【Java学习笔记】33：Set接口及其实现类

时间：2021-07-01 16:50:12

List和Set都是collection接口下的，本节学习Set接口及其两个实现类HashSet和TreeSet。

Set中的元素不可重复，无序。几个常用的方法remove()，clear()等和上一节的List一样，就不写了。


HashSet相当于一个哈希表(操作时间复杂度一般是常数级)，用hashcode和equals()保证不重复。

TreeSet相当于一个树(操作时间复杂度一般是对数级)，可以实现comparable接口以作排序。


*测试HashSet

package day33;

import java.util.HashSet;
import java.util.Iterator;

class Person
{
private int id;
private String name;

public Person(int id, String name) {
super();
this.id = id;
this.name = name;
}

@Override
public String toString() {
return id+":"+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 class Test {

public static void main(String[] args) {
HashSet<String> hs=new HashSet<String>();

//.添加
hs.add("three");
hs.add("one");
hs.add("two");
hs.add("one");//尝试重复添加

//.遍历
for(String e:hs)
{
System.out.print(e+" ");
}
System.out.println();

//.迭代器
Iterator<String> it=hs.iterator();
while(it.hasNext())
{
System.out.print(it.next()+" ");
}
System.out.println();

/**************************************/

HashSet<Person> hs2=new HashSet<Person>();

Person p1=new Person(1,"Tom");
Person p2=new Person(2,"Flora");
Person p3=new Person(3,"Json");
Person p4=new Person(2,"Flora");

//.添加
hs2.add(p1);
hs2.add(p2);
hs2.add(p3);
hs2.add(p4);

//.遍历
for(Person e:hs2)
{
System.out.print(e+" ");
}
System.out.println();

//.迭代器
Iterator<Person> it2=hs2.iterator();
while(it2.hasNext())
{
System.out.print(it2.next()+" ");
}
System.out.println();

}
}
运行结果：

one three two 
one three two 
3:Json 1:Tom 2:Flora 2:Flora 
3:Json 1:Tom 2:Flora 2:Flora 

可以看到泛型具体为String时是没有问题的，当用自己写的Person类时并没有识别出这两个2:Flora是相同的，而我们说Set是不能重复的，这就需要我们覆写equals()和hashCode()方法，以告诉它"如何判断相同"。

*覆写equals()和hashCode()方法

package day33;

import java.util.HashSet;
import java.util.Iterator;

class Person
{
private int id;
private String name;

public Person(int id, String name) {
super();
this.id = id;
this.name = name;
}

@Override
public boolean equals(Object obj) {
Person p1=(Person)obj;//强制转换
return p1.id==this.id;//自己为这个类定义的判定一致的方式
}

@Override
public int hashCode() {
//如何指定唯一的hashCode是一门学问，这里只是一个演示
return this.id+this.name.hashCode();
}

@Override
public String toString() {
return id+":"+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 class Test {

public static void main(String[] args) {

HashSet<Person> hs2=new HashSet<Person>();

Person p1=new Person(1,"Tom");
Person p2=new Person(2,"Flora");
Person p3=new Person(3,"Json");
Person p4=new Person(2,"Flora");

//.添加
hs2.add(p1);
hs2.add(p2);
hs2.add(p3);
hs2.add(p4);

//.遍历
for(Person e:hs2)
{
System.out.print(e+" ");
}
System.out.println();

//.迭代器
Iterator<Person> it2=hs2.iterator();
while(it2.hasNext())
{
System.out.print(it2.next()+" ");
}
System.out.println();

}
}
运行结果：

1:Tom 2:Flora 3:Json 
1:Tom 2:Flora 3:Json 

*用Eclipse自动生成这两个方法

【Java学习笔记】33：Set接口及其实现类

package day33;

import java.util.HashSet;
import java.util.Iterator;

class Person
{
private int id;
private String name;

public Person(int id, String name) {
super();
this.id = id;
this.name = name;
}

//@Override
//public boolean equals(Object obj) {
//Person p1=(Person)obj;//强制转换
//return p1.id==this.id;//自己为这个类定义的判定一致的方式
//}
//
//@Override
//public int hashCode() {
////如何指定唯一的hashCode是一门学问，这里只是一个演示
//return this.id+this.name.hashCode();
//}

