List of File to array in Java

Question

I'm wondering, why is there a problem to change Arraylist of File to an array.

public static void main(String[] args) {
    List<File> pl = new ArrayList<File>();
    pl.add(new File ("C:\\folder"));
    String[] k;
    k = pl.toArray(new String[pl.size()]);
    System.out.println(k);
}

In a simple example above I will get:

Exception in thread "main" java.lang.ArrayStoreException
    at java.lang.System.arraycopy(Native Method)
    at java.util.ArrayList.toArray(Unknown Source)
    at Main.main(Main.java:25)

and second question: Do I have to pass size of Arraylist? Because both version works fine with arraylist of strings:

k = pl.toArray(new String[pl.size()]);
k = pl.toArray(new String[] {});

What are you trying to do here? Are you trying to convert a list of files into a list of string contents? — Tim Biegeleisen
– Tim Biegeleisen, Commented Dec 15, 2018 at 14:03

Mureinik · Accepted Answer · 2018-12-15 14:04:38Z

3

You have a list of File objects, you can't just put them in an array of String, you'll need to explicitly convert them somehow (e.g. by calling File#getName()). The most convenient way of doing this is probably with a Stream:

k = p1.stream().map(File::getName).toArray(String[]::new);

answered Dec 15, 2018 at 14:04

Mureinik

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Comments

davidxxx · Accepted Answer · 2018-12-15 14:27:20Z

1

You don't need to convert Path to String when you want to map a List<Path> into an array.

Here what you could write (I add some comment about changes) :

public static void main(String[] args) {
    List<File> pl = new ArrayList<>(); // Diamond operator
    pl.add(new File ("C:\\folder"));
    File[] k = pl.toArray(new File[pl.size()]);
    k = pl.stream().toArray(File[]::new);  // equivalent to the previous line
    System.out.println(Arrays.toString(k)); // Array.toString is not overriden. So this is needed
}

answered Dec 15, 2018 at 14:27

davidxxx

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