hashmap可以用null为键值

import java.util.HashMap;
import java.util.Map;
import java.util.TreeMap;

public class TestMain {
	public static void main(String[] args) {

		// HashMap可以的键值可以是null, "".
		Map<String, String> strMap1 = new HashMap<String, String>();
		strMap1.put(null, "1");
		strMap1.put("", "2");
		strMap1.put(" ", "3");
		strMap1.put(null, "4");
		System.out.println(strMap1.get(null));
		for (String s : strMap1.keySet()) {
			System.out.println(s);
		}
		for (String s : strMap1.values()) {
			System.out.println(s);
		}
		
		// TreeMap的键值不能是null
		Map<String, String> strMap2 = new TreeMap<String, String>();
		strMap2.put(null, "1");
		//strMap2.put("", "2");
		//strMap2.put(" ", "3");
		//strMap2.put(null, "4");
		//System.out.println(strMap2.get(null));
		for (String s : strMap2.keySet()) {
			System.out.println(s);
		}
		for (String s : strMap2.values()) {
			System.out.println(s);
		}
	}
}