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java开发经常用到实体类作为orm层的数据交换对象,但是没有重写equals与hashCode会导致,在使用诸如set,map时的重复排序错误等等问题,所以equals与hashCode更有利于业务代码,方便使用map,set做去重,排序等等
注意,equals与hashCode中对于实体唯一性的判别,有时候只看id是否一致(实体id唯一性),有时候是多个属性决定唯一实体(id,name不能两个都一样)
import java.util.HashMap;
import java.util.Map;
import java.util.Objects;
public class Person {
private int id;
private String name;
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public Person(int id, String name) {
this.id = id;
this.name = name;
}
@Override
public int hashCode() {
// depends on equals, unique property
return Objects.hash(getId(),getName());
}
@Override
public boolean equals(Object obj) {
if(obj==null) return false;
if(!(obj instanceof Person)) return false;
Person person = (Person) obj;
// combine all property that can identify unique book
return Objects.equals(getId(),person.getId())
&& Objects.equals(getName(),person.getName());
}
public static void main(String[] args) {
Person one = new Person(1,"one");
Person two = new Person(1,"one");
Map<Person,Integer> map = new HashMap<>();
map.put(one,1);
map.put(two,2); // will override 1 because one equals to two
System.out.println(map.size());
System.out.println(map.get(one));
System.out.println(map.get(two));
}
}