【Java每日一題】201612015

package Dec2016;  
  
import java.util.HashSet;  
  
public class Ques1205 {  
  
    public static void main(String[] args){  
  
        StringBuffer sb1 = new StringBuffer("123");  
        StringBuffer sb2 = new StringBuffer("123");  
        StringBuffer sb3 = new StringBuffer("123");  
        StringBuffer sb4 = new StringBuffer("123");  
  
        HashSet<StringBuffer> set = new HashSet<StringBuffer>();  
        set.add(sb1);  
        set.add(sb2);  
        set.add(sb3);  
        set.add(sb4);  
  
        System.out.println(set.size());  
    }  
}

今日問題： 
請問主程式輸出結果是什麼？（點擊以下“【Java每日一題】20161205”查看20161202問題解析） 

題目原發佈於公眾號、簡書：【Java每日一題】20161205，【Java每日一題】20161205