单例模式:Ensure a class has only one instance,and provides a global point of access it。也就是说,如果将一个类设计成单例模式,那么它将只允许一个实例。下面可以看一下单例模式的通用代码:
package net.jerryblog.dp.sigleto;
public class Singleton {
private static final Singleton instance = new Singleton();
private Singleton(){
}
public static Singleton getInstance() {
return instance;
}
}
上面的情况下,一旦加载类,就会初始化instance,如果需要在用到instance的时候再实例化呢,可以采用懒汉式单例模式:
package net.jerryblog.dp.sigleto;
public class SingletonLazy {
private static SingletonLazy instance = null;
private SingletonLazy() {
}
public static synchronized SingletonLazy getInstance() {
if(instance == null) {
instance = new SingletonLazy();
}
return instance;
}
}
单例模式还有一种扩展形式:有上限的多例模式。看代码:(这个例子是《设计模式之禅》里面的。)
package net.jerryblog.dp.sigleto;
import java.util.ArrayList;
import java.util.List;
import java.util.Random;
public class Emperor {
private static List<String> nameList = new ArrayList<String>();
private static List<Emperor> empList = new ArrayList<Emperor>();
private static int curEmpNum = 0;
private static int maxEmpNum = 3;
private Emperor() {
}
private Emperor(String name) {
nameList.add(name);
}
static {
for(int i = 0; i < maxEmpNum; i++) {
empList.add(new Emperor("皇帝:" + (i+1)));
}
}
public static Emperor getInstance() {
Random random = new Random();
int index = random.nextInt(maxEmpNum);
return empList.get(index);
}
}