@Override
public String toString() {
return id+":"+name;
}

@Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result + id;
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 (id != other.id)
return false;
if (name == null) {
if (other.name != null)
return false;
} else if (!name.equals(other.name))
return false;
return true;
}

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 class Test {

public static void main(String[] args) {

HashSet<Person> hs2=new HashSet<Person>();

Person p1=new Person(1,"Tom");
Person p2=new Person(2,"Flora");
Person p3=new Person(3,"Json");
Person p4=new Person(2,"Flora");

//.添加
hs2.add(p1);
hs2.add(p2);
hs2.add(p3);
hs2.add(p4);

//.遍历
for(Person e:hs2)
{
System.out.print(e+" ");
}
System.out.println();

//.迭代器
Iterator<Person> it2=hs2.iterator();
while(it2.hasNext())
{
System.out.print(it2.next()+" ");
}
System.out.println();

}
}
运行结果：

1:Tom 3:Json 2:Flora 
1:Tom 3:Json 2:Flora


*测试TreeSet

package day33;

import java.util.TreeSet;

class Person
{
private int id;
private String name;

public Person(int id, String name) {
super();
this.id = id;
this.name = name;
}

@Override
public String toString() {
return id+":"+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 class Test {

public static void main(String[] args) {

TreeSet<String> ts=new TreeSet();

//.添加
ts.add("one");
ts.add("two");
ts.add("three");
ts.add("one");

//.遍历
for(String e:ts)
{
System.out.print(e+" ");
}
/***************************************/

TreeSet<Person> ts2=new TreeSet<Person>();

Person p1=new Person(1,"Tom");
Person p2=new Person(2,"Flora");
Person p3=new Person(3,"Json");
Person p4=new Person(2,"Flora");

//.添加
ts2.add(p1);
ts2.add(p2);
ts2.add(p3);
ts2.add(p4);

//.遍历
for(Person e:ts2)
{
System.out.print(e+" ");
}
}
}
这时出现了错误：

【Java学习笔记】33：Set接口及其实现类
放到TreeSet里的对象的类必须实现comparable接口(String本身已经实现了这个接口)，并且实现这个接口中的compareTo()方法。


*实现了comparable接口的TreeSet测试

package day33;

import java.util.TreeSet;

class Person implements Comparable<Person>
{
private int id;
private String name;

public Person(int id, String name) {
super();
this.id = id;
this.name = name;
}

@Override
public String toString() {
return id+":"+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;
}

@Override
public int compareTo(Person o) {
if(this.id>o.id)
return 1;
else if(this.id<o.id)
return -1;
return 0;
}

}

public class Test {

public static void main(String[] args) {

TreeSet<String> ts=new TreeSet();

//.添加
ts.add("one");
ts.add("two");
ts.add("three");
ts.add("one");

//.遍历
for(String e:ts)
{
System.out.print(e+" ");
}
/***************************************/

TreeSet<Person> ts2=new TreeSet<Person>();

Person p2=new Person(2,"Flora");
Person p1=new Person(1,"Tom");
Person p3=new Person(3,"Json");
Person p4=new Person(2,"Flora");

//.添加
ts2.add(p2);
ts2.add(p1);
ts2.add(p3);
ts2.add(p4);

//.遍历
for(Person e:ts2)
{
System.out.print(e+" ");
}
}
}
运行结果：

one three two 1:Tom 2:Flora 3:Json 


可以看到Person对象是升序排列的，如果要降序排列，那么只要修改为：

@Override
public int compareTo(Person o) {
if(this.id>o.id)
return -1;
else if(this.id<o.id)
return 1;
return 0;
}
运行结果：

one three two 3:Json 2:Flora 1:Tom 




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