单例设计、多例设计（2021-11-02）

  单例/多例设计，是控制实例化对象个数的设计操作。
  单例设计：将构造方法设为private，定义1个private static final的对象和对应static的get方法。多例设计只是定义多个private static final的对象。
  单例设计：

package package1;

public class Singleton {
    private static final Singleton INSTANCE = new Singleton();

    private Singleton(){};
    public static Singleton getInstance(){
        return INSTANCE;
    }
    public void print(){
        System.out.println("对象内的打印方法");
    }
}

  多例设计：

package package1;

public class Choice {
    private String color;
    private static final Choice RED = new Choice("红色");
    private static final Choice GREEN = new Choice("绿色");
    private static final Choice BLUE = new Choice("蓝色");


    private Choice(String color){
        this.color=color;
    }
    public static Choice getInstance(String str){
        switch(str){
            case "red": return RED;
            case "green": return GREEN;
            case "blue": return BLUE;
            default: return null;
        }
    }
    public String toString(){
        return this.color;
    }

}