Instantiating a TreeMap using a Comparator which should be able to access the said TreeMap

问题

I'm trying to instantiate a TreeMap using a Comparator which should be able to access the said TreeMap, i.e. the one it will be used for (I guess that "will" must precisly be the problem...):

final Map<String, Integer> map = new TreeMap<String, Integer>(new Comparator<String>() {

    @Override
    public int compare(String o1, String o2) {
        Integer i = map.get(o1);
        // Error: "the local variable map may not have been initialized"
        return ...;
    }

});

I can get why this error occurs, since when instantiating the Comparator<String>, the map variable isn't initialized yet, but is there any workaround solution?

A solution would have been a setComparator method in the TreeMap implementation, but its comparator field has been declared as final:

final Map<String, Integer> map = new TreeMap<String, Integer>();
Comparator<String> comparator = new Comparator<String>() {

    @Override
    public int compare(String o1, String o2) {
        Integer i = map.get(o1);
        return ...;
    }

};
// map.setComparator(comparator);

回答1:

Can't you implement Comparator on your class and pass this to the TreeMap constructor, example:

class MyClass implements Comparator<MyClass> {
private String property;
@Override // java.util.Comparator.compare
public int compare(MyClass o1,
            MyClass o2) {
return o1.getProperty().compare(o2.getProperty());
}
@Override // java.util.Comparator.equals
public boolean equals(Object o) {
    return this.getProperty().equals(o.getProperty());
}

public String getProperty() {
    return this.property;
}

public void setProperty(String myPropertyValue) {
    property = myPropertyValue;
}

TreeMap <String, MyClass> myMap = null;

public MyClass() {
  myMap = new TreeMap<String, MyClass>(this);
}

If you need further assistance, do leave a comment with more information as to your specific case.

回答2:

Two months later, after having relooked into the answer @hd1 gave me (which doesn't work as is actually), here is a possible solution:

public class SortedByValueMap<K, V extends Comparable<V>> implements Comparator<K> {

    private final Map<K, V> map = new TreeMap<K, V>();

    private class CustomTreeMap<KK extends K, VV extends V> extends TreeMap<KK, VV> {

        private static final long serialVersionUID = 9196929305071517886L;

        private CustomTreeMap(Comparator<KK> c) {
            super(c);
        }

        @Override
        public VV put(KK key, VV value) {
            map.put(key, value);
            return super.put(key, value);
        };

        @Override
        public VV remove(Object key) {
            map.remove(key);
            return super.remove(key);
        }

    }

    @Override
    public int compare(K o1, K o2) {
        return map.get(o1).compareTo(map.get(o2));
    }

    public Map<K, V> getMap() {
        return new CustomTreeMap<K, V>(this);
    }

}

Then:

Map<String, Integer> map = new SortedByValueMap<String, Integer>().getMap();
map.put("r", 2);
map.put("b", 0);
map.put("a", 1);
System.out.println(map); // prints {b=0, a=1, r=2}

But anyway, it has to be considered as a technical challenge solution rather than a real efficient tool though (since two identical parallel maps are created), so use it sparingly... ;)

来源：https://stackoverflow.com/questions/15807453/instantiating-a-treemap-using-a-comparator-which-should-be-able-to-access-the-sa

标签

java

sorting

map

comparator

treemap