java集合-set练习题
练习题
TreeSet练习
题目：定义一个Employee类，该类包含：private 成员变量 name age birthtday,其中birthday为MyDate类的对象。并为每一个属性定义 getter, setter 方法，并重写toString方法输出name age birthday
MyDate类包含： private 成员变量 year month day，并为每一个属性定义getter,setter方法
创建该类的5个对象，并把这些对象放入TreeSet集合中
分别按照以下两种方式对集合中的元素进行排序，并遍历输出
- 使Employee实现 Comparable接口，并按name排序
- 创建TreeSet时传入 Comparator对象，按生日日期的先后顺序
code
package com.collection.set.practice;
/**
* 定义一个Employee类，
* 该类包含：private 成员变量 name age birthtday,
* 其中birthday为MyDate类的对象。
* 并为每一个属性定义 getter, setter 方法，
* 并重写toString方法输出name age birthday
*/
public class Employee implements Comparable{
private String name;
private int age;
private MyDate birthday;
public Employee(String name, int age, MyDate birthday) {
this.name = name;
this.age = age;
this.birthday = birthday;
}
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;
}
public MyDate getBirthday() {
return birthday;
}
public void setBirthday(MyDate birthday) {
this.birthday = birthday;
}
@Override
public String toString() {
return "Employee{" +
"name='" + name + '\'' +
", age=" + age +
", birthday=" + birthday +
'}';
}
// 按照姓名排序
@Override
public int compareTo(Object o) {
if(o instanceof Employee){
Employee e = (Employee)o;
return this.name.compareTo(e.name);
}
//return 0;
throw new RuntimeException("传入的数据类型不一致");
}
}
package com.collection.set.practice;
/**
* MyDate类包含： private 成员变量 year month day，
* 并为每一个属性定义getter,setter方法
*/
public class MyDate implements Comparable{
private int year;
private int month;
private int day;
// constructor
public MyDate(int year, int month, int day) {
this.year = year;
this.month = month;
this.day = day;
}
// getter and setter
public int getYear() {
return year;
}
public void setYear(int year) {
this.year = year;
}
public int getMonth() {
return month;
}
public void setMonth(int month) {
this.month = month;
}
public int getDay() {
return day;
}
public void setDay(int day) {
this.day = day;
}
// toString
@Override
public String toString() {
return "MyDate{" +
"year=" + year +
", month=" + month +
", day=" + day +
'}';
}
@Override
public int compareTo(Object o) {
if(o instanceof MyDate){
MyDate m = (MyDate)o;
//比较年
int minusYear = this.getYear() - m.getYear();
if(minusYear != 0){
return minusYear;
}
// 比较月
int minusMonth = this.getMonth() - m.getMonth();
if(minusMonth != 0){
return minusMonth;
}
return this.getDay()-m.getDay();
}
throw new RuntimeException("not consistant");
}
}
package com.collection.set.practice;
import com.sun.org.apache.bcel.internal.generic.RETURN;
import java.util.Comparator;
import java.util.Iterator;
import java.util.TreeSet;
/**
* 创建该类的5个对象，并把这些对象放入TreeSet集合中
*
* 分别按照以下两种方式对集合中的元素进行排序，并遍历输出
*
* 1. 使Employee实现 Comparable接口，并按name排序
* 2. 创建TreeSet时传入 Comparator对象，按生日日期的先后顺序
*/
public class EmployeeTest {
public static void main(String[] args) {
//test1();
test2();
}
// 问题2：按生日日期的先后排序
public static void test2(){
TreeSet set = new TreeSet(new Comparator() {
@Override
public int compare(Object o1, Object o2) {
if(o1 instanceof Employee && o2 instanceof Employee){
Employee e1 = (Employee)o1;
Employee e2 = (Employee)o2;
MyDate b1 = e1.getBirthday();
MyDate b2 = e2.getBirthday();
// // 方式1：
// // 比较年
// int minusYear = b1.getYear() - b2.getYear();
// if(minusYear != 0){
// return minusYear;
// }
// // 比较月
// int minusMonth = b1.getMonth() - b2.getMonth();
// if(minusMonth != 0){
// return minusMonth;
// }
// return b1.getDay()-b2.getDay();
// 方式2：在MyDate类中实现
return b1.compareTo(b2);
}
//return 0;
throw new RuntimeException("传入的数据类型不一致");
}
});
// CREATE 5 OBJECTS
Employee e1 = new Employee("joe",24,new MyDate(1997,11,7));
