ToArray Method-Reprint

ToArray method may report null pointer exception and type conversion exception

Article One:

ArrayList provides a very handy way to convert a list to an array toarray. The ToArray has two overloaded methods:

1.list.toarray ();

2.list.toarray (t[] a);

For the first overloaded method, the list is converted directly to the object[] array;

The second method is to convert the list into an array of the type you want, and of course we'll use it as the same type as the list content.

Unidentified students like to use the first, is so written:

1234567

ArrayList<String> list=newArrayList<String>();        for(int i = 0; i < 10; i++) {            list.add(""+i);        }                String[] array= (String[]) list.toArray();

Result of a run, error:

Exception in thread "main" java.lang.ClassCastException: [Ljava.lang.Object; Cannot is cast to [Ljava.lang.String;

Why not convert object[] to string[] in a glance. The only thing that can be transformed is to take out every element and convert it, like this:

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object[] arr = List.toarray (); &NBSP;&NBSP;&NBSP;&NBSP;&NBSP;&NBSP;&NBSP;&NBSP; for   ( int   i = 0 ; I < Arr.length; i++) {              string e = (String) arr[i]; &NBSP;&NBSP;&NBSP;&NBSP;&NBSP;&NBSP;&NBSP;&NBSP;&NBSP;&NBSP;&NBSP;&NBSP; system.out.println (e); &NBSP;&NBSP;&NBSP;&NBSP;&NBSP;&NBSP;&NBSP;&NBSP;

So the first refactoring method is not so easy.

In fact, when you convert the list world to an array, the second refactoring method is more convenient and uses the following:

12	String[] array =newString[list.size()];        list.toArray(array);

Chapter Two:

public class Test {

public static void Main (string[] args) {

list<string> list = new arraylist<string> ();

List.add ("1");

List.add ("2");

String[] tt = (string[]) List.toarray (new string[0]);

}

}

This code is OK, but we see string[] tt = (string[]) List.toarray (new string[0]) The parameters are very strange, but remove this parameter new String[0] but run the times wrong ...

Two Root cause Analysis

The study found that ToArray has two methods:

Public object[] ToArray () {

Object[] result = new Object[size];

System.arraycopy (elementdata, 0, result, 0, size);

return result;

}

The ToArray method without parameters is a constructed object array and then a copy of the data, at which time the transformation will produce classcastexception, which is the root cause of the above problem.

Public object[] ToArray (Object a[]) {

if (A.length < size)

A = (object[]) java.lang.reflect.Array.newInstance (A.getclass (). Getcomponenttype (), size); System.arraycopy (elementdata, 0, a, 0, size);

if (A.length > Size)

A[size] = null;

return A;

}

The ToArray method with parameters, based on the type of the parameter array, constructs an empty array of the corresponding type, which is the same length as the size of ArrayList, although the method itself returns the result in the form of an object array. However, because the componenttype used in constructing arrays are consistent with the componenttype that need to be transformed, there is no transformation exception.

Three Solutions

So you can refer to the following three ways when using ToArray

1. long[] L = new Long[<total size>];

List.toarray (l);

2. long[] L = (long[]) List.toarray (new long[0]);

3. long[] A = new Long[<total size>];

Long[] L = (long[]) List.toarray (a);

Four Further consideration

The elements in the container have been restricted by generics, and the elements should be treated as generic types, but not in the current Java, when the direct string[] TT = (string[]) List.toarray (), run an error. Recall that the forced type conversion in Java is only for a single object, and to be lazy, it is not possible to convert the entire array into an array of another type, which is similar to the one that needs to be initialized.

ToArray Method-Reprint