List去重与equals/hashcode
List去重:
stackflow上关于list去除重复对象是否要重写equals和hashcode方法有争议;有的人认为重写这两个方法是具有侵入性的,有的人认为重写此二者使工作更明确的最佳实践(https://developer.jboss.org/wiki/EqualsandHashCode).有待研究.
使用set来为List去重的原理是利用set中元素不重复的特征,那使用Set来去重List中重复的Javabean一定复写此JavaBean的equals和hashcode方法吗?答案是否定的,为什么要重写equals和hashcode来去重,是因为原始的Object的equals的底层其实就是==,如果不重写equals,那么只要是new出来的bean,他们物理地址都是不同的.如果想要按照javabean中属性存在不同作为区分两者是否是重复元素的标准,那么未被复写的继承自Object的equals方法范围太大了,不够细致,所以要重写equals方法.
之所以重写equals时一般都要重写hashcode是为了提高大量对象是否重复时判断的效率,如果不重写hashcode,那就只有依次在对象见使用equals比较;重写hashcode 大大减少了比较的次数,效率提升.如果重写了equals而未重写hashcode方法，可能就会出现两个没有关系的对象equals相同的（因为equal都是根据对象的特征进行重写的），但hashcode确实不相同的。
对于两个对象，Java要求如下：
equals()相等，hashcode()一定相等；
quals()不等，hashcode()可能相等，也可能不等;
hashcode()不等，一定能推出equals()也不等；
hashcode()相等，equals()可能相等，也可能不等。
Student.class
package test1;
/**
* @author NSNP736
* @description StudentBean
*/
public class Student {
private String Local = "";
private String Id = "";
private String number = "";
private String Code = "";
private String rtype = "";
private String Id10digit = "";
private String nbRtype = "";
private String fullNm = "";
private String gender = "";
private String birthDt = "";
private String Status = "";
private String cusId = "";
public Student() {
super();
}
public Student(String local, String id, String number, String code, String rtype, String id10digit,
String nbRtype, String fullNm, String gender, String birthDt, String status, String cusId) {
super();
Local = local;
Id = id;
this.number = number;
Code = code;
this.rtype = rtype;
Id10digit = id10digit;
this.nbRtype = nbRtype;
this.fullNm = fullNm;
this.gender = gender;
this.birthDt = birthDt;
Status = status;
this.cusId = cusId;
}
public String getLocal() {
return Local;
}
public void setLocal(String local) {
Local = local;
}
public String getId() {
return Id;
}
public void setId(String id) {
Id = id;
}
public String getNumber() {
return number;
}
public void setNumber(String number) {
this.number = number;
}
public String getCode() {
return Code;
}
public void setCode(String code) {
Code = code;
}
public String getRtype() {
return rtype;
}
public void setRtype(String rtype) {
this.rtype = rtype;
}
public String getId10digit() {
return Id10digit;
}
public void setId10digit(String id10digit) {
Id10digit = id10digit;
}
public String getNbRtype() {
return nbRtype;
}
public void setNbRtype(String nbRtype) {
this.nbRtype = nbRtype;
}
public String getFullNm() {
return fullNm;
}
public void setFullNm(String fullNm) {
this.fullNm = fullNm;
}
public String getGender() {
return gender;
}
public void setGender(String gender) {
this.gender = gender;
}
public String getBirthDt() {
return birthDt;
}
public void setBirthDt(String birthDt) {
this.birthDt = birthDt;
}
public String getStatus() {
return Status;
}
public void setStatus(String status) {
Status = status;
}
public String getCusId() {
return cusId;
}
public void setCusId(String cusId) {
this.cusId = cusId;
}
@Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result + ((Code == null) ? 0 : Code.hashCode());
result = prime * result + ((Id == null) ? 0 : Id.hashCode());
result = prime * result + ((Id10digit == null) ? 0 : Id10digit.hashCode());
result = prime * result + ((Local == null) ? 0 : Local.hashCode());
result = prime * result + ((Status == null) ? 0 : Status.hashCode());
result = prime * result + ((birthDt == null) ? 0 : birthDt.hashCode());
result = prime * result + ((cusId == null) ? 0 : cusId.hashCode());
result = prime * result + ((fullNm == null) ? 0 : fullNm.hashCode());
result = prime * result + ((gender == null) ? 0 : gender.hashCode());
result = prime * result + ((nbRtype == null) ? 0 : nbRtype.hashCode());
result = prime * result + ((number == null) ? 0 : number.hashCode());
