Summary of the usage of equals and hashCode

一equals

##The equals method is Object-level. By default, it compares the memory addresses of two objects. Many classes have overridden this method to compare the actual contents of the objects. , generally compare whether the attribute values ​​of the same attributes of the same type of objects are the same.

2 hashCode

1. Hash table

The hash table is an array, Each element in the array is a one-way linked list. The nodes in the same one-way linked list have the same attributes. The attribute value is the subscript of the array. This same attribute is called hashCode hash value. .

2. The function and significance of hashCode hash value

Before hashCode is generated, the retrieval set needs to be compared one by one, which results in low query efficiency. If elements with the same attribute value in a collection are divided into a group based on a certain attribute, is stored centrally. When querying, the value of the attribute of the query object is first obtained, and the corresponding group is located based on the value, which greatly reduces the problem. It increases the query scope and improves the query efficiency. This is the background of hashCode.

The hashCode of two objects is equal, which only indicates that they are in the same group. It is not guaranteed to be the same. You must also compare the memory addresses of the two objects or pass ## The #equals method compares the specific contents. Only when this item is satisfied can it be determined that the two objects are the same.

It can be seen from the above that in the hash table, the same two objects must have the same hash value.

3.hashCode() method

The hashCode() method is Object-level and by default generates an int value based on the memory address of the object. This value has a one-to-one correspondence with the memory address. Since the same two objects are confirmed by equals

only to ensure that the attribute values ​​​​of the same attributes are the same, without rewriting the hashCode, the hash values ​​are different and are stored in are treated as two objects with different contents in the hash table, resulting in two elements with the same content appearing in the hash table, occupying unnecessary memory. Therefore, once the equals method is rewritten, it must be rewritten. Write the hashCode method and ensure that the same objects have equal values.

Three applications

Hash values ​​are mainly used in hash tables. The bottom layer of HashMap and HashSet is a hash table, so if you want to use When an object is stored in a HashMap or HashSet,

must override the equals and hashCode methods in the class so that the same objects have equal hash values.

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