具有两个值相同但存储桶索引不同的键对象的哈希图

Question

If Hashmap has two keys having same value, for eg:

HashMap map=new HashMap();
map.put("a","abc");
map.put("a","xyz");   

So here put two key with "a" value and suppose for first bucketindex=1 and second bucketindex=9

So my question is if bucket index for both is coming different after applying hashing algorithm, in this how to handle for not inserting duplicate key as it is already present and hashmap cannot have duplicate key. please suggest your view on this.

Answer 1

There won't be any such thing as "second bucket index".

I suggest you add something like System.out.println(map.toString()) in order to see what that second put() has done to your map.

EDIT:

In the method put(key,value) , the "bucket index" is computed as a function of the key element's value, not the value element's value (so "a" and "a" give the same index for the bucket). This function is supposed to be deterministic so feeding it the same value ("a" in your case), the same hashCode() will come out and subsequently, the same bucket index.

Answer 2

In Java if a hashing function returns the same hash, equality of two objects is determined by equals() method. And if the objects are found equal, the old one is simply replaced by the new one.

Instead, if the objects are not equal, they just get chained in a linked list (or a balanced tree) and the map contains both objects, because they are different .

So, back to your question: "if bucket index for both is coming different after applying hashing algorithm" - this is impossible for equal objects. Equal objects must have the same hash code.

Answer 3

To make @Erwin's answer more clear, here's the source code of HashMap from JDK

public V put(K key, V value) {
    return putVal(hash(key), key, value, false, true);
}

static final int hash(Object key) {
    int h;
    return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
}

Digging more deep you will find that the bucket index is calculated from key 's hash code.

To make it simple and straightforward, putting duplicate key with different values to the same HashMap will result just one single entry, which the second put is just overwriting the value of the entry.

Answer 4

如果您的问题是如何创建可为同一个键处理多个值的哈希映射，则您需要的是Map>，以便每次键相同时就向数组列表添加一个新值。