Java HashMap 允许重复吗？
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HashMap allows duplicates?
提问by Nani
I have a doubt regarding HashMap, as we all know HashMap allows one null key and value pair, My question here is
我对 HashMap 有疑问，众所周知，HashMap 允许一个空键和值对，我的问题是
If I wrote like this,
如果我这样写，
m.put(null,null);
m.put(null,a);
Will it throw a (error or exception) or will it override the value or what will be the value of returing??
它会抛出（错误或异常）还是会覆盖该值或返回的值是什么？？
回答by Suresh Atta
Doesn't allow duplicates in the sense, It allow to add you but it does'nt care about this key already have a value or not. So at present for one keythere will be only one value
从某种意义上说不允许重复，它允许添加你，但它不关心这个键是否已经有一个值。所以目前一个键只有一个值
It silently overrides the valuefor nullkey. No exception.
它以静默方式覆盖valuefornull键。没有例外。
When you try to get, the last inserted value with nullwill be return.
当您尝试获取时，null将返回最后插入的值。
That is not only with nulland for any key.
这不仅适用null于任何键。
Have a quick example
有一个简单的例子
Map m = new HashMap<String, String>();
m.put("1", "a");
m.put("1", "b"); //no exception
System.out.println(m.get("1")); //b
回答by user2864740
Each keyin a HashMapmust be unique.
HashMap 中的每个键都必须是唯一的。
When "adding a duplicate key" the old value (for the same key, as keys must be unique) is simply replaced; see HashMap.put:
当“添加重复键”时，旧值（对于相同的键，因为键必须是唯一的）被简单地替换；见HashMap.put：
Associates the specified value with the specified key in this map. If the map previously contained a mapping for the key, the old value is replaced.
Returns the previous value associated with key, or null if there was no mapping for key.
将指定值与此映射中的指定键相关联。如果映射先前包含键的映射，则旧值将被替换。
返回与 key 关联的前一个值，如果没有 key 的映射，则返回null。
As far as nulls: a single null keyis allowed (as keys must be unique) but the HashMap can have any number of null values, and a null key need not have a null value. Per the documentation:
至于空值：允许单个空键（因为键必须是唯一的）但 HashMap 可以具有任意数量的空值，并且空键不需要具有空值。根据文档：
[.. HashMap] permits null valuesand [a] null key.
[.. HashMap] 允许 null值和 [a] null key。
However, the documentation says nothingabout null/null needing to be a specific key/value pair or null/"a" being invalid.
但是，文档没有说明 null/null 需要是特定的键/值对或 null/"a" 无效。
回答by kai
Code example:
代码示例：
HashMap<Integer,String> h = new HashMap<Integer,String> ();
h.put(null,null);
h.put(null, "a");
System.out.println(h);
Output:
输出：
{null=a}
回答by user3094411
Hashmap type Overwrite that key if hashmap key is same key
哈希映射类型如果哈希映射键是相同的键，则覆盖该键
map.put("1","1111");
map.put("1","2222");
output
输出
key:value
1:2222
回答by dingjsh
HashMap don't allow duplicate keys,but since it's not thread safe,it might occur duplicate keys. eg:
HashMap 不允许重复键，但由于它不是线程安全的，因此可能会出现重复键。例如：
while (true) {
final HashMap<Object, Object> map = new HashMap<Object, Object>(2);
map.put("runTimeType", 1);
map.put("title", 2);
map.put("params", 3);
final AtomicInteger invokeCounter = new AtomicInteger();
for (int i = 0; i < 100; i++) {
new Thread(new Runnable() {
@Override
public void run() {
map.put("formType", invokeCounter.incrementAndGet());
}
}).start();
}
while (invokeCounter.intValue() != 100) {
Thread.sleep(10);
}
if (map.size() > 4) {
// this means you insert two or more formType key to the map
System.out.println( JSONObject.fromObject(map));
}
}
回答by BINIT kumar jha
m.put(null,null); // here key=null, value=null
m.put(null,a); // here also key=null, and value=a
Duplicate keys are not allowed in hashmap.
However,value can be duplicated.
哈希图中不允许有重复的键。
但是，值可以复制。
