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hashmap可以用null为键值

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10/02
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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);
        }
    }
}