Creating an array of Sets in Java

Question

I'm new to Java so I'm probably doing something wrong here, I want to create an array of Sets and I get an error (from Eclipse). I have a class:

public class Recipient 
{
String name;
String phoneNumber;

public Recipient(String nameToSet, String phoneNumberToSet)
{
    name = nameToSet;
    phoneNumber = phoneNumberToSet;
}

void setName(String nameToSet)
{
    name = nameToSet;
}

void setPhoneNumber(String phoneNumberToSet)
{
    phoneNumber = phoneNumberToSet;
}

String getName()
{
    return name;
}

String getPhoneNumber()
{
    return phoneNumber;
}
}

and I'm trying to create an array:

Set<Recipient>[] groupMembers = new TreeSet<Recipient>[100];

The error I get is "Cannot create a generic array of TreeSet"

What is wrong ?

Following SO question might be helpful. [http://stackoverflow.com/questions/529085/java-how-to-generic-array-creation][1] [1]: http://stackoverflow.com/questions/529085/java-how-to-generic-array-creation — Upul Bandara, Nov 03 '11 at 17:29

Mansoor Siddiqui · Accepted Answer · 2016-06-22T15:50:19.227

22

From http://www.ibm.com/developerworks/java/library/j-jtp01255/index.html:

you cannot instantiate an array of a generic type (new List<String>[3] is illegal), unless the type argument is an unbounded wildcard (new List<?>[3] is legal).

Rather than using an array, you can use an ArrayList:

List<Set<Recipient>> groupMembers = new ArrayList<Set<Recipient>>();

The code above creates an empty ArrayList of Set<Recipient> objects. You would still have to instantiate every Set<Recipient> object that you put into the ArrayList.

edited Jun 22 '16 at 15:50

answered Nov 03 '11 at 17:28

Mansoor Siddiqui

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I don't understand the first part of the answer: you wrote List[3] is both legal and illegal. what is the meaning of "unbounded wildcard " ? – Belgi Nov 03 '11 at 17:31
Apologies, stackoverflow was interpreting the generics (e.g. ``) as HTML tags. I have fixed the formatting. – Mansoor Siddiqui Nov 03 '11 at 17:32
thanks. I think you are missing > in , but its a very good answer. +1 – Belgi Nov 03 '11 at 17:36

score 0 · Answer 2 · answered Nov 03 '11 at 17:29

0

Arrays don't support Generics. Use an ArrayList:

ArrayList<Set<Recipient>> groupMembers = new ArrayList<Set<Recipient>>();

answered Nov 03 '11 at 17:29

Brian Roach

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score 0 · Answer 3 · answered Nov 03 '11 at 17:33

0

You might want to consider using Guava's Multimap where the key is the index. This will handle creating the Sets for each index as you need them.

SetMultimap

SetMultimap<Integer, Recipient> groupMembers;

answered Nov 03 '11 at 17:33

John B

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Creating an array of Sets in Java

3 Answers3

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