I remember one time I went to interview a Java software development engineer. The interviewer asked me about why JavaBean rewrites hashCode() method and equals method. I remember that I didn’t talk about the key points for a long time. Now, after thinking about it, I still don’t have a deep understanding of these two. Now I will summarize it specifically.The hashCode method is used to find a hash collection, and the equals method is used to determine whether two objects are equal.
1. Why do we need to override the hashCode() method and the equals() method? (Why)
Sometimes when judging objects in our business system, sometimes what we need is not a strict equality, but a business equality. In this case, the native equals method cannot meet our needs.The superclass (parent class) of JavaBean as we know is the Object class. The equals method in JavaBean is inherited from the method in Object. The equals() method defined in the Object class is used to compare whether the memory addresses of the two objects pointed to are consistent. It is not to compare whether the attribute values of the two objects are consistent, so we need to override the equals() method.
Source code of equals() method in Object class
public boolean equals(Object obj) {
return (this == obj);
}public class Demo {
public static void main(String[] args) {
Student stu1 = new Student("awu",22);
Student stu2 = new Student("awu",22);
((stu2));
/*Because Student's JavaBean does not override the equals() method about the equals value of the attribute value.
, so the default comparison is the address value, so the output result is false*/
}
}So why do you need to rewrite the hashCode method when rewriting the equals method?
Mainly common conventions:
a. When the java application is running, no matter when the hsahCode() method of the same object is called multiple times, the return value of the hashCode() method of this object must be the same int value.
b. If the return value of both objects equals() is true, their hashCode() must also return the same int value.
c. If the return value of the two objects equals() is false, their hashCode() returns must also be different.
Use HashSet to illustrate why this agreement is: when HashSet stores elements, it quickly finds the location to be stored according to the hashCode value of the element. If there are elements in this position, the two objects are compared through equals(). If the return value is true, it will not be placed; if the return value is false, then two elements will be stored in the same position in the form of a linked list, which will reduce the performance of HashSet because it cannot be quickly positioned. Another situation is that the hashCode() return values of the two objects are different, but equals() returns true. At this time, HashSet will save both objects, which is contrary to the rule that the Set collection does not repeat; therefore, when we rewrite the equals() method, we must rewrite the hashCode() method according to the b and c rules!(In fact, if only the equals method is rewritten, the two objects equals return true, but if the hashCode method is not rewritten, the collection will still insert elements. This way, duplicate elements appear in the collection.)
2. Under what circumstances do you need to override the hashCode() method and the equals() method? (When)
When we customize a class, we want to save its instance inWhen searching for a collection with a hash,We need to rewrite these two methods;
public class Student {
private String name;
private Integer age;
public Student(){
}
public Student(String name,Integer age){
= name;
= age;
}
public String getName() {
return name;
}
public void setName(String name) {
= name;
}
public Integer getAge() {
return age;
}
public void setAge(Integer age) {
= age;
}
@Override
public int hashCode(){
final int prime = 31;
int result = 17;
result = prime * result + ();
result = prime * result + age;
return result;
}
@Override
public boolean equals(Object obj){
if(this == obj)
return true;
if(obj == null)
return false;
if(getClass() != ())
return false;
final Student other = (Student)obj;
if(()){
return false;
}
if(()){
return false;
}
return true;
}
}public class Demo {
public static void main(String[] args) {
Student stu1 = new Student("awu",22);
Student stu3 = new Student("awu",33);
Student stu2 = new Student("awu",22);
Set set = new HashSet();
(stu1);
(stu2);
(stu3);
(());
/*The output result is 2*/
}
}If it is not a collection found with a Hash hash, even rewriting HashCode will not be of much practical use. For example, the following chestnut:
public class Demo {
public static void main(String[] args) {
Student stu1 = new Student("awu",22);
Student stu3 = new Student("awu",33);
Student stu2 = new Student("awu",22);
ArrayList list = new ArrayList();
(stu1);
(stu2);
(stu3);
(list .size());
/*The output result is 3*/
}
}3. How to rewrite these two methods? (How)
public class Student {
private String name;
private Integer age;
public Student(){
}
public Student(String name,Integer age){
= name;
= age;
}
public String getName() {
return name;
}
public void setName(String name) {
= name;
}
public Integer getAge() {
return age;
}
public void setAge(Integer age) {
= age;
}
@Override
public int hashCode(){
final int prime = 31;
int result = 17;
result = prime * result + ();
result = prime * result + age;
return result;
}
@Override
public boolean equals(Object obj){
if(this == obj)
return true;
if(obj == null)
return false;
if(getClass() != ())
return false;
final Student other = (Student)obj;
if(()){
return false;
}
if(()){
return false;
}
return true;
}
}