I should to search all keys in HashMap, that have values = null and delete them. How can I do this?

Question

I have such function and I should find all keys in references, that have values = null and delete them. How to do it?

public static void garbageCollector(Map references){
            if (references.containsValue(null) ==true){
                ????
            }

Thanks in advance.

If you want to do this, then don't put null values into the map in the first place. :) — allprog, Mar 26 '14 at 13:53
If you simply call containsValue(null) on the map, you didn't gain anything. You still have no entry point for deleting anything. The algorithm should be "Iterate over all entries in the map. With each entry, check if value == null. If yes, delete the entry." Are you able to transfer that into a code snippet? — Seelenvirtuose, Mar 26 '14 at 13:56
http://stackoverflow.com/questions/15872313/remove-a-key-in-hashmap-when-the-values-hashset-is-empty — giannis christofakis, Mar 26 '14 at 13:58
I have the Map references, which can contain some values, that = null. This is why i do the test on contain the null values in my Map. If it is true I should find keys of this values and delete them. — Kitty, Mar 26 '14 at 13:58

Raul Guiu · Answer 1 · 2014-03-27T14:54:25.810

You could use an iterator:

public static void garbageCollector(Map references){
    Iterator iterator = references.entrySet().iterator();
    while (iterator.hasNext()) {
        Map.Entry pair = (Map.Entry)iterator.next();
        if (pair.getValue() == null) {
            iterator.remove(); // avoids a ConcurrentModificationException
        }
    }
 }

And in Java 8 you could create a new Map with the null entries removed with:

references.entrySet().stream().filter(p -> p.getValue() != null).collect(toMap(Entry::getKey, Entry::getValue));

score 3 · Answer 2 · answered Mar 26 '14 at 14:17

3

references.values().removeAll(Collections.singleton(null));

This should simply do the job for you :)

answered Mar 26 '14 at 14:17

Shishir Kumar

7,981
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+1 Best and more concise answer so far. – giannis christofakis Mar 26 '14 at 14:31
1

This indeed works. I even did not try this: I was sure it will throw exception for some reason. BTW `references.keySet().removeAll()` works also. – AlexR Mar 26 '14 at 16:34

score 2 · Answer 3 · edited Apr 08 '14 at 12:20

I have prepared the demo as per your requirement. You can check this

public static void main(String args[]) {
    Map map = new HashMap();
    map.put(1, null);
    map.put(2, "val1");
    System.out.println("map is "+map);
    Iterator it = map.entrySet().iterator();
    while (it.hasNext()) {
         Map.Entry entry = (Map.Entry) it.next();
            Integer key = (Integer)entry.getKey();
            String value = (String)entry.getValue();
            if (value == null) 
                it.remove();

    }
    System.out.println("map is "+map);
}

AlexR · Answer 4 · 2014-03-26T14:42:43.947

1

Take a look on the following code sample:

public <K, V> static void garbageCollector(Map<K, V> map) {
    Collection<K> remove = new LinkedList<K>();
    for (Entry<K, V> e : map.entrySet()) {
        if (e.getValue() == null) {
            remove.add(e.getKey());
        }
    }
    for (K r : remove)  {
      map.remove(r);
    }
}

This is just one of a few ways to perform this.

edited Mar 26 '14 at 14:42

answered Mar 26 '14 at 13:58

AlexR

114,158
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Map doesn't has `removeAll` method – shem Mar 26 '14 at 14:01
@shem, u r right. I have fixed the code. – AlexR Mar 26 '14 at 14:43

score 0 · Answer 5 · answered Mar 26 '14 at 13:55

0

Loop through the keys, get the value of each key.
If that value is null, remove the key.

public static void garbageCollector(Map references){
    ArrayList lst = new ArrayList();
    for (Object k : references.keySet()){
        if (references.get(k)==null){
            lst.add(k);
        }
    }
    for (Object k : lst){
        references.remove(k);
    }
}

answered Mar 26 '14 at 13:55

peter.petrov

38,363
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Don't do it inside the loop you are going to cause a `ConcurrentModificationException`. – giannis christofakis Mar 26 '14 at 13:57
1

I know... Very quick downvoters. It's fixed now. – peter.petrov Mar 26 '14 at 13:58

score 0 · Answer 6 · answered Mar 26 '14 at 13:57

Map<String, Object> m = new HashMap<String, Object>();
LinkedList<String> toRemove = new LinkedList<String>();
for (Entry<String, Object> e : m.entrySet()) {
    if (e.getValue() == null) {
        toRemove.add(e.getKey());
    }
}
for (String key : toRemove) {
    m.remove(key);
}

score 0 · Answer 7 · answered Mar 26 '14 at 14:22

Use entrySet(). Specification says that

The set is backed by the map, so changes to the map are reflected in the set, and vice-versa.

So, you can edit set with entries instead of map and that is easier.

    HashMap<Object, Object> map = new HashMap();
    System.out.println("before");
    map.put(1, 2);
    map.put(2, null);
    map.put(3, null);

    for (Map.Entry<Object, Object> e : map.entrySet()) {
        System.out.println(e.getKey() + "=" + e.getValue());
    }

    for (Iterator<Map.Entry<Object, Object>> i = map.entrySet().iterator(); i.hasNext();) {
        if (i.next().getValue() == null) {
            i.remove();
        }
    }

    System.out.println("\nafter");
    for (Map.Entry<Object, Object> e : map.entrySet()) {
        System.out.println(e.getKey() + "=" + e.getValue());
    }

Output:

before
1=2
2=null
3=null

after
1=2

Shekhar Khairnar · Answer 8 · 2014-03-26T14:25:48.367

-2

you could write as:

Iterator<Map.Entry<String,String>> iter = map.entrySet().iterator();
    while (iter.hasNext()) {
        Map.Entry<String,String> entry = iter.next();
        if(null == (entry.getValue())){
            iter.remove();
        }
    }

edited Mar 26 '14 at 14:25

answered Mar 26 '14 at 13:59

Shekhar Khairnar

2,643
3
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2

Don't do it inside the loop you are going to cause a `ConcurrentModificationException`. – shem Mar 26 '14 at 14:00

I should to search all keys in HashMap, that have values = null and delete them. How can I do this?

8 Answers8