get Object by object variable value in Java

Question

I have Kvote class like this:

public class Kvote {
    private static int counter = 0;
    private double[] vrednost = new double[3];
    private int id; 

    public Kvote(double v1, double v2) {
        super();
        this.vrednost[1] = v1;
        this.vrednost[2] = v2;
        this.id = counter;
        counter++;

    }

    public static int getCounter(){
        return counter;
    }
}

I have created several objects of that class in Main:

Kvote k1 = new Kvote(2, 4.25);
Kvote k2 = new Kvote(1.85, 5);
Kvote k3 = new Kvote(1.62, 6.5);
Kvote k4 = new Kvote(1.72, 6);

So, every instance of class Kvote has some id, it's 0 for k1, 1 for k2, 2 for k3... Is it possible to access objects by it's id var value. For example if I ask for object with Id=2, I get object k3...

It's hard for me to explain why I need this, but I do. Basically, my alghorytm needs to work with infinite number of objects of class Kvote: k1,k2,k3,k4,k5,k6,k7....

try to add the item to a map using id as a key, please see my answer below — roeygol
– roeygol, Commented Jan 15, 2015 at 18:37

forgivenson · Accepted Answer · 2015-01-15 18:57:55Z

1

Try this:

HashMap<Integer, Kvote> map = new HashMap<Integer, Kvote>();
put(k1.getId(), k1);
put(k2.getId(), k2);
put(k3.getId(), k3);
put(k4.getId(), k4);

Then use the map to get values by id like:

map.get(2) => k3

also, you will need to add getId() function to Kvote class.

edited Jan 15, 2015 at 18:57

forgivenson

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answered Jan 15, 2015 at 18:39

roeygol

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Comments

forgivenson · Accepted Answer · 2015-01-15 18:45:33Z

1

If you are just giving them IDs in sequential order, then you can just use an ArrayList.

List<Kvote> myList = new ArrayList<Kvote>();
myList.add(new Kvote(2, 4.25));
myList.add(new Kvote(1.85, 5));
myList.add(new Kvote(1.62, 6.5));
myList.add(new Kvote(1.72, 6));

And to access the objects, just do this:

myList.get(3); // returns the 4th item in the list

However, if you want to have more flexibility about what your IDs are, then you can use a map, as @RoeyGolzarpoor suggests in his answer.

answered Jan 15, 2015 at 18:45

forgivenson

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Comments

wassgren · Accepted Answer · 2015-01-15 18:47:07Z

A java.util.Map is one of the most common data structures that supports key value pairs. There are multiple implementations available where one of them is the java.util.HashMap.

It works like this:

Map<Integer, Kvote> kvotes = new HashMap<>();

// Save it in the Map
Kvote kvote = new Kvote(123D, 213D);
kvotes.put(kvotes.getId(), kvote);

// Retrieve the kvote
Kvote kvote = kvotes.get(123);

The HashMap offers fast access to you elements and you can also iterate the available keys, values or entries.

// Print the kvotes
kvotes.values().stream().forEach(System.out.println(kvote.toString()));

I you wish to keep the order in which you added your elements (for algorithmical purposes) I suggest that you use a LinkedHashMap which will be able to remember the order for you. The HashMap does not hold the order.

aries23 · Accepted Answer · 2015-01-15 18:48:21Z

I'm not sure what you want as result, thought Roey's map solution is good and I'd just add that instead of having the counter inside the class, have a counter outside and pass its value to the constructor to set the value of id inside instantiation:

public class Kvote {

private double[] vrednost = new double[2];
private int id; 

public Kvote(double v1, double v2, int id) {
    this.vrednost[0] = v1;
    this.vrednost[1] = v2;
    this.id = id;
}

public static int getId(){
    return this.id;
}
}

In the main class:

int counter = 0;

Kvote k1 = new Kvote(2, 4.25, counter++);
Kvote k2 = new Kvote(1.85, 5, counter++);
Kvote k3 = new Kvote(1.62, 6.5, counter++);
Kvote k4 = new Kvote(1.72, 6, counter++);

Then put them in the map.

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