在HashMap與HashSet中,是不容許元素重複的。ide
在斷定時比較元素的hashCode()是否相等,equals()是否返回true,那麼問題來了this
這是一個Person類對象
public class Person {
private String id;
private String name;
private Integer age;
public String getId() {
return id;
}
public void setId(String id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public Integer getAge() {
return age;
}
public void setAge(Integer age) {
this.age = age;
}
@Override
public String toString() {
return "Person [id=" + id + ", name=" + name + ", age=" + age + "]";
}
@Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result + ((age == null) ? 0 : age.hashCode());
result = prime * result + ((id == null) ? 0 : id.hashCode());
result = prime * result + ((name == null) ? 0 : name.hashCode());
return result;
}
@Override
public boolean equals(Object obj) {
if (this == obj)
return true;
if (obj == null)
return false;
if (getClass() != obj.getClass())
return false;
Person other = (Person) obj;
if (age == null) {
if (other.age != null)
return false;
} else if (!age.equals(other.age))
return false;
if (id == null) {
if (other.id != null)
return false;
} else if (!id.equals(other.id))
return false;
if (name == null) {
if (other.name != null)
return false;
} else if (!name.equals(other.name))
return false;
return true;
}
/*@Override
public int hashCode() {
System.out.println("id = " + id + ";id.hashCode = " + id.hashCode());
return id.hashCode();
}
@Override
public boolean equals(Object obj) {
System.out.println("equals");
Person p = (Person)obj;
if(this.id == p.id) {
return true;
}else{
return false;
}
}*/
}get
這是一個Test類hash
public class TestHashCode {
public static void main(String[] args) {
Set<Person> set = new HashSet<Person>();
Person p1 = new Person();
p1.setId("11");
p1.setName("張三");
p1.setAge(20);
Person p2 = new Person();
p2.setId("11");
p2.setName("李四");
p2.setAge(30);
set.add(p1);
set.add(p2);
System.out.println(p1.hashCode() == p2.hashCode());
System.out.println(p1.equals(p2));
for (Person person : set) {
System.out.println(person.toString());
}
}
}class
若是Person的id屬性做爲主鍵,且只重寫equals方法,咱們認爲id相等的兩個對象爲同一個對象方法
此時p1.equals(p2)返回true,可是hashCode()卻不相等,而hashSet判斷惟一的依據是hashCode()值相等且equals()返回trueim
因此咱們的預期是p1和p2爲同一個對象,可是hashSet認爲他們是兩個對象。static
因此咱們須要同時重寫hashCode()和equals()方法。new