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此自定義的Valuecomparator通過其值對TreeMap進行排序。但是,在搜索TreeMap是否具有某個鍵時,它不能容忍nullpointexception。我如何修改比較器來處理零點?使TreeMap Comparator容忍null
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"));
}
}
啊,是的,使它小於非零! 我試過'if((a == null)||(b == null)){return -1;}'。但是這並不能正確地對地圖進行排序。爲什麼? – alvas 2012-03-07 07:12:17
因爲它違反了'x.equals(x)''時'comparator.compare(x,x)== 0'的約束,特別是當'x == null'時。 – 2012-03-07 11:54:12