HashSet插入流程分析
我们都用过java里面的HashSet,也都知道这是通过哈希表(数组+链表)实现的。但是我们在使用过程中，很难感觉到散列表的特征，可以通过实验分析。
元素类型
public class Person {
private String name;
private int age;
public Person()
{
}
public Person(String name, int age) {
super();
this.name = name;
this.age = age;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
@Override
public String toString() {
return "Person [name=" + name + ", age=" + age + "]";
}
}
插入相同的元素
public class HashSetTest2 {
public static void main(String[] args) {
// TODO Auto-generated method stub
HashSet<Person> hs=new HashSet();
Person p1=new Person("张三",20);
Person p2=new Person("李四",22);
Person p3=new Person("王五",24);
hs.add(p1);
hs.add(p2);
hs.add(p3);
hs.add(p1);
System.out.println(hs.toString());
}
}
[Person [name=李四, age=22], Person [name=王五, age=24], Person [name=张三, age=20]]
众所周知，显然不行
插入值相同的不同元素，引用地址不一样，成员变量值一样
public class HashSetTest2 {
public static void main(String[] args) {
// TODO Auto-generated method stub
HashSet<Person> hs=new HashSet();
Person p1=new Person("张三",20);
Person p2=new Person("李四",22);
Person p3=new Person("王五",24);
Person p4=new Person("张三",20);
hs.add(p1);
hs.add(p2);
hs.add(p3);
hs.add(p4);
System.out.println(p1.hashCode());
System.out.println(p4.hashCode());
System.out.println(hs.toString());
}
}
366712642
1311053135
[Person [name=王五, age=24], Person [name=张三, age=20], Person [name=李四, age=22], Person [name=张三, age=20]]
可以，哈希值不一样
如果我们重写hashcode函数（默认哈希值和地址有关）
public class Person {
private String name;
private int age;
public Person()
{
}
public Person(String name, int age) {
super();
this.name = name;
this.age = age;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
@Override
public String toString() {
return "Person [name=" + name + ", age=" + age + "]";
}
@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;
}
}
public class HashSetTest2 {
public static void main(String[] args) {
// TODO Auto-generated method stub
HashSet<Person> hs=new HashSet();
Person p1=new Person("张三",20);
Person p2=new Person("李四",22);
Person p3=new Person("王五",24);
Person p4=new Person("张三",20);
hs.add(p1);
hs.add(p2);
hs.add(p3);
hs.add(p4);
System.out.println(p1.hashCode());
System.out.println(p4.hashCode());
System.out.println(hs.toString());
}
}
776470
776470
[Person [name=李四, age=22], Person [name=王五, age=24], Person [name=张三, age=20], Person [name=张三, age=20]]
HashSet认为，这两个哈希值相同，但是不是一个元素，可以加入。因为默认的equals函数只与地址有关。
现在不重写hashcode，只重写equals
public class Person {
private String name;
private int age;
public Person()
{
}
public Person(String name, int age) {
super();
this.name = name;
this.age = age;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
@Override
public String toString() {
return "Person [name=" + name + ", age=" + age + "]";
}
@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 != other.age)
return false;
if (name == null) {
if (other.name != null)
return false;
} else if (!name.equals(other.name))
return false;
return true;
}
}
366712642
1311053135
[Person [name=王五, age=24], Person [name=张三, age=20], Person [name=李四, age=22], Person [name=张三, age=20]]
哈希值不同，自然可以插入。
两个方法都重写
package myset;
public class Person {
private String name;
private int age;
public Person()
{
}
public Person(String name, int age) {
super();
this.name = name;
this.age = age;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
@Override
public String toString() {
return "Person [name=" + name + ", age=" + age + "]";
}
@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;
}
@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 != other.age)
return false;
if (name == null) {
if (other.name != null)
return false;
} else if (!name.equals(other.name))
return false;
return true;
}
}
776470
776470
[Person [name=李四, age=22], Person [name=王五, age=24], Person [name=张三, age=20]]
终于插不进去了。
可见。HashSet插入过程是先比较哈希值，与原有元素都不同的话，直接插入。
相同的话，使用equals方法，相等就不插入。
另外需要说明，remove方法也是先哈希后equals