Examining bytecode, I've noticed javac seems to duplicate checkcast
instructions when casting to array types.
Cast.java:
class Cast {
void test(Object a) {
Object[] b = (Object[])b;
}
}
javap disassembly of the javac compiled version
void test(java.lang.Object);
Code:
0: aload_1
1: checkcast #2; //class "[Ljava/lang/Object;"
4: checkcast #2; //class "[Ljava/lang/Object;"
7: astore_2
8: return
Testing jikes shows the expected single cast
void test(java.lang.Object);
Code:
0: aload_1
1: checkcast #10; //class "[Ljava/lang/Object;"
4: astore_2
5: return
checkcast
is supposed to raise an exception if the object can't be treated as the requested type and otherwise does nothing, so I don't see why it might help to double the cast. I haven't looked at the JDK sources to see how it's produced, and if that helps explain the why (maybe it's meant as a hint).