Employee e2 = new Employee("pepper",67,new MyDate(1945,1,23));
Employee e3 = new Employee("amy",15,new MyDate(2005,3,6));
Employee e4 = new Employee("jenny",26,new MyDate(1995,8,20));
Employee e5 = new Employee("lisa",25,new MyDate(1996,9,7));
set.add(e1);
set.add(e2);
set.add(e3);
set.add(e4);
set.add(e5);
Iterator iterator = set.iterator();
while(iterator.hasNext()){
System.out.println(iterator.next());
}
// // 问题1：使用自然排序
// public static void test1(){
// TreeSet set = new TreeSet();
// // CREATE 5 OBJECTS
// Employee e1 = new Employee("joe",24,new MyDate(1997,11,7));
// Employee e2 = new Employee("pepper",67,new MyDate(1945,1,23));
// Employee e3 = new Employee("amy",15,new MyDate(2005,3,6));
// Employee e4 = new Employee("jenny",26,new MyDate(1995,8,20));
// Employee e5 = new Employee("lisa",25,new MyDate(1996,9,7));
// set.add(e1);
// set.add(e2);
// set.add(e3);
// set.add(e4);
// set.add(e5);
//
// Iterator iterator = set.iterator();
// while(iterator.hasNext()){
// System.out.println(iterator.next());
// }
//
}
}
在List内去除重复数字值，要求尽量简单
hashSet过滤重复数据，效率较高
package com.collection.set;
import java.util.ArrayList;
import java.util.HashSet;
import java.util.List;
public class Demo01 {
public static void main(String[] args) {
List list = new ArrayList();
list.add(new Integer(1));
list.add(new Integer(1));
list.add(new Integer(2));
list.add(new Integer(2));
list.add(new Integer(4));
List list2 = duplicateList(list);
for(Object integer:list2){
System.out.println(integer);
}
}
private static List duplicateList(List list) {
HashSet set = new HashSet();
set.addAll(list);
return new ArrayList(set);
}
}
面试题
pubilc void test(){
HashSet set = new HashSet();
Person p1 = new Person(1001,"AA");
Person p2 = new Person(1002,"BB");
set.add(p1);
set.add(p2);
p1.name = "CC";
set.remove(p1);
System.out.println(set);
set.add(new Person(1001,"AA"));
System.out.println(set);
}
先计算 HashCode，再 equals()
code
package com.collection.set.practice;
public class Person {
int id;
String name;
public Person(int id, String name) {
this.id = id;
this.name = name;
}
public Person() {
}
@Override
public String toString() {
return "Person{" +
"id=" + id +
", name='" + name + '\'' +
'}';
}
@Override
public boolean equals(Object o) {
if (this == o) return true;
if (o == null || getClass() != o.getClass()) return false;
Person person = (Person) o;
if (id != person.id) return false;
return name != null ? name.equals(person.name) : person.name == null;
}
@Override
public int hashCode() {
int result = id;
result = 31 * result + (name != null ? name.hashCode() : 0);
return result;
}
}
package com.collection.set.practice;
import java.util.HashSet;
public class Test {
public static void main(String[] args) {
test1();
}
public static void test1(){
HashSet set = new HashSet();
Person p1 = new Person(1001,"AA");
Person p2 = new Person(1002,"BB");
set.add(p1);
set.add(p2);
// System.out.println(set);
// [Person{id=1001, name='AA'}, Person{id=1002, name='BB'}]
p1.name = "CC";
// System.out.println(set);
// [Person{id=1001, name='CC'}, Person{id=1002, name='BB'}]
set.remove(p1);
System.out.println(set);
// [Person{id=1002, name='BB'}, Person{id=1001, name='CC'}]
// p1是索引，remove计算P1的哈希值，可能找到的不是开始1001的索引，可能为空
set.add(new Person(1001,"CC"));
System.out.println(set);
// [Person{id=1002, name='BB'}, Person{id=1001, name='CC'}, Person{id=1001, name='CC'}]
// 拿 1001 和 "CC" 计算哈希值
set.add(new Person(1001,"AA"));
System.out.println(set);
// [Person{id=1002, name='BB'}, Person{id=1001, name='CC'}, Person{id=1001, name='CC'}, Person{id=1001, name='AA'}]
}
}