单例模式

单例模式中需要解决的重要问题是方法的同步问题，同步的粒度有多大等。在本例子中同在获得类的实例的时候使用了同步，代码如下：
public class Singleton{
  private Singleton(){
    generator = new Random();
  }
  public void setSeed(int seed){
    generator.setSeed(seed);
  }
  public int nextInt(){
    return generator.nextInt();
  }
  public static synchronized Singleton getInstance(){
    if (instance == null) {
      instance = new Singleton();
    }
    return instance;
  }
  private Random generator;
  private static Singleton instance;
}
客户端调用的代码：
package singleton;
public class Client{
    public static void main(String[] args){
       Singleton s1 = Singleton.getInstance();
       System.out.println(s1.toString());
       for(int i=0;i<10;i++){
           Singleton s2 = Singleton.getInstance();
           System.out.println("The randomed number is "+s2.toString());
       }
    }
}