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  • 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);
            }
        }
    }
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  • 原文地址:https://www.cnblogs.com/Seachal/p/7111843.html
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