CODEWITHSUNDEEP
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armnotstrong
armnotstrong

Reputation: 9065

Is there a way to change the key of a map in-place with java?

I want to change the key of a specific origin key name of a map, and I want to find a way to modify it just in place, so that it will not copy things around while doing it. I already know that I can only delete a entry of the map through iterate using only a Iterator. But when I put things back to the map while iterating the map. It can't be done. Anyway, I will put my sample code below, hope some one could give me a hint of doing this.

public class TestMapModify {
    public static void main(String[] argvs){
        Map<String, Object> m = new HashMap<String, Object>();
        m.put("a",1);
        m.put("b",2);
        for(Iterator<Map.Entry<String, Object>> it = m.entrySet().iterator();it.hasNext();){
            Map.Entry<String, Object> entry = it.next();
            if(entry.getKey().equals("a")){
                m.put("c", entry.getValue());
                it.remove();
            }
        }
        System.out.println(m);
    }
}

On the above code, I want to change the key "a" to "c" and leaving it's value unmodified. But this code snippet will give a exception:

Exception in thread "main" java.util.ConcurrentModificationException
    at java.util.HashMap$HashIterator.remove(HashMap.java:944)
    at me.armnotstrong.TestMapModify.main(TestMapModify.java:21)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:606)
    at com.intellij.rt.execution.application.AppMain.main(AppMain.java:134)

EDIT

I am aware that what cause this problem, but I want to find the "best" or alternative way to do that.

Upvotes: 0

Views: 434

Answers (1)

Naman Gala
Naman Gala

Reputation: 4692

@Durandal has already answered this answer. I didn't read it earlier.

You can put new values into another map and then add it to existing map after iteration.

public class TestMapModify {
    public static void main(String[] argvs) {
        Map<String, Object> m = new HashMap<String, Object>();
        Map<String, Object> anotherMap = new HashMap<String, Object>();

        m.put("a", 1);
        m.put("b", 2);
        for (Iterator<Map.Entry<String, Object>> it = m.entrySet().iterator(); it.hasNext();) {
            Map.Entry<String, Object> entry = it.next();
            if (entry.getKey().equals("a")) {
                anotherMap.put("c", entry.getValue());
                it.remove();
            }
        }
        m.putAll(anotherMap);
        System.out.println(m);
    }
}

Upvotes: 1

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