Learn from the 'identity mapping' framework in the Java class library
Learn from the 'identity mapping' framework in the Java class library
Identity Mapping is a design pattern often used in the Java class library to create the uniqueness of the object.In the development process, we often need to establish a correlation between different objects, but because the characteristics of the object make the value of the directly comparison object may not be able to accurately determine the equal nature of the object.At this time, the accurate comparison of the object can be achieved through the identity mapping framework.
Some Java libraries have provided the realization of identity mapping frameworks, the most commonly used is the HashMap class of Java.HashMap uses hash tables to store key values pairs. The uniqueness of the key is determined by the HashCode () method and the EQUALS () method.
To use the identity mapping framework, we need to ensure the uniqueness of the object.In order to achieve uniqueness, objects need to rewrite the HashCode () and Equals () methods.The HashCode () method is responsible for returning the hash code of the object, and the Equals () method is used to compare whether the object is equal.When rewriting these two methods, the following rules need to be guaranteed:
1. If the two objects are equal (the Equals () method returns true), then their hash code must be the same;
2. If the two objects are not equal, their hash code does not have different requirements.
Next, you can obtain the object by putting the object into the identity mapping framework.The identity mapping framework will determine the uniqueness of the object based on the HashCode () method of the object and the equals () method, and return the corresponding value.The following is a simple example of using HashMap to implement identity mapping:
import java.util.HashMap;
public class IdentityMappingExample {
public static void main(String[] args) {
HashMap<Person, String> identityMapping = new HashMap<>();
Person person1 = new Person("John", 25);
Person person2 = new Person("Jane", 30);
identityMapping.put(person1, "Employee");
identityMapping.put(person2, "Manager");
System.out.println(identityMapping.get(person1)); // 输出：Employee
System.out.println (IdentityMapping.get (Person2)); // Output: Manager
}
}
class Person {
private String name;
private int age;
public Person(String name, int age) {
this.name = name;
this.age = age;
}
@Override
public int hashCode() {
return age * name.hashCode();
}
@Override
public boolean equals(Object obj) {
if (this == obj) {
return true;
}
if (obj == null || getClass() != obj.getClass()) {
return false;
}
Person otherPerson = (Person) obj;
return age == otherPerson.age && name.equals(otherPerson.name);
}
}
In the above example, we created a HashMap object IdentityMapping to store the Person object and the corresponding role.According to the uniqueness of the Person object, we can obtain their roles through Person1 and Person2 objects and output results.
By understanding the identity mapping framework in the Java library, we can better understand the association and uniqueness between objects.The identity mapping framework is not only one of the common design models in Java development, but also plays an important role in large -scale applications.
