public class ListToArrayTest {
public static void main(String args[]) {
ArrayList<String> list = new ArrayList<String>();
String group[];
list.add("aaa");
list.add("bbb");
list.add("ccc");
list.add("ddd");
// 1
// Object[] temp;
// temp = list.toArray();
// System.out.println(temp.length);
// group=new String[temp.length];
// for(int i=0;i<temp.length;i++){
// group[i]=(String) temp[i];
// }
//
// 2
group =new String[list.size()];
group= (String[]) list.toArray();
//Object to String
System.out.println("group size is "+group.length);
}
}
报错的是：
Exception in thread "main" java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to [Ljava.lang.String;
at ListToArrayTest.main(ListToArrayTest.java:23)
oboject的强转不是应该可以的吗？谢谢！
------解决方案--------------------
- Java code
import java.util.ArrayList;public class ListToArrayTest { public static void main(String args[]) { ArrayList<String> list = new ArrayList<String>(); String group[]; list.add("aaa"); list.add("bbb"); list.add("ccc"); list.add("ddd"); // 1 // Object[] temp; // temp = list.toArray(); // System.out.println(temp.length); // group=new String[temp.length]; // for(int i=0;i<temp.length;i++){ // group[i]=(String) temp[i]; // } // // 2 group = new String[list.size()]; list.toArray(group); // Object to String System.out.println("group size is " + group.length); for(String s: list) System.out.println(s); }}
------解决方案--------------------
- Java code
public static void main(String args[]) { ArrayList<String> list = new ArrayList<String>(); String group[]; list.add("aaa"); list.add("bbb"); list.add("ccc"); list.add("ddd"); String[] arr = new String[list.size()]; list.toArray(arr); for (String s : arr) { System.out.println(s); } }
------解决方案--------------------
使用 public <T extends java/lang/Object> T[] toArray(T[]);
------解决方案--------------------
List list = new ArrayList();
list.add("aaa");
list.add("bbb");
list.add("ccc");
list.add("ddd");
int size = list.size();
String[] str = (String[])list.toArray(new String[size]);
------解决方案--------------------
list.toArray(group);
------解决方案--------------------
下面是这两种方法的区别，查源码和注释就明白了
- Java code
/** * Returns an array containing all of the elements in this list * in proper sequence (from first to last element). * * <p>The returned array will be "safe" in that no references to it are * maintained by this list. (In other words, this method must allocate * a new array). The caller is thus free to modify the returned array. * * <p>This method acts as bridge between array-based and collection-based * APIs. * * @return an array containing all of the elements in this list in * proper sequence */ public Object[] toArray() { return Arrays.copyOf(elementData, size); } /** * Returns an array containing all of the elements in this list in proper * sequence (from first to last element); the runtime type of the returned * array is that of the specified array. If the list fits in the * specified array, it is returned therein. Otherwise, a new array is * allocated with the runtime type of the specified array and the size of * this list. * * <p>If the list fits in the specified array with room to spare * (i.e., the array has more elements than the list), the element in * the array immediately following the end of the collection is set to * <tt>null</tt>. (This is useful in determining the length of the * list <i>only</i> if the caller knows that the list does not contain * any null elements.) * * @param a the array into which the elements of the list are to * be stored, if it is big enough; otherwise, a new array of the * same runtime type is allocated for this purpose. * @return an array containing the elements of the list * @throws ArrayStoreException if the runtime type of the specified array * is not a supertype of the runtime type of every element in * this list * @throws NullPointerException if the specified array is null */ public <T> T[] toArray(T[] a) { if (a.length < size) // Make a new array of a's runtime type, but my contents: return (T[]) Arrays.copyOf(elementData, size, a.getClass()); System.arraycopy(elementData, 0, a, 0, size); if (a.length > size) a[size] = null; return a; }