java - NavigableMap : java. lang.IllegalArgumentException:键超出范围

Question

您能解释一下这段代码吗？

TreeMap<String, Integer> myMap = new TreeMap<String, Integer>();
        myMap.put("a", 50);
        myMap.put("b", 60);
        myMap.put("c", 70);

        NavigableMap<String, Integer> myMap2 = myMap.headMap("d", true);
        myMap.put("e", 90);
        myMap2.put("f", 100);//exception here
        System.out.println(myMap.size() + " " + myMap2.size());

输出消息:

java.lang.IllegalArgumentException: key out of range
    at java.util.TreeMap$NavigableSubMap.put(TreeMap.java:1407)
    at SomeClass.main(SomeClass.java:13)

我不明白为什么我有它。

Answer 1

来自JavaDoc on TreeMap.headMap :

The returned map will throw an IllegalArgumentException on an attempt to insert a key outside its range.