5

Is there any way to find if my HashMap<String, String> contains entry ( key,value) with value="x" and to go through all entries sequentially ?

ROMANIA_engineer
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Dayanne
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    Please take a look into Map interface first – Petro Semeniuk Mar 08 '11 at 09:29
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    If you change your question to mean the opposite, you should somehow mention that in your edited question or all the existing answers will look quite out-of-place. – Joachim Sauer Mar 08 '11 at 09:40
  • [Possible duplicate: Detailed explanation given ][1] [1]: http://stackoverflow.com/questions/1383797/java-hashmap-how-to-get-key-from-value – cryptickp Jul 30 '13 at 16:12

6 Answers6

8

HashMap.containsKey()

This is what HashMap is made for in the first place...

(Not sure what you mean by "to go through all entries sequentially", though. There's only 1 entry per key.)

Edit:

Now that you edited the question, the answer is no! :(

If you need that feature, design your own two-way HashMap that stores the location of each value in the other's value (hopefully that made sense), and then use that class. HashMaps aren't designed for this.

user541686
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5

There is the containsValue() method, but for common implementations this just internally iterates through all values and compares them to the parameter.

Paŭlo Ebermann
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1

A common pattern is to use

if(hashMap.containsKey(key)) {
    Object o = hashMap.get(key);

}

however if you know none of the values are null (Many Map collections do not allow null) then you can do the following which is more efficient.

Object o = hashMap.get(key);
if (o != null) {

}

BTW: containsKey is the same as

Set<Key> keys = hashMap.keySet();
boolean containsKey = keys.contains(key);
Peter Lawrey
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0

Use HashMap.containsKey() to know if it contains a given key. Use HashMap.keySet() or HashMap.entrySet() to retreive the collection of entries or values and iterate sequentially on it.

greydet
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0

You could find the information you are looking for, but it would be inefficient:

Object key;
Object val;
HashMap hm = new HashMap();
for (Iterator iter = hm.entrySet().iterator(); iter.hasNext();) {
  Map.Entry e = (Map.Entry) iter.next();
  if (key.equals(e.getKey()) && val.equals(e.getValue())) {
    // do something
  }
}

As suggested in some of the other answers, you might consider a better data structure for the problem you are trying to solve.

whatsleep
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You might like to consider using a bidirectional map, such as the one in Google's Guava library: http://guava-libraries.googlecode.com/svn/trunk/javadoc/index.html?com/google/common/collect/BiMap.html