Compare words in two arrayList<String> java

Question

I've got two ArrayList of String and i want to count words that are in both of them.

Each ArrayList contains keyphrases, for example strings of few words like:

un'altra recessione

strada lunga

presidente barack obama

[...]

I tried with this two methods but i get a NullPointerException

 public boolean containsKeyword(String testKeyword, ArrayList<String> list) 
{ return list.contains(testKeyword);
}

public int compareKeywords(Quotidiano quotidiano){
    int sameKeywords=0;
    for(int i=0; i<this.Keywords.size(); i++){
            if(this.containsKeyword(this.getKeyword(i), quotidiano.Keywords))
             sameKeywords++;
          }

    return sameKeywords;
}

Thanks a lot for answers!

---

[EDIT]

It worked using intersection, thank you! But it would be better if it compares single words.

For example, if i have keyphrases

"spread of italy",

"spread under 300",

I think intersection is null, instead i would like "spread" returned. How can I do it?

Answer 1

Use Apache ListUtils Intersection

public static java.util.List intersection(java.util.List list1,
                                      java.util.List list2)

PS: This throws java.lang.NullPointerException - if either list is null

Answer 2

You should check for null whenever you are getting an input from unknown source. You should not count that the input is OK.

public boolean containsKeyword(String testKeyword, ArrayList<String> list) 
{ 
    return (list != null) ? list.contains(testKeyword) : false;
}

public int compareKeywords(Quotidiano quotidiano){
    if(quotidiano == null || this.Keywords == null) return 0;
    int sameKeywords=0;
     for(int i=0; i<this.Keywords.size(); i++){
        if(this.containsKeyword(this.getKeyword(i), quotidiano.Keywords))
         sameKeywords++;
    }

    return sameKeywords;
}

checking this.Keyword is optional as I assume that's your code and you are aware it should always be different then null.

Answer 3

NullPointerException occurs when you are using member of any object and that object is not yet initialized. Make sure both the lists are initialized and are not null.