java中根据map的value进行排序

Question

public class Testing {

    public static void main(String[] args) {
        HashMap<String,Double> map = new HashMap<String,Double>();
        ValueComparator bvc =  new ValueComparator(map);
        TreeMap<String,Double> sorted_map = new TreeMap<String,Double>(bvc);
        sorted_map.putAll(map);
    }
}
class ValueComparator implements Comparator<String> {
    Map<String, Double> base;
    public ValueComparator(Map<String, Double> base) {
        this.base = base;
    }
    // Note: this comparator imposes orderings that are inconsistent with equals.
    public int compare(String a, String b) {
        if (base.get(a) >= base.get(b)) {
            return -1;
        } else {
            return 1;
        } // returning 0 would merge keys
    }
}

Answer 1

sorted_map在初始化的时候，你给了它Comparator， 所以sorted_map.putAll(map)的时候，实际上是把map里的每个entry放进sorted_map里，每放一个，sorted_map就会调用Comparator的compare方法来比较一下，决定放在哪个位置，这样sorted_map的entry就都是按照Comparator的顺序规则来排列的啦。

Answer 2