ArrayList toArray method class cast exception

Question

I don't understand why this is giving me a class cast exception:

List<String> names = new ArrayList<String>();
for (ApplicationInfo app : apps) {
    names.add("" + app.loadLabel(packageManager));
}
return (String[]) names.toArray();

the log is:

01-23 16:56:18.746: E/AndroidRuntime(4027): Caused by: java.lang.ClassCastException: java.lang.Object[] cannot be cast to java.lang.String[]

but I clearly defined names to be a list of String and not Object

**EDIT:

oh wait, okay i see in the API it returns array of objects. how could i do this without doing:

List<String> names = new ArrayList<String>();
for (ApplicationInfo app : apps) {
    names.add("" + app.loadLabel(packageManager));
}
String[] list = new String[names.size()];
return names.toArray(list);

and making a new array each time?

you can loop on Array list and push string in your array and return it — mohammed momn, Jan 24 '14 at 01:09

score 4 · Answer 1 · answered Jan 24 '14 at 01:11

4

Based on the javadoc, you can do this:

String[] list = names.toArray(new String[0]);

answered Jan 24 '14 at 01:11

aysonje

2,595
1
14
16

I like the solution from javadoc. – Dino Tw Mar 28 '18 at 20:45

score 3 · Accepted Answer · answered Jan 24 '14 at 01:29

3

The class cast exception happened just because toArray() returns Object[]. Surely Object[] cannot cast to String[].

Insteadly, you can use this:

String[] list = toArray(new String[names.size()]);

which returns the right cast(String[]).

answered Jan 24 '14 at 01:29

sun

66
4

score 1 · Answer 3 · answered Jan 24 '14 at 01:08

1

You just have to, either by you directly, or indirectly. Someone got to create a array (with correct size) to hold the result.

new String[0] is for dummy, but you will create two objects.

If you believe an additional array object do matter, you should REUSE the String[] object, or just use the List without convert to array.

answered Jan 24 '14 at 01:08

Dennis C

24,511
12
71
99

Over 10k rep and you post an answer rather than mark as a dup? If this answer was better or more complete than the duplicate then that would fly but this... – Boris the Spider Jan 24 '14 at 01:11

score 1 · Answer 4 · answered Jan 24 '14 at 01:36

You are working on generics and at run time it is not able to identify what type of array to create and what is the size to be allocated to that array.

A very simple working solution.

ApplicationInfo a1  = new ApplicationInfo("label1");
    ApplicationInfo a2  = new ApplicationInfo("label2");
    ApplicationInfo a3  = new ApplicationInfo("label3");

    List<ApplicationInfo> infos = new ArrayList<ApplicationInfo>();
    infos.add(a1);
    infos.add(a2);
    infos.add(a3);

    ArrayList<String> labels = new ArrayList<String>();
    for (ApplicationInfo info : infos) {
        labels.add(info.getLabel().toString());
    }

    String[] lbls = labels.toArray(new String[labels.size()]);
    for (int i = 0; i < lbls.length; i++) {
        System.out.println(lbls[i]);
    }

score 0 · Answer 5 · answered Jan 24 '14 at 01:08

0

You could do this instead:

String[] list = new String[0];
return names.toArray(list);

You don't need to allocate the additional space for the size of your array.

answered Jan 24 '14 at 01:08

Dana

490
1
4
11

First of all this answer provides no explanation. Secondly this question is obviously a duplicate. Thirdly, why allocate two arrays? – Boris the Spider Jan 24 '14 at 01:09
@BoristheSpider she needs more points on stack overflow :'( – David T. Jan 24 '14 at 20:56

ArrayList toArray method class cast exception

5 Answers5