up vote 37 down vote favorite
17

I'm working in the android environment and have tried the following code, but it doesn't seem to be working.

String [] stockArr = (String[]) stock_list.toArray();

If I define as follows:

String [] stockArr = {"hello", "world"};

it works. Is there something that I'm missing?

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B.T.W., This is not related to Android - it is a pure Java matter – RonK Mar 21 '11 at 6:06
5  
use String [] stockArr = (String[]) stock_list.toArray(new String[0]); refer java doc here – Nishant Mar 21 '11 at 6:11

3 Answers

up vote 80 down vote accepted

Use like this.

    ArrayList<String> stock_list = new ArrayList<String>();
    stock_list.add("stock1");
    stock_list.add("stock2");
    String[] stockArr = new String[stock_list.size()];
    stockArr = stock_list.toArray(stockArr);
    for(String s : stockArr)
        System.out.println(s);
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1  
Thanks your anser help me. – iDroid Explorer Feb 2 '12 at 5:10
7  
To provide an explanation as to what is going on here, the JVM doesn't know how to blindly downcast Object[] (the result of toArray()) to String[]. To let it know what your desired object type is, you can pass a typed array into toArray(). The typed array can be of any size (new String[1] is valid), but if it is too small then the JVM will resize it on it's own. – dhackner Apr 6 '12 at 7:41
2  
@dhackner - "... the JVM doesn't know how to blindly downcast Object[] to String[]". Or more precisely, it is not allowed to do that. If it could do that, it would violate Java type safety. – Stephen C Nov 8 '12 at 4:14
Thanks for the answer. – Code Enthusiastic Jan 26 at 12:53
Thanks for sharing, really works great. Simple yet very useful knowledge – Diljeet Feb 23 at 22:45
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up vote 45 down vote

Try this

String[] arr = list.toArray(new String[list.size()]);
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up vote 24 down vote

What is happening is that stock_list.toArray() is creating an Object[] rather than a String[] and hence the typecast is failing.

The correct code would be:

  String [] stockArr = stockList.toArray(new String[stockList.size()]);

or even

  String [] stockArr = stockList.toArray(new String[0]);

For more details, refer to the javadocs for the two overloads of List.toArray.

(From a technical perspective, the reason for this API behaviour / design is that an implementation of the List<T>.toArray() method has no information of what the <T> is at runtime. All it knows is that the raw element type is Object. By contrast, in the other case, the array parameter gives the base type of the array. (If the supplies array is big enough, it is used. Otherwise a new array of the same type and a larger size is allocated and returned as the result.)

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2  
Why doesn't this work? String[] gs=(String[]) stockList.toArray(); – Ashwin Jun 29 '12 at 10:02
@Ashwin - that is explained in the first line of my Answer. And if you want the legalistic "why", it is because this is how the toArray() is specified by the javadoc. – Stephen C Aug 22 '12 at 6:54

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