TIJ chapter 7: Reusing Classess

Composition, inheritance and delegation are 3 main methods to reuse classes.

Delegation is not a familiar concept for me, and it is not directly supported. However, I think I have used its content before. It just put a member object in the new class and expose all method to the new one.

About inheritance, it always calls the super() constructor even if you don’t called it literally in derived class. As a result, if the base class don’t have a constructor without any parameters, and you don’t call base class constructor literally in derived class, you will get a compile time error. read more

TIJ chapter 6: Access Control

This chapter is not a complex one. It just tells the access control involved in Java.

There are 4 access level in Java, public, protected, package-private, private. (By accident they all start with letter “p”). Package-private is the default one without any modifier. The only thing one should note is the protected access level implies package-private ones(Actually this is true for all 4 levels, one implies the other one by order).

If one doesn’t allocate a package for a class, this class will belong to the default package. Default package in a same directory is viewed as one package. read more

Thinking in Java Chapter 1&2: Object

After one year of programing with Java, I am gradually familiar with OOP. Learning java for me, whose first programming language is C, was much more than change the name system, like field, method, reference, etc. It is almost a renewal of concept. Although so, my knowledge of Java was founded from all kinds of sources, and a book like Thinking in Java seems useful for me to introduce systematically introduce the thought behind Java.

It is important to design classes appropriately. For people who is not familiar with OOP, it can easily happen that use classes as structs and designs methods as “functions” in the outer classes. Abusing inheritance may also be a popular mistake. When the relationship between classes is “have-a”, one should use composition(or aggregation when dynamically). We should also think about it whether the relationship is “is-a” or “is-like-a”. For the former one, we may only override methods of base class, while the latter one may involves implicating new methods. When we upcasting objects to their base classes types, only methods we use substitution can be called. Otherwise, we will have to downcast the reference to call the unique methods. Although so, both the two models are useful, and the choice will be obvious for certain problems. read more