CODEWITHSUNDEEP
javaarraysarraylist
nishantc1527
nishantc1527

Reputation: 376

ArrayList's toArray() Method not Working Properly

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?

Upvotes: 0

Views: 1228

Answers (3)

Matthieu
Matthieu

Reputation: 3098

It's all written in the javadoc:

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

Upvotes: 2

Sravanth Pujari
Sravanth Pujari

Reputation: 54

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);

Upvotes: 1

Elliott Frisch
Elliott Frisch

Reputation: 201439

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));

Upvotes: 3

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