result = prime * result + ((rtype == null) ? 0 : rtype.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;
Student other = (Student) obj;
if (Code == null) {
if (other.Code != null)
return false;
} else if (!Code.equals(other.Code))
return false;
if (Id == null) {
if (other.Id != null)
return false;
} else if (!Id.equals(other.Id))
return false;
if (Id10digit == null) {
if (other.Id10digit != null)
return false;
} else if (!Id10digit.equals(other.Id10digit))
return false;
if (Local == null) {
if (other.Local != null)
return false;
} else if (!Local.equals(other.Local))
return false;
if (Status == null) {
if (other.Status != null)
return false;
} else if (!Status.equals(other.Status))
return false;
if (birthDt == null) {
if (other.birthDt != null)
return false;
} else if (!birthDt.equals(other.birthDt))
return false;
if (cusId == null) {
if (other.cusId != null)
return false;
} else if (!cusId.equals(other.cusId))
return false;
if (fullNm == null) {
if (other.fullNm != null)
return false;
} else if (!fullNm.equals(other.fullNm))
return false;
if (gender == null) {
if (other.gender != null)
return false;
} else if (!gender.equals(other.gender))
return false;
if (nbRtype == null) {
if (other.nbRtype != null)
return false;
} else if (!nbRtype.equals(other.nbRtype))
return false;
if (number == null) {
if (other.number != null)
return false;
} else if (!number.equals(other.number))
return false;
if (rtype == null) {
if (other.rtype != null)
return false;
} else if (!rtype.equals(other.rtype))
return false;
return true;
}
@Override
public String toString() {
return "Student [Local=" + Local + ", Id=" + Id + ", number=" + number + ", Code=" + Code + ", rtype=" + rtype
+ ", Id10digit=" + Id10digit + ", nbRtype=" + nbRtype + ", fullNm=" + fullNm + ", gender=" + gender
+ ", birthDt=" + birthDt + ", Status=" + Status + ", cusId=" + cusId + "]";
}
}
去重方法:
package test1;
import java.util.ArrayList;
import java.util.HashSet;
import java.util.Iterator;
import java.util.LinkedHashSet;
import java.util.List;
import java.util.Random;
import java.util.Set;
import java.util.stream.Collectors;
import org.apache.log4j.Logger;
import org.junit.Assert;
/**
* @author zhangdi
* @description Remove duplicates from Lists
*/
public class TestListMethod {
private final static Logger log = Logger.getLogger(TestListMethod.class);
@SuppressWarnings("unused")
public static void main(String[] args) {
List<Student> orgList = generateListContatinsDuplicates();//why cant be write as a universally method by param to generate wanted Object?
List<Student> removeDuplicateBeanOfList = removeDuplicateBeanOfList(orgList);// 无序
List<Student> removeDuplicateBeanOfList2 = removeDuplicateBeanOfList2(orgList);//有序
List<Student> removeDuplicateBeanOfList3 = removeDuplicateBeanOfList3(orgList);//有序
List<String> generateListContatinsDuplicates = generateListContatinsDuplicates("1",5000);
List<String> removeDupliStringList = removeDupliStringList(generateListContatinsDuplicates);
List<String> removeDupliStringList8 = removeDupliStringListJAVA8(generateListContatinsDuplicates);
List<Student> generateListContatinsDuplicates2 = generateListContatinsDuplicates(new Student(),5000);
List<Student> removeDuplicateBeanOfList4 = removeDuplicateBeanOfList(generateListContatinsDuplicates2);//无序
List<Student> removeDuplicateBeanOfList5 = removeDuplicateBeanOfList2(generateListContatinsDuplicates2);//有序
List<Student> removeDuplicateBeanOfList6 = removeDuplicateBeanOfList3(generateListContatinsDuplicates2);//有序
}
/**
* @return
* @description generate List may contains Duplicates
*/
public static List<Student> generateListContatinsDuplicates(){
long startTime = System.currentTimeMillis();
List<Student> list = new ArrayList<Student>();
