一樣。有時(shí)可能不用初始化的這么早。

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."