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JQ+ 953425458590 (type casting)

 
Greenhorn
Posts: 7
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Hello all,
On JQ+ there is a question as follows:
public class TestClass
{
public static void main(String args[])
{
A[] a, a1;
B[] b;
a = new A[10]; a1 = a;
b = new B[20];
a = b; // 1
b = (B[]) a; // 2
b = (B[]) a1; // 3
}
}
class A { }
class B extends A { }
..............
The choices are
1) Compile Time error at line 3
2) The program will throw java.lang.ClassCastException at line 2 when run
3) The program will throw java.lang.ClassCastException at line 3 when run
4) The program will compile and run, if the (B[]) cast at line 2 and the whole line 3 is removed
5) The cast at line 2 is needed
..............
The answers are 3 and 5. I don't quite understand why it will throw a ClassCastException at line 3 if it does not do so at line 2. If I had variables A a = new A(); and B b = new B(), a = b does not need a cast, but b = a would not work. b = (B) a is required. The only thing I can see causing the specified behaviour is line 1, but I don't understand why it is so. Can someone please clarify this for me? Thanks.
 
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