for (int i = 0; i < 10; i++) {
// local,id,number,code,rtype,Id10digit,nbRtype,fullNm,gender,birthDt,Status,cusId
list.add(new Student(String.valueOf(i), "71", "C999", "001", "P", "", "02", "zhangsanfeng", "M", "19950212","02", "C01"));
if (new Random().nextBoolean()) {
list.add(new Student(String.valueOf(i), "71", "C999", "001", "P", "", "02", "zhangsanfeng", "M","19950212", "02", "C01"));
}
}
long endTime = System.currentTimeMillis();
long time = endTime-startTime;
log.debug("generateListContatinsDuplicates time:"+time);
return list;
}
/**
* @param <T>
* @return
* @description generate Duplicates
*/
@SuppressWarnings("unchecked")
public static <T> List<T> generateListContatinsDuplicates(T str,int count){
long startTime = System.currentTimeMillis();
List<T> list = new ArrayList<T>();
if(str!=null && str instanceof Student){
List<Student> studentList = new ArrayList<Student>();
for (int i = 0; i < count; i++) {
// local,id,number,code,rtype,Id10digit,nbRtype,fullNm,gender,birthDt,Status,cusId
studentList.add(new Student(String.valueOf(i), "71", "C999", "001", "P", "", "02", "zhangsanfeng", "M", "19950212",
"02", "C01"));
if (new Random().nextBoolean()) {
studentList.add(new Student(String.valueOf(i), "71", "C999", "001", "P", "", "02", "zhangsanfeng", "M",
"19950212", "02", "C01"));
}
}
list = (List<T>) studentList;
}
if(str!=null && str instanceof String){
List<String> strList = new ArrayList<String>();
for (int i = 0; i < count; i++) {
strList.add("String"+String.valueOf(i));
if (new Random().nextBoolean()) {
strList.add("String"+String.valueOf(i));
}
}
list = (List<T>) strList;
}
long endTime = System.currentTimeMillis();
long time = endTime-startTime;
log.debug("generateListContatinsDuplicates(T) time:"+time);
return list;
}
/**
* @param stringList
* @return
* @description remove repeated String
*/
public static List<String> removeDupliStringList(List<String> stringList) {
long startTime = System.currentTimeMillis();
Set<String> set = new LinkedHashSet<>();
set.addAll(stringList);
stringList.clear();
stringList.addAll(set);
long endTime = System.currentTimeMillis();
long time = endTime-startTime;
log.debug("removeDupliStringList(List<String> stringList) time:"+time);
return stringList;
}
/**
* @param stringList
* @return
* @description remove repeated String use java8 API
*/
public static List<String> removeDupliStringListJAVA8(List<String> stringList) {
long startTime = System.currentTimeMillis();
@SuppressWarnings("unused")
List<String> unique = stringList.stream().distinct().collect(Collectors.toList());
long endTime = System.currentTimeMillis();
long time = endTime-startTime;
log.debug("removeDupliStringListJAVA8(List<String> stringList) time:"+time);
return stringList;
}
/**
* @param list
* @return
* @description remove repeated javabean
*/
public static List<Student> removeDuplicateBeanOfList(List<Student> list) {
long startTime = System.currentTimeMillis();
// 重复数据
List<Student> list2 = new ArrayList<>();
/*
for (int i = 0; i < 10; i++) {
// local,id
// ,number,code,rtype,Id10digit,nbRtype,fullNm,gender,birthDt,Status,cusId
list.add(new Student(String.valueOf(i), "71", "C999", "001", "P", "", "02", "zhangsanfeng", "M", "19950212",
"02", "C01"));
if (new Random().nextBoolean()) {
list.add(new Student(String.valueOf(i), "71", "C999", "001", "P", "", "02", "zhangsanfeng", "M",
"19950212", "02", "C01"));
}
}*/
// hashset
HashSet<Student> set = new HashSet<>();
for (Student pol : list) {
boolean add = set.add(pol);
if (!add) {
list2.add(pol);
}
}
// 比较
Assert.assertEquals(list.size(), list2.size() + set.size());
System.out.println(
"list.size():" + list.size() + " " + "list2.size():" + list2.size() + " " + "set.size():" + set.size());
