EJB HashMap如何实现比较两个对象的方法

Question

This is an EJB project. I need to implement a function that compares two or more objects and I can add or remove only one object once I click "Add" or "Remove" button. Therefore, my idea about it is to store objects in the HashMap, then compare them and take the best. But when I run my methods, the HashMap is null. Methods cannot add objects in it. How can make the HashMap is operated. My codes is attached below.

@Stateful
public class ComparePropertySessionBean implements ComparePropertySessionBeanRemote{

public HashMap<Integer, Double> map = new HashMap<Integer, Double>();

@Override
public int getBestPerRoom() {
    Iterator<Integer> iterator = map.keySet().iterator();
    int i = 0;
    double ave = 10000000000.00;
    if (iterator.hasNext()) {
        Integer key = iterator.next();
        if (map.get(key) <  ave) {
            i = key;
            ave = map.get(key);
        }
    }

    return i;
}

@Override
public void addCompareProperty(int propertyId, double price, int noofbedrooms) {

    if (!map.containsKey(propertyId)) {
        map.put(propertyId, price/noofbedrooms);

    }

}

@Override
public void removeCompareProperty(int propertyId) {
    if (map.containsKey(propertyId)) {
        map.remove(propertyId);
    }
}

}

Answer 1

map itself could only be null here if some other code outside this class was setting it to null. (Which is unlikely, but you should really make the map field private on principle).

It would be good to see the stack trace, but the big problem I'm seeing is a logic error in getBestPerRoom() : This method works if the map has at least one entry, but if it's empty it will return 0. If your code then tries to call the other methods with this as a propertyId , they will get a null entry and (if they aren't checking carefully) will get a NPE.
To avoid these kinds of problems, you need to properly handle the case that no entries are present.

More generally though, there are much better ways to structure this code - OO principles suggest you should use a Property class to encapsulate all the information about the different properties. You can then sort them by price by writing a Comparator (or just store them in a TreeSet).
This would also allow you to easily store your Properties in a database using JPA, if you're writing an EJB application.