How to use Comparator in Java to sort

Question

I learned how to use the comparable but I m having difficulty with the Comparator  I am having a error in my code    Exception in thread  main  java lang ClassCastException  New People cannot be cast to java lang Comparable  at java util Arrays mergeSort Unknown Source   at java util Arrays sort Unknown Source   at java util Collections sort Unknown Source   at New TestPeople main TestPeople java 18    Here is my code   import java util Comparator   public class People implements Comparator      private int id     private String info     private double price      public People int newid  String newinfo  double newprice           setid newid          setinfo newinfo          setprice newprice            public int getid            return id           public void setid int id           this id   id           public String getinfo            return info           public void setinfo String info           this info   info           public double getprice            return price           public void setprice double price           this price   price           public int compare Object obj1  Object obj2           Integer p1     People  obj1  getid           Integer p2     People  obj2  getid            if  p1  gt  p2               return 1           else if  p1  lt  p2              return -1           else              return 0                     import java util ArrayList  import java util Collections   public class TestPeople       public static void main String   args            ArrayList peps   new ArrayList             peps add new People 123   M   14 25            peps add new People 234   M   6 21            peps add new People 362   F   9 23            peps add new People 111   M   65 99            peps add new People 535   F   9 23             Collections sort peps            for  int i   0  i  lt  peps size    i                 System out println peps get i                        I believe it has to do something with the casting in the compare method but I was playing around with it and still could not find the solution

User · Answer

Java 8 added a new way of making Comparators that reduces the amount of code you have to write, Comparator.comparing. Also check out Comparator.reversed

Here's a sample

import org.junit.Test;

import java.util.ArrayList;
import java.util.Comparator;
import java.util.List;

import static org.junit.Assert.assertTrue;

public class ComparatorTest {

    @Test
    public void test() {
        List<Person> peopleList = new ArrayList<>();
        peopleList.add(new Person("A", 1000));
        peopleList.add(new Person("B", 1));
        peopleList.add(new Person("C", 50));
        peopleList.add(new Person("Z", 500));
        //sort by name, ascending
        peopleList.sort(Comparator.comparing(Person::getName));
        assertTrue(peopleList.get(0).getName().equals("A"));
        assertTrue(peopleList.get(peopleList.size() - 1).getName().equals("Z"));
        //sort by name, descending
        peopleList.sort(Comparator.comparing(Person::getName).reversed());
        assertTrue(peopleList.get(0).getName().equals("Z"));
        assertTrue(peopleList.get(peopleList.size() - 1).getName().equals("A"));
        //sort by age, ascending
        peopleList.sort(Comparator.comparing(Person::getAge));
        assertTrue(peopleList.get(0).getAge() == 1);
        assertTrue(peopleList.get(peopleList.size() - 1).getAge() == 1000);
        //sort by age, descending
        peopleList.sort(Comparator.comparing(Person::getAge).reversed());
        assertTrue(peopleList.get(0).getAge() == 1000);
        assertTrue(peopleList.get(peopleList.size() - 1).getAge() == 1);
    }

    class Person {

        String name;
        int age;

        Person(String n, int a) {
            name = n;
            age = a;
        }

        public String getName() {
            return name;
        }

        public int getAge() {
            return age;
        }

        public void setName(String name) {
            this.name = name;
        }

        public void setAge(int age) {
            this.age = age;
        }
    }



}

User · Answer

Do not waste time implementing Sorting Algorithm by your own  Instead  use     Collections sort     to sort data

User · Answer

You should use the overloaded sort peps  new People    method  import java util ArrayList  import java util Collections  import java util Comparator  import java util List   public class Test        public static void main String   args                 List lt People gt  peps   new ArrayList lt  gt              peps add new People 123   M   14 25            peps add new People 234   M   6 21            peps add new People 362   F   9 23            peps add new People 111   M   65 99            peps add new People 535   F   9 23             Collections sort peps  new People   new ComparatorId              for  int i   0  i  lt  peps size    i                          System out println peps get i                       class People          private int id         private String info         private double price          public People                              public People int newid  String newinfo  double newprice               setid newid              setinfo newinfo              setprice newprice                    public int getid                return id                   public void setid int id               this id   id                   public String getinfo                return info                   public void setinfo String info               this info   info                   public double getprice                return price                   public void setprice double price               this price   price                   class ComparatorId implements Comparator lt People gt                     Override         public int compare People obj1  People obj2                   Integer p1   obj1 getid                   Integer p2   obj2 getid                    if  p1  gt  p2                       return 1                   else if  p1  lt  p2                      return -1                   else                      return 0

User · Answer

Two corrections    You have to make an ArrayList of People objects   ArrayList lt People gt  preps   new ArrayList lt People gt       After adding the objects to the preps  use    Collections sort preps  new CompareId        Also  add a CompareId class as    class CompareId implements Comparator         public int compare Object obj1  Object obj2              People t1    People obj1            People t2    People obj2             if  t1 marks  gt  t2 marks                return 1             else               return -1

User · Answer

Use People implements Comparable lt People gt  instead  this defines the natural ordering for People   A Comparator lt People gt  can also be defined in addition  but People implements Comparator lt People gt  is not the right way of doing things   The two overloads for Collections sort are different     lt T extends Comparable lt   super T gt  gt  void sort List lt T gt  list    Sorts Comparable objects using their natural ordering   lt T gt  void sort List lt T gt  list  Comparator lt   super T gt  c    Sorts whatever using a compatible Comparator    You re confusing the two by trying to sort a Comparator  which is again why it doesn t make sense that Person implements Comparator lt Person gt    Again  to use Collections sort  you need one of these to be true    The type must be Comparable  use the 1-arg sort  A Comparator for the type must be provided  use the 2-args sort    Related questions   When to use Comparable vs Comparator Sorting an ArrayList of Contacts     Also  do not use raw types in new code  Raw types are unsafe  and it s provided only for compatibility   That is  instead of this   ArrayList peps   new ArrayList       BAD    No generic safety    you should ve used the typesafe generic declaration like this   List lt People gt  peps   new ArrayList lt People gt        GOOD      You will then find that your code doesn t even compile   That would be a good thing  because there IS something wrong with the code  Person does not implements Comparable lt Person gt    but because you used raw type  the compiler didn t check for this  and instead you get a ClassCastException at run-time     This should convince you to always use typesafe generic types in new code  Always   See also   What is a raw type and why shouldn t we use it

User · Answer

There are a couple of awkward things with your example class    it s called People while it has a price and info  more something for objects  not people   when naming a class as a plural of something  it suggests it is an abstraction of more than one thing     Anyway  here s a demo of how to use a Comparator lt T gt    public class ComparatorDemo        public static void main String   args            List lt Person gt  people   Arrays asList                  new Person  Joe   24                   new Person  Pete   18                   new Person  Chris   21                     Collections sort people  new LexicographicComparator             System out println people           Collections sort people  new AgeComparator             System out println people            class LexicographicComparator implements Comparator lt Person gt         Override     public int compare Person a  Person b            return a name compareToIgnoreCase b name            class AgeComparator implements Comparator lt Person gt         Override     public int compare Person a  Person b            return a age  lt  b age   -1   a age    b age   0   1           class Person        String name      int age       Person String n  int a            name   n          age   a              Override     public String toString             return String format   name  s  age  d    name  age             EDIT  And an equivalent Java 8 demo would look like this   public class ComparatorDemo        public static void main String   args            List lt Person gt  people   Arrays asList                  new Person  Joe   24                   new Person  Pete   18                   new Person  Chris   21                     Collections sort people   a  b  - gt  a name compareToIgnoreCase b name            System out println people           Collections sort people   a  b  - gt  a age  lt  b age   -1   a age    b age   0   1           System out println people

User · Answer

For the sake of completeness  here s a simple one-liner compare method   Collections sort people  new Comparator lt Person gt           Override     public int compare Person lhs  Person rhs              return Integer signum lhs getId   - rhs getId

User · Answer

For the sake of completeness   Using Java8  people sort Comparator comparingInt People  getId      if you want in descending order  people sort Comparator comparingInt People  getId  reversed

User · Answer

You want to implement Comparable  not Comparator  You need to implement the compareTo method  You re close though  Comparator is a  3rd party  comparison routine  Comparable is that this object can be compared with another   public int compareTo Object obj1      People that    People obj1    Integer p1   this getId      Integer p2   that getid       if  p1  gt  p2       return 1        else if  p1  lt  p2      return -1        else    return 0       Note  you may want to check for nulls in here for getId  just in case

User · Answer

public static Comparator lt JobSet gt  JobEndTimeComparator   new Comparator lt JobSet gt                  public int compare JobSet j1  JobSet j2                    int cost1   j1 cost                  int cost2   j2 cost                  return cost1-cost2

User · Answer

Here is my answer for a simple comparator tool  public class Comparator   public boolean isComparatorRunning    false  public void compareTableColumns List lt String gt  tableNames        if  isComparatorRunning            isComparatorRunning   true          try               for  String schTableName   tableNames                    Map lt String  String gt  schemaTableMap   ComparatorUtil getSchemaTableMap schTableName                    Map lt String  ColumnInfo gt  primaryColMap   ComparatorUtil getColumnMetadataMap DbConnectionRepository getConnectionOne    schemaTableMap                   Map lt String  ColumnInfo gt  secondaryColMap   ComparatorUtil getColumnMetadataMap DbConnectionRepository getConnectionTwo    schemaTableMap                   ComparatorUtil publishColumnInfoOutput  Comparing table      schemaTableMap get CompConstants TABLE NAME                    compareColumns primaryColMap  secondaryColMap                           catch  Exception e                ComparatorUtil publishColumnInfoOutput  ERROR  e getMessage                       isComparatorRunning   false           public void compareColumns Map lt String  ColumnInfo gt  primaryColMap  Map lt String  ColumnInfo gt  secondaryColMap        try           boolean isEqual   true          for Map Entry lt String  ColumnInfo gt  entry   primaryColMap entrySet                  String columnName   entry getKey                ColumnInfo primaryColInfo   entry getValue                ColumnInfo secondaryColInfo   secondaryColMap remove columnName               if secondaryColInfo    null                       column is not present in Secondary Environment                 ComparatorUtil publishColumnInfoOutput  ALTER   primaryColInfo                   isEqual   false                  continue                            if  primaryColInfo equals secondaryColInfo                     isEqual   false                     Column not equal in secondary env                 ComparatorUtil publishColumnInfoOutput  MODIFY   primaryColInfo                                   if  secondaryColMap isEmpty                  isEqual   false              for Map Entry lt String  ColumnInfo gt  entry   secondaryColMap entrySet                         column is not present in Primary Environment                 ComparatorUtil publishColumnInfoOutput  DROP   entry getValue                                      if isEqual                ComparatorUtil publishColumnInfoOutput  --Exact Match                    catch  Exception e            ComparatorUtil publishColumnInfoOutput  ERROR  e getMessage              public void compareTableColumnsValues String primaryTableName  String primaryColumnNames  String primaryCondition  String primaryKeyColumn           String secTableName  String secColumnNames  String secCondition  String secKeyColumn        if  isComparatorRunning            isComparatorRunning   true          Connection conn1   DbConnectionRepository getConnectionOne            Connection conn2   DbConnectionRepository getConnectionTwo             String query1   buildQuery primaryTableName  primaryColumnNames  primaryCondition  primaryKeyColumn           String query2   buildQuery secTableName  secColumnNames  secCondition  secKeyColumn           try               Map lt String Map lt String  Object gt  gt  query1Data   executeAndRefactorData conn1  query1  primaryKeyColumn               Map lt String Map lt String  Object gt  gt  query2Data   executeAndRefactorData conn2  query2  secKeyColumn                for Map Entry lt String Map lt String  Object gt  gt  entry   query1Data entrySet                      String key   entry getKey                    Map lt String  Object gt  value   entry getValue                    Map lt String  Object gt  secondaryValue   query2Data remove key                   if secondaryValue    null                        ComparatorUtil publishColumnValuesInfoOutput  NO SUCH VALUE AVAILABLE IN SECONDARY DB    value toString                         continue                                    compareMap value  secondaryValue  key                              if  query2Data isEmpty                      ComparatorUtil publishColumnValuesInfoOutput  Extra Values in Secondary table      Map query2Data values    values   toString                             catch  Exception e                ComparatorUtil publishColumnValuesInfoOutput  ERROR  e getMessage                       isComparatorRunning   false           private void compareMap Map lt String  Object gt  primaryValues  Map lt String  Object gt  secondaryValues  String columnIdentification        for Map Entry lt String  Object gt  entry   primaryValues entrySet              String key   entry getKey            Object value   entry getValue            Object secValue   secondaryValues get key           if value  null  amp  amp  secValue  null  amp  amp   String valueOf value  equalsIgnoreCase String valueOf secValue                  ComparatorUtil publishColumnValuesInfoOutput columnIdentification     Secondary Table does not match value     value     for column     key                         if value  null  amp  amp  secValue  null                ComparatorUtil publishColumnValuesInfoOutput columnIdentification     Values not available in primary table for column    key                     if value  null  amp  amp  secValue  null                ComparatorUtil publishColumnValuesInfoOutput columnIdentification     Values not available in Secondary table for column    key                      private String buildQuery String tableName  String column  String condition  String keyCol        if      equalsIgnoreCase column             String   keyColArr   keyCol split               for String key  keyColArr                if  column contains key trim                       column      key trim                                      StringBuilder queryBuilder   new StringBuilder        queryBuilder append  select   column   from    tableName       if  ComparatorUtil isNullorEmpty condition             queryBuilder append   where 1 1 and   condition             return queryBuilder toString       private Map lt String Map lt String  Object gt  gt  executeAndRefactorData Connection connection  String query  String keyColumn        Map lt String Map lt String  Object gt  gt  result   new HashMap lt String  Map lt String Object gt  gt         try           PreparedStatement preparedStatement   connection prepareStatement query           ResultSet resultSet   preparedStatement executeQuery            resultSet setFetchSize 1000           if  resultSet    null  amp  amp   resultSet isClosed                  while  resultSet next                      Map lt String  Object gt  columnValueDetails   new HashMap lt String  Object gt                     int columnCount   resultSet getMetaData   getColumnCount                    for  int i 1  i lt  columnCount  i                          String columnName   String valueOf resultSet getMetaData   getColumnName i                        Object columnValue   resultSet getObject columnName                       columnValueDetails put columnName  columnValue                                     String   keys   keyColumn split                       String newKey                       for int j 0  j lt keys length  j                          newKey    String valueOf columnValueDetails get keys j                                       result put newKey   columnValueDetails                                  catch  SQLException e            ComparatorUtil publishColumnValuesInfoOutput  ERROR  e getMessage               return result         Utility Tool for the same  public class ComparatorUtil    public static Map lt String  String gt  getSchemaTableMap String tableNameWithSchema        if isNullorEmpty tableNameWithSchema             return null            Map lt String  String gt  result   new LinkedHashMap lt  gt         int index   tableNameWithSchema indexOf           String schemaName   tableNameWithSchema substring 0  index       String tableName   tableNameWithSchema substring index 1       result put CompConstants SCHEMA NAME  schemaName       result put CompConstants TABLE NAME  tableName       return result     public static Map lt String  ColumnInfo gt  getColumnMetadataMap Connection conn  Map lt String  String gt  schemaTableMap        try           String schemaName   schemaTableMap get CompConstants SCHEMA NAME           String tableName   schemaTableMap get CompConstants TABLE NAME           ResultSet resultSetConnOne   conn getMetaData   getColumns null  schemaName  tableName  null           Map lt String  ColumnInfo gt  resultSetTwoColInfo   getColumnInfo schemaName  tableName  resultSetConnOne           return resultSetTwoColInfo        catch  SQLException e            e printStackTrace              return null        Number Type mapping    12-----VARCHAR    3-----DECIMAL    93-----TIMESTAMP    1111-----OTHER    public static Map lt String  ColumnInfo gt  getColumnInfo String schemaName  String tableName  ResultSet columns        try           Map lt String  ColumnInfo gt  tableColumnInfo   new LinkedHashMap lt String  ColumnInfo gt             while  columns next                  ColumnInfo columnInfo   new ColumnInfo                columnInfo setSchemaName schemaName               columnInfo setTableName tableName               columnInfo setColumnName columns getString  COLUMN NAME                 columnInfo setDatatype columns getString  DATA TYPE                 columnInfo setColumnsize columns getString  COLUMN SIZE                 columnInfo setDecimaldigits columns getString  DECIMAL DIGITS                 columnInfo setIsNullable columns getString  IS NULLABLE                 tableColumnInfo put columnInfo getColumnName    columnInfo                     return tableColumnInfo        catch  Exception e            e printStackTrace              return null     public static boolean isNullOrEmpty Object obj        if  obj    null          return true      if  String valueOf obj  equalsIgnoreCase  NULL             return true      if  obj toString   trim   length      0          return true      return false       public static boolean isNullorEmpty String str        if str    null          return true      if str trim   length      0           return true      return false     public static void publishColumnInfoOutput String type  ColumnInfo columnInfo        String str    ALTER TABLE   columnInfo getSchemaName       columnInfo getTableName        switch type toUpperCase              case  ALTER               if  NUMBER  equalsIgnoreCase columnInfo getDatatype        DATE  equalsIgnoreCase columnInfo getDatatype                       str      ADD    columnInfo getColumnName        columnInfo getDatatype                       else                   str      ADD    columnInfo getColumnName        columnInfo getDatatype        columnInfo getColumnsize                                    break          case  DROP               str      DROP    columnInfo getColumnName                     break          case  MODIFY               if  NUMBER  equalsIgnoreCase columnInfo getDatatype        DATE  equalsIgnoreCase columnInfo getDatatype                       str      MODIFY    columnInfo getColumnName        columnInfo getDatatype                       else                   str      MODIFY    columnInfo getColumnName        columnInfo getDatatype        columnInfo getColumnsize                                    break            publishColumnInfoOutput str      public static Map lt Integer  String gt  allJdbcTypeName   null   public static Map lt Integer  String gt  getAllJdbcTypeNames         Map lt Integer  String gt  result   new HashMap lt Integer  String gt         if allJdbcTypeName    null          return allJdbcTypeName      try           for  Field field   java sql Types class getFields                  result put  Integer  field get null   field getName                     catch  Exception e            e printStackTrace              return allJdbcTypeName result     public static String getStringPlaces String   attribs        String params           for int i 0  i lt attribs length  i      params                return params substring 0  params length  -1          Column Info Class   public class ColumnInfo   private String schemaName  private String tableName  private String columnName  private String datatype  private String columnsize  private String decimaldigits  private String isNullable

User · Answer

Here s a super short template to do the sorting right away    Collections sort people new Comparator lt Person gt         Override    public int compare final Person lhs Person rhs           TODO return 1 if rhs should be before lhs              return -1 if lhs should be before rhs             return 0 otherwise  meaning the order stays the same                if it s hard to remember  try to just remember that it s similar  in terms of the sign of the number  to    lhs-rhs    That s in case you want to sort in ascending order   from smallest number to largest number

User · Answer

The solution can be optimized in following way  Firstly  use a private inner class as the scope for the fields is to be the enclosing class TestPeople so as the implementation of class People won t get exposed to outer world  This can be understood in terms of creating an APIthat expects a sorted list of people Secondly  using the Lamba expression java 8  which reduces the code  hence development effort  Hence code would be as below   import java util ArrayList  import java util Collections  import java util Comparator   public class TestPeople       public static void main String   args            ArrayList lt People gt  peps   new ArrayList lt  gt       Be specific  to avoid                                                        classCast Exception          TestPeople test   new TestPeople             peps add test new People 123   M   14 25            peps add test new People 234   M   6 21            peps add test new People 362   F   9 23            peps add test new People 111   M   65 99            peps add test new People 535   F   9 23                           Collections sort peps                          for  int i   0  i  lt  peps size    i                System out println peps get i                              The above code can be replaced by followin           peps sort  People p1  People p2  - gt  p1 getid   - p2 getid              peps forEach  p  - gt  System out println       p toString                  private class People           private int id            Override         public String toString                 return  People  id     id      info     info      price     price                           private String info          private double price           public People int newid  String newinfo  double newprice                setid newid               setinfo newinfo               setprice newprice                      public int getid                 return id                     public void setid int id                this id   id                     public String getinfo                 return info                     public void setinfo String info                this info   info                     public double getprice                 return price                     public void setprice double price                this price   price

User · Answer

Here s an example of a Comparator that will work for any zero arg method that returns a Comparable   Does something like this exist in a jdk or library   import java lang reflect Method  import java util Comparator   public class NamedMethodComparator implements Comparator lt Object gt                   instance variables             private String methodName       private boolean isAsc                 constructor             public NamedMethodComparator String methodName  boolean isAsc            this methodName   methodName          this isAsc   isAsc                        Method to compare two objects using the method named in the constructor               Override     public int compare Object obj1  Object obj2            Comparable comp1   getValue obj1  methodName           Comparable comp2   getValue obj2  methodName           if  isAsc                return comp1 compareTo comp2             else               return comp2 compareTo comp1                                  implementation             private Comparable getValue Object obj  String methodName            Method method   getMethod obj  methodName           Comparable comp   getValue obj  method           return comp             private Method getMethod Object obj  String methodName            try               Class   signature                   Method method   obj getClass   getMethod methodName  signature               return method            catch  Exception exp                throw new RuntimeException exp                        private Comparable getValue Object obj  Method method            Object   args               try               Object rtn   method invoke obj  args               Comparable comp    Comparable  rtn              return comp            catch  Exception exp                throw new RuntimeException exp

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