JavaScript style array filling in Java

Question

In JavaScript, the following:

var a = [];
a[20] = "hello";
console.log(JSON.stringify(a));

would yield:

[null,null,null,null,null,null,null,null,null,null,
null,null,null,null,null,null,null,null,null,null,"hello"]

Is there a list type in Java that will auto expand when setting values beyond it's current bounds? A map isn't practical because I also need to know the array dimension.

@Sarthak Mittal - ArrayList does not autofill. You cannot set an element past the end. — jalynn2
– jalynn2, Commented Dec 5, 2014 at 17:13
@jalynn2 yeah you are right, but what do you mean by, "You cannot set an element past the end"? — Sarthak Mittal
– Sarthak Mittal, Commented Dec 5, 2014 at 17:15
@SarthakMittal If there are x elements in the list, then you can't add another element on position y if y > x. In other words: if there are 5 elements in a list, list.add(10, "blub"); will fail. — Tom
– Tom, Commented Dec 5, 2014 at 17:17
If the ArrayList is empty, you may only set into element 0. If it has one item, you may only set into element 0 and 1, etc. If you try to set into a greater index, you will get IndexOutOfBoundsException. — jalynn2
– jalynn2, Commented Dec 5, 2014 at 17:18

Zo72 · Accepted Answer · 2014-12-05 17:20:28Z

2

In the standard JDK there is not such a class.

Your best bet is probably to create a wrapper around an ArrayList and provide methods like set(int index,Object value)

Its implementation would look like this:

public void set(int index,Object value) {
   while (list.size() <= index) {
       list.add(null); // filling the gaps
   }   
   list.set(index,value); 
}

answered Dec 5, 2014 at 17:20

Zo72

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Alexis C. · Accepted Answer · 2014-12-05 18:01:56Z

1

Such implementation is not provided in the standard JDK, but you can use a GrowthList (from Apache Commons Collections).

List<String> list = new GrowthList<>(); //[]
list.add(5, "test"); //[null, null, null, null, null, test]

answered Dec 5, 2014 at 18:01

Alexis C.

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Community · Accepted Answer · 2017-05-23 10:33:29Z

0

You can use Arrays.fill.Take a look at this response.

edited May 23, 2017 at 10:33

CommunityBot

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answered Dec 5, 2014 at 17:16

cesar_sr91

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1 Comment

AaronF Over a year ago

Arrays.fill functions on ordinary, static arrays, and throws an exception if the index is out of bounds. It expands nothing.

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