398 APPENDIX A: Introduction to Java
The actual object type in line 4 of Listing A-22 is ClassB, so at runtime the JVM checks whether
methodA( ) exists in ClassB. On not finding methodA( ) in ClassB (because it does not exist in ClassB),
the JVM checks for the existence of methodA( ) in the hierarchy of ClassB, because the JVM
considers the fact that there must be methodA( ) existing somewhere in the hierarchy of ClassB;
otherwise, the compiler would not have approved the call. Since methodA( ) exists in ClassA, the JVM
executes methodA( ) in ClassA at runtime.
Summary
This appendix introduced you to the basics of Java and object-oriented programming. You learned
how classes are the basic building blocks of object-oriented programs and how you can instantiate
objects from classes. Next, you were introduced to the three pillars of object-oriented programming:
encapsulation, inheritance, and polymorphism.