List去重与equals/hashcode

偏执的太偏执、 2022-05-27 07:08 232阅读 0赞

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][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_stackoverflow.com_questions_5741038_remove-duplicates-from-arraylists];  
[https://blog.csdn.net/jiaobuchong/article/details/54412094][https_blog.csdn.net_jiaobuchong_article_details_54412094]

[https_developer.jboss.org_wiki_EqualsandHashCode]: https://developer.jboss.org/wiki/EqualsandHashCode
[https_stackoverflow.com_questions_5741038_remove-duplicates-from-arraylists]: https://stackoverflow.com/questions/5741038/remove-duplicates-from-arraylists
[https_blog.csdn.net_jiaobuchong_article_details_54412094]: https://blog.csdn.net/jiaobuchong/article/details/54412094