Using Arrays.asList to create List from Array

Question

This post shows that the code below creates a List from an array.

double[] features = new double[19];
List<Double> list = new ArrayList(Arrays.asList(features));

I'm expecting list to contain 19 elements, each of which is 0.0. However, after running the code above, list only contains 1 element, which is [0.0, 0.0, ..., 0.0]. I'm running Java 6 and not sure if this is related.

I don't know why your question was re-opened. http://stackoverflow.com/questions/1467913/arrays-aslist-not-working-as-it-should — Sotirios Delimanolis, Nov 15 '14 at 21:33

score 2 · Accepted Answer · answered Nov 15 '14 at 21:27

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Don't use Raw Types. Your features is empty. And you can't make a collection of a primitive type double, you need Double.

Double[] features = new Double[19]; // <-- an Object type
Arrays.fill(features, Double.valueOf(1)); // <-- fill the array
List<Double> list = new ArrayList<Double>(Arrays.asList(features));
System.out.println(list);

answered Nov 15 '14 at 21:27

Elliott Frisch

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Diamond operator `<>` was introduced in Java 7 but OP states that he is using Java 6 – halex Nov 15 '14 at 21:30
`Your features is empty.` Not really and filling that array is not necessary for this example. (repost due to evil typo) – Tom Nov 15 '14 at 21:55
@Tom Or it would be 19 null(s). – Elliott Frisch Nov 15 '14 at 21:56
:D right, you had to fix the type to correct the code, already forgot that. But "his" `features` isn't empty :P. – Tom Nov 15 '14 at 22:01

score 2 · Answer 2 · answered Nov 15 '14 at 21:37

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Every array is also an object. You call to Arrays.asList creates a List with a single element, which is the entire array. So you are creating a List<double[]>, not a List<Double>. Since you were using a raw type, the compiler did not spot this error and compiled with only a warning message. If you would have typed new ArrayList<Double>(Arrays.asList(features)), your program would not have compiled.

answered Nov 15 '14 at 21:37

Hoopje

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I observed that it didn't compile with `new ArrayList(Arrays.asList(features))` but failed to understand what the problem was. Thanks for pointing it out. – goldfrapp04 Nov 15 '14 at 21:41

Using Arrays.asList to create List from Array

2 Answers2