Contains not finding partial string in List or Set

Question

I currently have a Set that contains three strings ("car", "two dollar", "foo"). I then execute the following against a passed HashMap<String, Double>.

if (getSet().contains(currentHashMapItem.getKey()) == true) {
    System.out.println(currentHashMapItem.getKey());
}

The first key is "car", that matches as expected and displays. However, the second key is "dollar", but doesn't display. Now my understanding of how contains works is it will return true if the string it is comparing exists in the Set that's being returned in getSet(), but no dice. Even tried an ArrayList<String> as well with no luck. Anyone ran into this before? Am I trying to cut too sharp a corner and forced to use a regex or iterator loop?

contains() uses equals(). And "dollar" is clearly not equal to "two dollar". So that's expected. Read [the javadoc](http://docs.oracle.com/javase/7/docs/api/java/util/Collection.html#contains%28java.lang.Object%29). — JB Nizet, Jul 13 '15 at 16:20
@JBNizet dammit I knew it was something simple I was overlooking. Thanks JB. — canadiancreed, Jul 13 '15 at 16:21
You might want to look into this thread as well. http://stackoverflow.com/questions/6645379/partially-match-strings-in-case-of-list-containsstring — theguywhodreams, Jul 13 '15 at 16:23
BTW `==true` is redundant, and in most cases provides risk of making mistake like `while(variable = true)` where instead of `==` we use `=` which is assignment, not comparison, causing infinite loop. — Pshemo, Jul 13 '15 at 16:27

score 2 · Accepted Answer · edited May 23 '17 at 10:30

2

The problem is that your Set does not contain "dollar" but "two dollar". Therefore it contains a string which contains the key but does not directly contain the key. As the Set.contains() method uses .equals(), this will fail unless the keys are identical.

You can iterate through the set of keys and see if any String.contains() your key.

As an additional option, you could make a new Set class with your own contains() method (either overriding or overloading the current one) which uses the String.contains() method of comparison instead of .equals(). This could be done three ways:

Implement the Set interface
-Convenient and could simply overload the boolean contains(Object) method with a boolean contains(String) option (also look at using the AbstractSet class)

Composition
-Allows use of a distinct type of Set (such as HashSet, TreeSet, etc), instead of the more general interface

Inheritence
-Similar to composition but a less elegant and appropriate option
-Only do this if you have other reasons to want a child class

edited May 23 '17 at 10:30

Community

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answered Jul 13 '15 at 16:18

River

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Composition would be a better option instead of extending from Set. – Chetan Kinger Jul 13 '15 at 16:32
If you are implementing `Set`, that's not a problem. My comment was for extending from a concrete `Set` implementation because your answer said *you could create a child class of Set* – Chetan Kinger Jul 13 '15 at 16:37
@CKing does the `Set` interface not already have an implementation for `contains()`? I'm assuming you'd have to do something more than just implementing the interface to get a custom `contains()` method. – River Jul 13 '15 at 16:39
What I am saying is basically : `class SetPartialContains implements Set { private Set set = new HashSet<>(); public boolean contains(String o) { for(String s : set) { if(s.contains(o)) return true; } } }` – Chetan Kinger Jul 13 '15 at 16:52
@CKing ah, I would've thought that would throw an error as `boolean contains(Object)` already has an implementation in `Set`. But I suppose that it can be overloaded by taking a `String` parameter instead. – River Jul 13 '15 at 17:07
1

@CKing I've added all 3 options to my answer, with some info on the pro and cons of each. I do think the implementation of the interface looks easiest though. Thanks for the suggestions. – River Jul 13 '15 at 17:19
1

Went with this answer as I felt it was the most complete and descriptive. – canadiancreed Jul 15 '15 at 15:53

score 1 · Answer 2 · answered Jul 13 '15 at 16:20

1

The key is "two dollar". That does not match "dollar" so the key is not found. To do what you want, you would have to retrieve every key in the set, and use keyString.contains("dollar"); to look for partial matches.

answered Jul 13 '15 at 16:20

rossum

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score 1 · Answer 3 · answered Jul 13 '15 at 16:21

The contains method will use the equals method of String class which looks for an exact match. Therefore, "dollar".equals("two dollar"); will return false.

What you want to do is iterate through the Set and use String.contains instead :

Set<String> set = getSet();
for(String s : set) {
  if(currentHashMapItem.getKey().contains(s)) {
     System.out.println(s);
  }
}

score 1 · Answer 4 · answered Jul 13 '15 at 16:21

contains returns true if the collection (in your case - a Set) contains the object passed to it. Your set does not contain the string "dollar" - it contains a longer string, "two dollars". If you want to test whether a collection contains a given substring, you'd have to implement it yourself:

public static boolean containsSubString (Collection<String> col, String str) {
    for (String s: col) {
        if (s.contains(str)) {
            return true;
        }
    }
    return false;
}

Viktor Mellgren · Answer 5 · 2015-07-14T08:38:34.673

-1

contains on Set is true if key is equal to the parameter

contains on String is true if string contains parameter

edited Jul 14 '15 at 08:38

answered Jul 13 '15 at 16:20

Viktor Mellgren

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`getSet()` seems to return a `Set` and not a `Map` – Chetan Kinger Jul 13 '15 at 16:26
@CKing thanks, missed that for some reason, was quite obvious it was a Set :D – Viktor Mellgren Jul 14 '15 at 08:39

Contains not finding partial string in List or Set

5 Answers5