The Java Language Specification, Third Edition

15.12.4 Runtime Evaluation of Method Invocation EXPRESSIONS
478
DISCUSSION
As an example of such a situation, consider the declarations:
class C { abstract T id(T x); }
class D extends C { String id(String x) { return x; } }
Now, given an invocation
C c = new D();
c.id(new Object()); // fails with a ClassCastException
The erasure of the actual method being invoked, D.id(), differs in its signature from
that of the compile-time method declaration, C.id(). The former takes an argument of
type String while the latter takes an argument of type Object. The invocation fails with a
ClassCastException before the body of the method is executed.
Such situations can only arise if the program gives rise to an unchecked warning
(§5.1.9).
Implementations can enforce these semantics by creating bridge methods. In the
above example, the following bridge method would be created in class D:
Object id(Object x) { return id((String) x); }
This is the method that would actually be invoked by the Java virtual machine in
response to the call c.id(new Object()) shown above, and it will execute the cast and
fail, as required.
If the method m is a native method but the necessary native, implementationdependent
binary code has not been loaded or otherwise cannot be dynamically
linked, then an UnsatisfiedLinkError is thrown.
If the method m is not synchronized, control is transferred to the body of the
method m to be invoked.
If the method m is synchronized, then an object must be locked
before the transfer of control. No further progress can be made until
the current thread can obtain the lock. If there is a target reference,
then the target must be locked; otherwise the Class object for class
S, the class of the method m, must be locked. Control is then transferred to the
body of the method m to be invoked. The object is automatically unlocked when
execution of the body of the method has completed, whether normally or abruptly.
The locking and unlocking behavior is exactly as if the body of the method were
embedded in a synchronized statement (§14.19).
DRAFT