设计模式-单例模式

public class Single01 {    private final static Single01 single=new Single01();    private Single01(){    }    public static Single01 getSingle(){        return single;    }}

public class Single02 {    private static Single02 single = null;    private Single02() {    }    public static synchronized Single02 getSingle() {        if (single == null) {            single = new Single02();            return single;        } else {            return single;        }    }}

public class Single03 {    private static volatile Single03 single = null;    private Single03() {    }    public static Single03 getSingle() {        if (single == null) {            synchronized (Single03.class) {                if (single == null) {                    single = new Single03();                }            }        }        return single;    }}