// return Arrays.asList(set.toArray());//this clause not return a
// List<Student> but a List<Object>
long endTime = System.currentTimeMillis();
long time = endTime-startTime;
log.debug("removeDuplicateBeanOfList(List<Student> list) time:"+time);
return new ArrayList<Student>(set);
}
/**
* @param list
* @return
* @description https://stackoverflow.com/questions/5741038/remove-duplicates-from-arraylists
* You may need to override "equals()" so that these elements are considered equals
*/
public static List<Student> removeDuplicateBeanOfList2(List<Student> list) {
long startTime = System.currentTimeMillis();
//remove duplicates if any
Set<Student> setItems = new LinkedHashSet<Student>(list);
list.clear();
list.addAll(setItems);
long endTime = System.currentTimeMillis();
long time = endTime-startTime;
log.debug(" removeDuplicateBeanOfList2(List<Student> list) time:"+time);
return list;
}
/**
* @param list
* @return
* @description if one property of List's javabean repeat is the standard to remove DuplicateBean ,may need below method
*/
public static List<Student> removeDuplicateBeanOfList3(List<Student> list) {
long startTime = System.currentTimeMillis();
List<Student> result = new ArrayList<Student>();
Set<String> titles = new HashSet<String>();
for( Student item : list ) {
if( titles.add( item.toString())) {
result.add( item );
}
}
long endTime = System.currentTimeMillis();
long time = endTime-startTime;
log.debug("removeDuplicateBeanOfList3(List<Student> list) time:"+time);
return result;
}
/**
* @param list
* @return
* @description truely "remove" DuplicateBean not rely on overriding equals() and hashcode()
*/
@SuppressWarnings("unchecked")
public static List<Student> removeDuplicateBeanOfList4(List<Student> list) {
long startTime = System.currentTimeMillis();
Set<String> subtitles = new HashSet<String>();
Iterator<Student> it = null;
for (it = list.iterator(); it.hasNext(); ) {
if (!subtitles.add(it.next().toString())) {
it.remove();
}
}
long endTime = System.currentTimeMillis();
long time = endTime-startTime;
log.debug("removeDuplicateBeanOfList4(List<Student> list) time:"+time);
return (List<Student>) it;
}
/**
* @param list
* @return
* @description use java8API to remove Duplicates
*/
public static List<Student> removeDuplicateBeanOfListJAVA8(List<Student> list) {
long startTime = System.currentTimeMillis();
List<Student> deduped;
deduped = list.stream()
.distinct()
.collect(Collectors.toCollection(ArrayList::new));
long endTime = System.currentTimeMillis();
long time = endTime-startTime;
log.debug("removeDuplicateBeanOfListJAVA8(List<Student> list) time:"+time);
return deduped;
}
/**
* @param list
* @return
* @description use java8API to remove Duplicates
*/
public static List<Student> removeDuplicateBeanOfListJAVA8_2(List<Student> list) {
long startTime = System.currentTimeMillis();
List<Student> personList = new ArrayList<>();
// 去重
list.stream().forEach(
p -> {
if (!personList.contains(p)) {
personList.add(p);
}
}
);
System.out.println(personList);
long endTime = System.currentTimeMillis();
long time = endTime-startTime;
log.debug("removeDuplicateBeanOfListJAVA8(List<Student> list) time:"+time);
return personList;
}
}
log4j.properties:
# \u4E5F\u5C31\u662F\u8BF4\u53EA\u663E\u793A\u6BD4\u5927\u4E8E\u7B49\u4E8E\u5F53\u524D\u7EA7\u522B\u7684\u4FE1\u606F
log4j.rootLogger=debug,appender1
#log4j.rootLogger=info,appender1
#log4j.rootLogger=error,appender1
# \u8F93\u51FA\u5230\u63A7\u5236\u53F0
log4j.appender.appender1=org.apache.log4j.ConsoleAppender
# \u6837\u5F0F\u4E3A TTCCLayout
log4j.appender.appender1.layout=org.apache.log4j.TTCCLayout参考:https://stackoverflow.com/questions/5741038/remove-duplicates-from-arraylists;
https://blog.csdn.net/jiaobuchong/article/details/54412094
