Java - Returning Collections

Question

Problem: Write a method called personIndex() where you pass in an ArrayList of person objects. Place each person object with its corresponding id number in HashMap. Return a collection of the person objects in order.

Could someone explain / show how I would do this?

This is my code:

public Collection<person> personIndex(ArrayList<Person> parameterArr)
{
    HashMap<String, Person> answerMap = new HashMap<String, Person>();

    for (Person p: parameterArr)
    {
        answerMap.put(p.getID(), p)
    }
     return answerMap.values();
}

Question: Is my code correct? Will it return a collection?

I want to know if this is a syntactically correct way to return a collection, and also how would you write this method?

PROBLEM:
when you write the method nothing is being returned but the compiler says everything is syntactically correct

Thank you!

You write some code.. Run into trouble.. and we will be happy to help.. :) — TheLostMind, Dec 23 '15 at 05:33
public Collection personIndex(ArrayList parameterArr) { HashMap answerMap = new HashMap(); for (Person p: parameterArr) { answerMap.put(p.getID(), p) } return answerMap.values(); } — SophieGele, Dec 23 '15 at 05:37
Possible duplicate of [Sorting HashMap,?> by key](http://stackoverflow.com/questions/5855783/sorting-hashmap-by-key) — Atri, Dec 23 '15 at 05:41
Did you run your code and test it? What is the expected result and what are you getting? — Atri, Dec 23 '15 at 05:42
I want to know if this is a syntactically correct way to return a collection, and also how would you write this method? — SophieGele, Dec 23 '15 at 05:45
The Java compiler `javac`, or your favorite IDE, will tell you right away if your code is syntactically correct. (Hint: it's not, why is `person` with a lowercase `p` in `Collection` ?) — Erwin Bolwidt, Dec 23 '15 at 05:48
My bad I meant to make that capitalized, but other than that does the code work? — SophieGele, Dec 23 '15 at 05:49
this question was on an old exam and I got points docked off for it being a wrong way to write the method — SophieGele, Dec 23 '15 at 05:50
@SophieGele - *Return a collection of the person objects in order.* . You need a `LinkedHashMap` — TheLostMind, Dec 23 '15 at 05:58
@SophieGele - And you can simply return the same `parameterArr`. :P — TheLostMind, Dec 23 '15 at 05:59
@VinodMadyalkar oh hello, LinkedHashMap hasn't been taught yet. So what would be the simplest way to do it? — SophieGele, Dec 23 '15 at 06:05
@SophieGele - Replace `HashMap answerMap = new HashMap();` with `Map answerMap = new LinkedHashMap();` and tadaaaa. Your ids will be stored in the order they are added :) — TheLostMind, Dec 23 '15 at 06:06
@SophieGele - BTW why do even need a Map?. You are not doing anything with it — TheLostMind, Dec 23 '15 at 06:07
when you write the method nothing is being returned but the compiler says everything is syntactically correct — SophieGele, Dec 23 '15 at 06:33

score 0 · Answer 1 · edited May 23 '17 at 10:27

It seems you don't have any problem in the method you have shown other than not capitalizing "person" in your method return type to represent the Person class.

After the enhanced-for loop is finished you have a result of a hashmap with the keys being the person IDs of type String and the values being the Person objects. If you want to return the hashmap with its keys in order (1... 2... 3... 4), you can see this question how to sort Map values by key in Java. Since String already implements the Comparable interface various methods in the question I linked will have a pre-defined sorting method (from the String class) to sort the strings.

After sorting the hashmap you can get the values as a collection and return that, since now it will be sorted depending on the IDs.

In the future please show your attempt at ordering the keys in the hashmap.

Java - Returning Collections

1 Answers1