Why doesn't list.get(0).equals(null) work?

Question

The first index is set to null (empty), but it doesn't print the right output, why?

//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
    }
}

A trick: `System.out.println(Arrays.asList(null,"High","High","High","High","High"));` does what you want without all that extra code. I say its the "same" because you probably didn't know you could print nulls — Pyrolistical, Mar 31 '10 at 22:46

score 7 · Accepted Answer · answered Mar 31 '10 at 22:42

7

I believe what you want to do is change,

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

to

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

answered Mar 31 '10 at 22:42

Anthony Forloney

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No problem, see cletus' answer for a more in depth reason why it didn't work as expected. – Anthony Forloney Mar 31 '10 at 22:46

score 7 · Answer 2 · answered Mar 31 '10 at 22:42

7

You want:

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
    }
  }
}

The expression choice.get(0).equals(null) should throw a NullPointerException because choice.get(0) is null and you try and call a function on it. For this reason anyObject.equals(null) will always return false.

answered Mar 31 '10 at 22:42

cletus

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942

While returning true for `equals(null)` violates the API contract, it is in fact quite possible. – Michael Borgwardt Mar 31 '10 at 22:52

Why doesn't list.get(0).equals(null) work?

2 Answers2