To check if List> is empty or not

Question

The answer from a library (DBpediaLookupClient.variableBindings()) results in

List<Map<String, String>>

I am trying to check is if the list if empty or not. What I have tried so far is

if (dbpedialookup.variableBindings() != null && !dbpedialookup.variableBindings().isEmpty()) {
    System.out.println("Results Present);
} else {
    System.out.println("No Results");
}

But, on items where the resultant List is []. Its throwing,

Exception in thread "main" java.lang.NullPointerException

How do we check for null in such a situation?

How do you test resultant that resultant List is [] if it is throwing an Exception? I suggest you check references one after another. — Witold Kaczurba, Oct 28 '16 at 05:21
The check for `variableBindings()` not returning `null` and testing `isEmpty()` only if `variableBindings()` does not return `null` looks correct. The only viable alternative is that `dbpedialookup` is `null`. Please add this check too. — Thomas Kläger, Oct 28 '16 at 05:27

score 2 · Accepted Answer · answered Oct 28 '16 at 05:19

2

Look like the top object is null so

if (dbpedialookup != null && 
    dbpedialookup.variableBindings() != null &&
   !dbpedialookup.variableBindings().isEmpty()) {

answered Oct 28 '16 at 05:19

Scary Wombat

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score 1 · Answer 2 · answered Oct 28 '16 at 05:20

The following is the pattern:

if (myList != null && !myList.isEmpty()) {
    System.out.println("Isn't empty");
} else {
    System.out.println("Is empty");
}

If you want to check variables in the list use the following:

if (myList != null) {
    for (Map<String, String> map : myList) {
        if (map != null) {
            //perform on map
        }
    }
}

score 0 · Answer 3 · answered Oct 28 '16 at 05:22

0

Use Optional to tidy things up:

if (Optional.ofNullable(dbpedialookup).map(DBpediaLookupClient::variableBindings).map(List::isEmpty).orElse(true))
    // it's empty / null

answered Oct 28 '16 at 05:22

Bohemian

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522
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I'm not sure I agree that that's tidier than Scary Wombat's approach; but even if it is, I feel confident that this answer won't actually help the OP understand what the problem was. – ruakh Oct 28 '16 at 05:34

To check if List> is empty or not

3 Answers3