Question

第一个索引设置为null（空），但它不打印正确的输出，为什么？

//set the first index as null and the rest as "High"
String a []= {null,"High","High","High","High","High"};

//add array to arraylist
ArrayList<Object> choice = new ArrayList<Object>(Arrays.asList(a)); 

for(int i=0; i<choice.size(); i++){
   if(i==0){
       if(choice.get(0).equals(null))
           System.out.println("I am empty");  //it doesn't print this output
    }
}

Answer 1

我相信你想做的就是改变，

if(choice.get(0).equals(null))

到

if(choice.get(0) == null))

Answer 2

你想：

for (int i=0; i<choice.size(); i++) {
  if (i==0) {
    if (choice.get(0) == null) {
      System.out.println("I am empty");  //it doesn't print this output
    }
  }
}

表达式choice.get(0).equals(null)应抛出NullPointerException，因为choice.get(0)为null，您尝试在其上调用函数。因此，anyObject.equals(null)将始终返回false。

为什么list.get（0）.equals（null）不起作用？

2 个答案: