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 > p2) {
return 1;
} else if (p1 < 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 < 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
This question is related to
java
sorting
comparator
Use People implements Comparable<People> instead; this defines the natural ordering for People.
A Comparator<People> can also be defined in addition, but People implements Comparator<People> is not the right way of doing things.
The two overloads for Collections.sort are different:
<T extends Comparable<? super T>> void sort(List<T> list)
Comparable objects using their natural ordering<T> void sort(List<T> list, Comparator<? super T> c)
ComparatorYou're confusing the two by trying to sort a Comparator (which is again why it doesn't make sense that Person implements Comparator<Person>). Again, to use Collections.sort, you need one of these to be true:
Comparable (use the 1-arg sort)Comparator for the type must be provided (use the 2-args sort)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<People> peps = new ArrayList<People>(); // 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<Person>), 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.
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<Object> {
//
// 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);
}
}
}
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<People> peps = new ArrayList<>();// 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 < peps.size(); i++){
* System.out.println(peps.get(i)); }
*/
// The above code can be replaced by followin:
peps.sort((People p1, People p2) -> p1.getid() - p2.getid());
peps.forEach((p) -> 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;
}
}
}
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<People> 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, new People().new ComparatorId());
for (int i = 0; i < 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<People>
{
@Override
public int compare(People obj1, People obj2) {
Integer p1 = obj1.getid();
Integer p2 = obj2.getid();
if (p1 > p2) {
return 1;
} else if (p1 < p2){
return -1;
} else {
return 0;
}
}
}
}
For the sake of completeness, here's a simple one-liner compare method:
Collections.sort(people, new Comparator<Person>() {
@Override
public int compare(Person lhs, Person rhs) {
return Integer.signum(lhs.getId() - rhs.getId());
}
});
Do not waste time implementing Sorting Algorithm by your own. Instead; use
Collections.sort() to sort data.
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 > p2 ){
return 1;
}
else if (p1 < p2){
return -1;
}
else
return 0;
}
Note, you may want to check for nulls in here for getId..just in case.
Two corrections:
You have to make an ArrayList of People objects:
ArrayList<People> preps = new ArrayList<People>();
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 > t2.marks)
return 1;
else
return -1;
}
}
There are a couple of awkward things with your example class:
price and info (more something for objects, not people);Anyway, here's a demo of how to use a Comparator<T>:
public class ComparatorDemo {
public static void main(String[] args) {
List<Person> 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<Person> {
@Override
public int compare(Person a, Person b) {
return a.name.compareToIgnoreCase(b.name);
}
}
class AgeComparator implements Comparator<Person> {
@Override
public int compare(Person a, Person b) {
return a.age < 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);
}
}
And an equivalent Java 8 demo would look like this:
public class ComparatorDemo {
public static void main(String[] args) {
List<Person> people = Arrays.asList(
new Person("Joe", 24),
new Person("Pete", 18),
new Person("Chris", 21)
);
Collections.sort(people, (a, b) -> a.name.compareToIgnoreCase(b.name));
System.out.println(people);
Collections.sort(people, (a, b) -> a.age < b.age ? -1 : a.age == b.age ? 0 : 1);
System.out.println(people);
}
}
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;
}
}
}
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());
Here's a super short template to do the sorting right away :
Collections.sort(people,new Comparator<Person>(){
@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.
Here is my answer for a simple comparator tool
public class Comparator {
public boolean isComparatorRunning = false;
public void compareTableColumns(List<String> tableNames) {
if(!isComparatorRunning) {
isComparatorRunning = true;
try {
for (String schTableName : tableNames) {
Map<String, String> schemaTableMap = ComparatorUtil.getSchemaTableMap(schTableName);
Map<String, ColumnInfo> primaryColMap = ComparatorUtil.getColumnMetadataMap(DbConnectionRepository.getConnectionOne(), schemaTableMap);
Map<String, ColumnInfo> 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<String, ColumnInfo> primaryColMap, Map<String, ColumnInfo> secondaryColMap) {
try {
boolean isEqual = true;
for(Map.Entry<String, ColumnInfo> 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<String, ColumnInfo> 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<String,Map<String, Object>> query1Data = executeAndRefactorData(conn1, query1, primaryKeyColumn);
Map<String,Map<String, Object>> query2Data = executeAndRefactorData(conn2, query2, secKeyColumn);
for(Map.Entry<String,Map<String, Object>> entry : query1Data.entrySet()) {
String key = entry.getKey();
Map<String, Object> value = entry.getValue();
Map<String, Object> 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<String, Object> primaryValues, Map<String, Object> secondaryValues, String columnIdentification) {
for(Map.Entry<String, Object> entry : primaryValues.entrySet()) {
String key = entry.getKey();
Object value = entry.getValue();
Object secValue = secondaryValues.get(key);
if(value!=null && secValue!=null && !String.valueOf(value).equalsIgnoreCase(String.valueOf(secValue))) {
ComparatorUtil.publishColumnValuesInfoOutput(columnIdentification+" : Secondary Table does not match value ("+ value +") for column ("+ key+")");
}
if(value==null && secValue!=null) {
ComparatorUtil.publishColumnValuesInfoOutput(columnIdentification+" : Values not available in primary table for column "+ key);
}
if(value!=null && 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<String,Map<String, Object>> executeAndRefactorData(Connection connection, String query, String keyColumn) {
Map<String,Map<String, Object>> result = new HashMap<String, Map<String,Object>>();
try {
PreparedStatement preparedStatement = connection.prepareStatement(query);
ResultSet resultSet = preparedStatement.executeQuery();
resultSet.setFetchSize(1000);
if (resultSet != null && !resultSet.isClosed()) {
while (resultSet.next()) {
Map<String, Object> columnValueDetails = new HashMap<String, Object>();
int columnCount = resultSet.getMetaData().getColumnCount();
for (int i=1; i<=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<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<String, String> getSchemaTableMap(String tableNameWithSchema) {
if(isNullorEmpty(tableNameWithSchema)) {
return null;
}
Map<String, String> result = new LinkedHashMap<>();
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<String, ColumnInfo> getColumnMetadataMap(Connection conn, Map<String, String> 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<String, ColumnInfo> 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<String, ColumnInfo> getColumnInfo(String schemaName, String tableName, ResultSet columns) {
try {
Map<String, ColumnInfo> tableColumnInfo = new LinkedHashMap<String, ColumnInfo>();
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<Integer, String> allJdbcTypeName = null;
public static Map<Integer, String> getAllJdbcTypeNames() {
Map<Integer, String> result = new HashMap<Integer, String>();
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<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;
public static Comparator<JobSet> JobEndTimeComparator = new Comparator<JobSet>() {
public int compare(JobSet j1, JobSet j2) {
int cost1 = j1.cost;
int cost2 = j2.cost;
return cost1-cost2;
}
};
Source: Stackoverflow.com