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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().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 orgList = generateListContatinsDuplicates();//why cant be write as a universally method by param to generate wanted Object? List removeDuplicateBeanOfList = removeDuplicateBeanOfList(orgList);// 无序 List removeDuplicateBeanOfList2 = removeDuplicateBeanOfList2(orgList);//有序 List removeDuplicateBeanOfList3 = removeDuplicateBeanOfList3(orgList);//有序 List generateListContatinsDuplicates = generateListContatinsDuplicates("1",5000); List removeDupliStringList = removeDupliStringList(generateListContatinsDuplicates); List removeDupliStringList8 = removeDupliStringListJAVA8(generateListContatinsDuplicates); List generateListContatinsDuplicates2 = generateListContatinsDuplicates(new Student(),5000); List removeDuplicateBeanOfList4 = removeDuplicateBeanOfList(generateListContatinsDuplicates2);//无序 List removeDuplicateBeanOfList5 = removeDuplicateBeanOfList2(generateListContatinsDuplicates2);//有序 List removeDuplicateBeanOfList6 = removeDuplicateBeanOfList3(generateListContatinsDuplicates2);//有序 } /** * @return * @description generate List may contains Duplicates */ public static List generateListContatinsDuplicates(){ long startTime = System.currentTimeMillis(); List list = 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")); } } long endTime = System.currentTimeMillis(); long time = endTime-startTime; log.debug("generateListContatinsDuplicates time:"+time); return list; } /** * @param * @return * @description generate Duplicates */ @SuppressWarnings("unchecked") public static List generateListContatinsDuplicates(T str,int count){ long startTime = System.currentTimeMillis(); List list = new ArrayList (); if(str!=null && str instanceof Student){ List studentList = new ArrayList (); 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 ) studentList; } if(str!=null && str instanceof String){ List strList = new ArrayList (); 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 ) 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 removeDupliStringList(List stringList) { long startTime = System.currentTimeMillis(); Set set = new LinkedHashSet<>(); set.addAll(stringList); stringList.clear(); stringList.addAll(set); long endTime = System.currentTimeMillis(); long time = endTime-startTime; log.debug("removeDupliStringList(List stringList) time:"+time); return stringList; } /** * @param stringList * @return * @description remove repeated String use java8 API */ public static List removeDupliStringListJAVA8(List stringList) { long startTime = System.currentTimeMillis(); @SuppressWarnings("unused") List unique = stringList.stream().distinct().collect(Collectors.toList()); long endTime = System.currentTimeMillis(); long time = endTime-startTime; log.debug("removeDupliStringListJAVA8(List stringList) time:"+time); return stringList; } /** * @param list * @return * @description remove repeated javabean */ public static List removeDuplicateBeanOfList(List list) { long startTime = System.currentTimeMillis(); // 重复数据 List 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 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 but a List log4j.properties:
# \u4E5F\u5C31\u662F\u8BF4\u53EA\u663E\u793A\u6BD4\u5927\u4E8E\u7B49\u4E8E\u5F53\u524D\u7EA7\u522B\u7684\u4FE1\u606Flog4j.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
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