checkcast : ensure type of an object or array : index : visitTypeInsn()

Description

checkcast checks that the top item on the operand stack (a reference to an object or array) can be cast to a given type. For example, if you write in Java:

return ((String)obj);

then the Java compiler will generate something like:

aload_1                        ; push -obj- onto the stack
checkcast java/lang/String     ; check its a String
areturn                        ; return it

checkcast is actually a shortand for writing Java code like:

if (! (obj == null  ||  obj instanceof <class>)) {
    throw new ClassCastException();
}
// if this point is reached, then object is either null, or an instance of
// <class> or one of its superclasses.

In Jasmin, checkcast takes a single parameter, <type>. <type> is either the name of a Java class or interface, or it is the type descriptor of an array. At runtime, the symbolic name given by <type> is resolved to a Java class (see Chapter 7 for a description of how classes are resolved). Next, checkcast examines the top item on the stack. If objectref belongs to <type> (or one of its subclasses), or if it is null, the objectref is left on the stack and execution continues at the subsequent instruction. If not, a ClassCastException is thrown.

Exceptions

ClassCastException - the object on the stack is not an instance of the specified class

Example

; push object in local variable 1 onto stack
aload_1
; check if it is an instance of Throwable or one of its subclasses.
checkcast java/lang/Throwable
; if execution reaches here, the object in local variable 1
; is still on the stack, and is either null or a Throwable object.

; ---
; Note that checkcast can also be used to check that an array belongs to a given type, 
; e.g. to check that local variable 1 contains an array of Strings, use:
aload_1
checkcast [Ljava/lang/String;
; if execution reaches here, the  object on the stack is an array of Strings, or it is null.

See also

instanceof

Stack

Before	After
objectref	objectref
...	...

Bytecode

Type	Description
u1	checkcast opcode = 0xC0 (192)
u2	index