如何通过索引获取TreeMap的键？

Question

So I'm trying to get the key of a TreeMap by an int index.

Here's my code:

TreeMap<String, Integer> map = new TreeMap<String, Integer>(Collections.reverseOrder());
map.put("hi", 1);
map.put("hi2", 5);

System.out.println("Key: " + (String) map.keySet().toArray()[0] + "\nValue: " + map.get(0));

But when I execute this, I get the error:

java.lang.Integer cannot be cast to java.lang.String

So I'm trying to get the key: "hi2"

Answer 1

The problem is

"\nValue: " + map.get(0)

The argument for get must be a String , not an Integer like 0 . So do this:

String key = map.keySet().toArray()[0];
Integer value = map.get(key);
System.out.println("Key: " + key + "\nValue: " + value);

Answer 2

It seems to me that the error comes from the latter part of the println statement:

println(...map.get(0))

Where the integer 0 has a type conflict with the map, where the key type is String .

[edit]

• As for the original question about getting the key by index, I think you did it correctly: map.keySet().toArray()[0]
• For the error complaining about key type conflict in map.get() , you can correct the parameter type, as suggested by @Tichodroma's answer above.