HashMap 中的重复值

我遇到了大麻烦，创建了一个 hashMap 并使用相同的键插入了两个值 StringBuilder作为map的键。 现在，虽然尝试使用 StringBuilder 对象检索数据工作正常，但在其他情况下它无法返回任何值。我在下面给出的代码中列出了三种情况，

`

class MainClass {

public static void main(String[] args) {

    MainClass m = new MainClass();

    StringBuilder sb = new StringBuilder("sb");
    StringBuilder sb1 = new StringBuilder("sb");

    Map<StringBuilder, String> map = new HashMap<StringBuilder, String>();
    map.put(sb, "a");
    map.put(sb1, "b");
    System.out.println("----Inside Main method---- mapValue"+map);

    System.out.println("Expected value a, coming also => " + map.get(sb)); //a
    System.out.println("Expected value b, coming also => " + map.get(sb1)); //b
    System.out.println("Expected value a, not coming  => " + map.get("sb")); // why null ?


    m.receiveMap(map, sb, sb1);

}

public void receiveMap(Map<StringBuilder, String> map, StringBuilder refSb,StringBuilder refSb1) {
    StringBuilder sb = new StringBuilder("sb");
    StringBuilder sb1 = new StringBuilder("sb");
    System.out.println("----Inside receiveMap method mapValue"+map);
    System.out.println("Expected value a, not coming  => " + map.get(sb)); // why null ?
    System.out.println("Expected value b, not coming  => " + map.get(sb1)); // why null ?

    System.out.println("Expected value a, coming also => " + map.get(refSb)); // o/p - a
    System.out.println("Expected value b, coming also => " + map.get(refSb1)); // o/p -b
}

} `

在方法中receiveMap

System.out.println("Expected value a, not coming  => " + map.get(sb)); // why null ?

因为你正在创造新的StringBuilder某人在方法中具有不同的 hashcode 与 StringBuilder sb 进行比较PSVM.

StringBuilder 不会覆盖 equals 和 hashCode

  StringBuilder sb = new StringBuilder("sb");
  StringBuilder sb1 = new StringBuilder("sb");
  Set s = new HashSet();
  s.add(sb);
  s.add(sb1);
  System.out.println(s); 

设置不允许重复但仍然打印[sb, sb]因为 StringBuilder 不会覆盖 equals 和 hashcode 方法。