what is hashcode
Hashcodeis an algorithm that maps data of any size to fixed-size values. It converts data into fixed-length hash values or hash codes, often used as a fast lookup for data indexing.
Hash codes can be used in many fields, including cryptography, data integrity checks, and search algorithms. In programming, hash codes are often used in hash table data structures to quickly look up where the payload of a key or value resides, thereby improving program performance and efficiency.
Some advantages of using hashcodes are:
- Hash codes are fixed-length, thus greatly reducing the time and space complexity of processing each element.
- A good hash function can map large amounts of data into smaller hash tables, making lookups faster.
- Hash codes can also be used to verify the integrity of data. For example, in web development, we can use hash codes to verify that a file has not been tampered with.
However, there are also some disadvantages:
- For different data input values, it is possible to get the same hash code, that is, a hash collision. Therefore, in order to avoid this situation, corresponding solutions need to be adopted, such as chained hashing or open addressing.
- Computing the hash code requires a certain computational cost, especially when the input data set is large, which may cause performance degradation.
The role of hashcode in Java, when to use it
hashCode()is a method of a Java object that returns the object's hash code value. The hash code value is an integer calculated from a specific state of an object, and is used as an index of a key-value pair in a data structure such as a hash table and hashMap. The function of hashCode() is to improve the performance of HashMap and some other collection classes.
hashCode()Common cases of using in Java , such as:
- When implementing a hash table, you need to override
hashCode()the method to ensure that the keys of the hash table can be stored and retrieved correctly. - When using a custom object as the key of the HashMap, you need to override the
hashCode()andequals()methods to ensure that equal objects have the same hashCode().
Although Java can automatically generate a hash code based on the memory address of an object, in practice, this mechanism may be unstable or perform poorly. Therefore, it is usually recommended to override this method for a class in order to generate a better hash code.
example
Suppose you have a Personclass with two properties: nameand age. Then you could create a method for that class hashCode()to generate a hash code for quick lookup when you use a hash table or other data structure.
public class Person {
private String name;
private int age;
public Person(String name, int age) {
this.name = name;
this.age = age;
}
// 重写hashCode() 方法
@Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result + age;
result = prime * result + ((name == null) ? 0 : name.hashCode());
return result;
}
}
In the above code, we override hashCode()the method and use nameand ageproperty to calculate the hash code. Note that we use a constant prime(31), and namespecial handling for the property, since it can be null. Finally, we combine all the pieces and return the result.
Summarize
In Java, hashCode()a method is a method that returns the hash code value of an object. Hash code is a mechanism for fast lookup based on the hash code value of an object when looking up an object in a hash table.
When an object is created, the system will generate a unique hash code value for the object, and when the method is used to get()retrieve the object from the hash table, the system will also quickly locate the object according to the hash code value of the object.
ObjectThe hash code value can be obtained through the method of the class in Java hashCode(). But it should be noted that when rewriting the method of a custom object equals(), the method is usually rewritten hashCode()to ensure that if two objects return true, the return value of equals()their method must also be equal.hashCode()