Updating a java map entry

Question

I'm facing a problem that seems to have no straighforward solution.

I'm using java.util.Map, and I want to update the value in a Key-Value pair.

Right now, I'm doing it lik this:

private Map<String,int> table = new HashMap<String,int>();
public void update(String key, int val) {
    if( !table.containsKey(key) ) return;
    Entry<String,int> entry;
    for( entry : table.entrySet() ) {
        if( entry.getKey().equals(key) ) {
            entry.setValue(val);
            break;
        }
    }
}

So is there any method so that I can get the required Entry object without having to iterate through the entire Map? Or is there some way to update the entry's value in place? Some method in Map like setValue(String key, int val)?

jrh

You can remove the first "if" statement. It still causes a full iteration through the map if the key is not present. If you remove it, you will still iterate through the whole map and not execute anything if the key isn't present. Right now if the key is present, you iterate through the map, find that it is present, and then iterate through the entries again. — Sandy Simonton, Jul 23 '15 at 23:48

score 165 · Accepted Answer · edited May 01 '18 at 21:51

165

Use

table.put(key, val);

to add a new key/value pair or overwrite an existing key's value.

From the Javadocs:

V put(K key, V value): Associates the specified value with the specified key in this map (optional operation). If the map previously contained a mapping for the key, the old value is replaced by the specified value. (A map m is said to contain a mapping for a key k if and only if m.containsKey(k) would return true.)

edited May 01 '18 at 21:51

Dave Jarvis

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answered Jun 30 '09 at 07:05

skaffman

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For reference, in Java 8 there is a putIfAbsent() method in case someone did not want to update. – Arcturus Jun 06 '17 at 12:09

score 13 · Answer 2 · edited Feb 23 '17 at 10:36

13

If key is present table.put(key, val) will just overwrite the value else it'll create a new entry. Poof! and you are done. :)

you can get the value from a map by using key is table.get(key); That's about it

edited Feb 23 '17 at 10:36

Maytham Fahmi

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answered Jun 30 '09 at 07:13

Priyank

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score 6 · Answer 3 · answered Jun 30 '09 at 07:10

6

You just use the method

public Object put(Object key, Object value)

if the key was already present in the Map then the previous value is returned.

answered Jun 30 '09 at 07:10

mkoeller

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Updating a java map entry

3 Answers3