ArrayList's toArray() Method not Working Properly

Question

When looking through ArrayList's methods, I saw a method called toArray(). I tried out this method using the following code:

ArrayList<Integer> a = new ArrayList<>();
// Assigning random elements to the ArrayList
int[] b = a.toArray();

However, this showed the following exception in the compiler:

Incompatible types.
Required: int[]
Found: java.lang.Object[]

The next thing I tried next is down-casting it to int[]

ArrayList<Integer> a = new ArrayList<>();
// Assigning random elements to the ArrayList
int[] b = (int[]) a.toArray();

This showed another error:

Cannot cast java.lang.Object[] to int[]

The last thing I tried is making it an Integer[] instead, and down-casting it to Integer[]

ArrayList<Integer> a = new ArrayList<>();
// Assigning random elements to the ArrayList
Integer[] b = (Integer[]) a.toArray();

This one compiled, but as soon as I ran it it produced ClassCastException. How do I use this toArray() method without errors?

Elliott Frisch · Accepted Answer · 2019-08-18 03:49:34Z

3

List can only hold reference types (like Integer). Integer is a wrapper type. To convert a List<Integer> to an int[] there is an option using a Stream to map the Integer values to int and then collect to an int[]. Like,

int[] b = a.stream().mapToInt(Integer::intValue).toArray();
System.out.println(Arrays.toString(b));

answered Aug 18, 2019 at 3:49

Elliott Frisch

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Comments

Matthieu · Accepted Answer · 2019-08-18 08:10:50Z

2

It's all written in the javadoc:

Integer[] b = a.toArray(new Integer[0]);

edited Aug 18, 2019 at 8:10

answered Aug 18, 2019 at 3:31

Matthieu

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4 Comments

nishantc1527 Over a year ago

I tried changing this to int[] b = (int[]) a.toArray(new int[0]);, but it said there is no such method for primitive type arrays. Is there any way to do this?

MK. Over a year ago

you dont need the cast Integer[] b = a.toArray(new Integer[0]);

MK. Over a year ago

@nishantc1527 the reason that you need to pass the stupid new Integer[0] thing is to make it invoke generic version of toArray and let it know which type it should return. Type inference doesn't work with primitive types, so new int[0] won't work. Bottom line is that if you want type safety you should stay away from arrays.

Matthieu Over a year ago

@MK true (for both statements). I usually try to rework my data structures when I find myself needing to call toArray(). Especially for a list of boxed primitives where Java seems terribly cumbersome.

Sravanth Pujari · Accepted Answer · 2019-08-18 03:54:37Z

1

Extending the answer of @Matthieu, It seems you need to pass new Integer[]. Attaching an example link given in geeksforgeeks.

Integer[] arr = new Integer[a.size()]; 
arr = a.toArray(arr);

answered Aug 18, 2019 at 3:54

Sravanth Pujari

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