public class XXXX{
private List<Integer>[] a;
public XXXX(int num)
{
a = new ....?
}
}
How should i new the a ?
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7Arrays and generics don't go well together.NPE– NPE2013-03-26 12:00:24 +00:00Commented Mar 26, 2013 at 12:00
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what is a? is it a array of List<Integer>Arun P Johny– Arun P Johny2013-03-26 12:00:33 +00:00Commented Mar 26, 2013 at 12:00
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Why would you even need such a construct? (apart from an interview or exam question...)ppeterka– ppeterka2013-03-26 12:03:07 +00:00Commented Mar 26, 2013 at 12:03
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You cannot create an array of a parameterised type in Java. See: stackoverflow.com/questions/3903196/…Jesper– Jesper2013-03-26 12:08:22 +00:00Commented Mar 26, 2013 at 12:08
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1Please do a thorough search on SO for your questions before asking them stackoverflow.com/questions/217065/…Code2Interface– Code2Interface2013-03-26 12:29:53 +00:00Commented Mar 26, 2013 at 12:29
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6 Answers
You can create an array of lists but you cannot use type during initalization.
List<Integer>[] lists=new List[10];
//insertion
for(int i=0;i<10;i++){
lists[i]=new ArrayList<Integer>();
lists[i].add(i);
lists[i].add(i+1);
}
//printing
for(List<Integer> list:lists){
System.out.println(list.size());
}
Why this works? Because the lists variable points to an array data structure whose data type is List<Integer>. This array contains a set of references to different objects of type List<Integer>, and this is why if we try to run lists[i]=new ArrayList<String>(); it will not compile. However when we initialize the array itself we don't need to provide the type of the List objects as List since from JVM point of view a List of Integer objects and a List of Object objects will require the same number of bytes as logn as their sizes is same. The only constraint comes when we set a array member to a value (of type List - it has to be List<Integer> not anything else)
You can type cast the List[] to a List<Integer>[] but the end result and the JVM behavior is the same.
3 Comments
Nikolay Kuznetsov
why would someone need this mess?
Jit B
In production level java nobody would require this (we are all about Collections and Collections of Collections) but it's good food for thought.
Jit B
nobody uses arrays unless they 'need' to work with primitives. arrays in java might have been useful if they were kept in stack.
NPE: "Arrays and generics don't go well together"
Go for list of lists
List<List<Integer>> a = new ArrayList<List<Integer>>();
or double array
int[][] a = new int[5][5];
answered Mar 26, 2013 at 12:04
Nikolay Kuznetsov
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Comments
This works
int arraySize = 10;
List<Integer>[] a = (List<Integer>[]) new List[arraySize];
It create an array (of size 10) that can contains List of Integer
1 Comment
xchiltonx
Don't bother reading the initial comments as they refer to an incorrect link and people asking why why why. This worked perfectly, simple question and simple answer. I was grouping my four List<Integer> together and an array was the perfect choice. Good answer.
a = (List<Integer>[]) new List[num];
for(int i = 0; i < num; i++)
a[i] = new ArrayList<Integer>();
Really though, why not declare a as a List<List<Integer>>?
Comments
you can do as List<Integer> a=new ArrayList<Integer>();
but a should not be generic
Comments
List is an interface in java!
you can give references of instances like ArrayList, linket list, vector and stack to a variable with List datatype
so you can use one of the following,
a = new ArrayList<Integer>();
a = new LinkedList<Integer>();
a = new Vector()<Integer>;
a = new Stack()<Integer>;
3 Comments
Jesper
Look closely at the question.
a is an array of lists. Java does not allow you to make an array of a parameterised type.
Nomesh Gajare
yes you are right jasper! i did not observe keenly!
ppeterka
And what is with the syntax on the last two?