Problem with hash map

[Shridhar Raghavan]

Hi,
I am maintaing a map of key, value pairs. The key is a POJO and the value is a list

So it goes Map - HashMap<POJO, LIst>

While browsing an array,

some elements of the array describe the pojo and the rest describe the object that go in the list
So,

new POJO(). setProp1(arr[0]);
new POJO(). setProp2(arr[1]);
new POJO(). setProp2(arr[2]);....

new Object.setProp1(arr[3]);
new Object.setProp1(arr[4]);
new Object.setProp1(arr[5]);....

Now in case i find a pojo which is similar to the POJO in the hashmap, the corresponding object that gets created goes into the list of the key.

The problem i'm facing is when i call

Map<TaskPOJO, List<TimeEntryView>> excelResultSet = new HashMap<TaskPOJO, List<TimeEntryView>>();
if (excelResultSet.containsKey(somePOJOObject)) {

}

the condition always evaluates to fall inspite of me having overriden the POJO's .equals method.
As a result semantically similar POJOs are treated as new keys.


PLease Help asap.

[Akhilesh Trivedi]

Can you please tell the relation between the POJO and 'array' you mentioned?

[John Jai]

Can you change HashMap to TreeMap and try using a compareTo() method like below?

import java.util.HashMap;
import java.util.Map;
import java.util.TreeMap;

public class MapComparison {
	public static void main(String...args){		
	
	Map<Person,Integer> hash = new HashMap<Person,Integer>();
	Map<Person,Integer> tree = new TreeMap<Person,Integer>();
	
	
	hash.put(new Person(1),1);
	hash.put(new Person(2),2);
	hash.put(new Person(1),1);
	System.out.println(hash.size());
	
	tree.put(new Person(1),1);
	tree.put(new Person(2),2);
	tree.put(new Person(1),1);
	System.out.println(tree.size());
	
	System.out.println("Does hasmap contains Person(1)? -> " + hash.containsKey(new Person(1)));
	System.out.println("Does treemap contains Person(1)? -> " + tree.containsKey(new Person(1)));
	
	}
	
}

class Person implements Comparable<Person>{
	int id;
	Person (int id){
		this.id = id;
	}
	@Override
	public boolean equals(Object obj) {		
		System.out.println("Invoking equals()....");
		return this.id == ((Person)obj).id;
	}
	
	@Override
	public int compareTo(Person o) {
		System.out.println("Invoking compareTo()...");
		if (this.id == o.id) {
			return 0;
		}
		else 
			return -1;
	}	
}

[Shridhar Raghavan]

package main.java.reports;

import java.util.Date;

public class TaskPOJO {
	private Date created_date;
	private String name;

	public String getName() {
		return name;
	}

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


	@Override
	public boolean equals(Object obj) {
		if ((obj instanceof TaskPOJO)) {
			return ((TaskPOJO) obj).getName().equalsIgnoreCase(name);
		}
		else {
			return super.equals(obj);
		}
	}

	
}

\

private void modifyResultSet(List<Object> resultSet) {
		Map<TaskPOJO, List<TimeEntryView>> excelResultSet = new HashMap<TaskPOJO, List<TimeEntryView>>();
		Set<String> tasks = new HashSet<String>();
		for (int i = 0; i < resultSet.size(); i++) {
			Object[] arr = (Object[]) resultSet.get(i);
			// Present query 3 - Original hours,4 - Calculated estimate,8 - Name
			TaskPOJO keyPOJO = new TaskPOJO();
			keyPOJO.setName(arr[8].toString());
			if (excelResultSet.containsKey(keyPOJO)) {
				TimeEntryView view = new TimeEntryView();
				view.setName(arr[7].toString());
				excelResultSet.get(keyPOJO).add(view);

			}
			else {
				TaskPOJO pojo = new TaskPOJO();
				pojo.setOriginal_estimate(new Double(arr[3].toString()).intValue());
				pojo.setEstimated_hours(new Double(arr[4].toString()).intValue());
				pojo.setName(arr[8].toString());
				TimeEntryView view = new TimeEntryView();
				view.setName(arr[7].toString());
				List<TimeEntryView> viewList = new ArrayList<TimeEntryView>();
				viewList.add(view);
				excelResultSet.put(pojo, viewList);
				TaskPOJO pj = new TaskPOJO();
				pj.setName("First Task");
				System.out.println(pj.equals(pojo));
				System.out.println(excelResultSet.keySet().contains(pj));

			}
		}

Here we go!!!

The esssential bit are something this way. the method receives an array list. Each object in the list is an array. So a multi dimensional array if you will. Now each row in this multidimensional defines certain attributes. Requirement is to group all those guys with the same column values together in a key value sort of relationship. My key shall be a pojo over those grouping column values. The value is a list of all those objects that share the same column values.


Thanks. I will try it out the tree thing meanwhile.

[Shridhar Raghavan]

The tree map doesnt work?

[Shridhar Raghavan]

Any help?

[Joanne Neal]

Shridhar Raghavan wrote:the condition always evaluates to fall inspite of me having overriden the POJO's .equals method.

You need to override the hashCode method as well.

[Shridhar Raghavan]

Oops i goofed up with the comparable equalsTo. == and .equals error. It works now.

But could you explain the difference. The HashMap containsKey() api says the .equals method is invoked to determine if the key is contained. But the overriden .equals of my key object was never called? What am i missing?

[amit punekar]

Hello,
AFAIK being a HashMap first it checks if HashCodes are same for your keys. If they are same then only it will go and call equals() to see if Objects are equal.

Regards,
Amit

[Shridhar Raghavan]

Got it. Forgot the basics. Thanks

[John Jai]

Joanne's right... Corrected to implement the hashCode() method.

import java.util.HashMap;
import java.util.Map;
import java.util.TreeMap;

public class MapComparison {
	public static void main(String...args){		
	
	Map<Person,Integer> hash = new HashMap<Person,Integer>();
	Map<Person,Integer> tree = new TreeMap<Person,Integer>();
	
	
	hash.put(new Person(1),1);
	hash.put(new Person(2),2);
	hash.put(new Person(1),1);
	System.out.println(hash.size());
	
	tree.put(new Person(1),1);
	tree.put(new Person(2),2);
	tree.put(new Person(1),1);
	System.out.println(tree.size());
	
	System.out.println("Does hasmap contains Person(1)? -> " + hash.containsKey(new Person(1)));
	System.out.println("Does treemap contains Person(1)? -> " + tree.containsKey(new Person(1)));
	
	}
	
}

class Person implements Comparable<Person>{
	int id;
	Person (int id){
		this.id = id;
	}
	@Override
	public boolean equals(Object obj) {		
		System.out.println("Invoking equals()....");
		return this.id == ((Person)obj).id;
	}
	
	
	@Override
	public int hashCode() {
		int hash = 7;
		hash = 31 * hash + id;	
		return hash;			
	}
	
	@Override
	public int compareTo(Person o) {
		System.out.println("Invoking compareTo()...");
		if (this.id == o.id) {
			return 0;
		}
		else 
			return -1;
	}	
}