This customized Valuecomparator sorts the TreeMap by it's value. But it doesn't tolerate nullpointexception when searching whether the TreeMap has a certain key. How do i modify the comparator to handle the nullpoint?
import java.io.IOException;
import java.util.Comparator;
import java.util.HashMap;
import java.util.Map;
import java.util.TreeMap;
public class TestTreeMap {
public static class ValueComparator<T> implements Comparator<Object> {
Map<T, Double> base;
public ValueComparator(Map<T, Double> base) {
this.base = base;
}
@Override
public int compare(Object a, Object b) {
/*if (((Double) base.get(a) == null) || ((Double) base.get(b) == null)){
return -1;
} */
if ((Double) base.get(a) < (Double) base.get(b)) {
return 1;
} else if ((Double) base.get(a) == (Double) base.get(b)) {
return 0;
} else {
return -1;
}
}
}
public static void main(String[] args) throws IOException {
Map<String, Double> tm = new HashMap<String, Double>();
tm.put("John Doe", new Double(3434.34));
tm.put("Tom Smith", new Double(123.22));
tm.put("Jane Baker", new Double(1378.00));
tm.put("Todd Hall", new Double(99.22));
tm.put("Ralph Smith", new Double(-19.08));
ValueComparator<String> vc = new ValueComparator<String>(tm);
TreeMap<String, Double> sortedTm =
new TreeMap<String, Double>(vc);
sortedTm.putAll(tm);
System.out.println(sortedTm.keySet());
System.out.println(sortedTm.containsKey("John Doe"));
// The comparator doesn't tolerate null!!!
System.out.println(!sortedTm.containsKey("Doe"));
}
}
This is not rocket science ...
Insert this in the place of the commented out code:
This treats
null
as a smaller value than any non-nullDouble
instance.Your version is incorrect:
This says "if a is null or b is null then a is smaller than b".
That leads to bogus relationships like
This sort of thing causes the tree invariants to break when there are nulls in the set / map, and leads to inconsistent and unstable key ordering ... and worse.
The requirements for a valid
compare
method are set out in the javadocs. The mathematical version is that the method must define a total order over the domain of all possible input values.