A method and computer system that implements polymorphic inline caches having locking slots. Each time the polymorphic method is called having a new object type, a new slot in the polymorphic inline cache is created. By comparing the object types of new methods with object type values already in the cache, it can readily be determined if a new slot has to be created to accommodate a new object type. Preferably, if two threads wish to access the polymorphic inline cache at the same time, the second thread will wait until the first thread accesses and creates and/or uses its slot because it is likely that the second thread will call the method of the same type. Altematively, the second thread may access and create and/or use another slot of the polymorphic inline cache because the first slot is locked to all objects other than its type.