OOP questions

[Panphobia]

What is constructor chaining? Also what is Encapsulation, Polymorphism, and Overloading, I am sure I know what they are, I just don't know the terms.

[Insectoid]

You might try Wikipedia.

[evildaddy911]

POLYMORPHISM: Extending a class and modifying the original class's methods to more suit the new method
example: make an "items" class for a RPG, then extend it and make a "sword" class.


OVERLOADING: have multiple methods with the same name but different parameters. the compiler will use the parameters you used to determine which method you meant and use it.
example:
public double square (double num){}
&
public int square (int num){}

not sure about the other two

[Insectoid]

Well, there isn't a wiki on constructor chaining, but it's pretty simple. Say you have a class A. Class B extends class A. The first thing you have to do in class B's constructor is call class A's constructor.

[Panphobia]

ahhh so like using the super functions am I correct? and does this() call the current class's constructor?

[Insectoid]

No. Calling this() inside a function returns a reference to itself.

[TerranceN]

Actually in Java calling this() DOES call other constructors, as seen here.

[md]

TerranceN @ 2013-01-26, 2:29 pm wrote:

Actually in Java calling this() DOES call other constructors, as seen here.

It can be used to call other constructors within the same class (as per your example). To call a superclass' constructor you need to use super()

[Panphobia]

can you use this() to chain constructors in the current class?

[Panphobia]

Also what is an advantage of constructor chaining? Why do we use it anyway?

[Dreadnought]

Not sure if this applies to Java, but in C++ you can't access elements of a parent class in the child class' constructor. So the child class' constructor will have to call the parent class' constructor in the initialization list.

[DemonWasp]

To clarify:

In Java, you use this(args) to call a second constructor from a first constructor. You can do this repeatedly, but each call to this() must be the first statement in the constructor. This strategy is commonly used for default parameters:

Java:

class Example {     public Example() { // constructor A         this ( DEFAULT_NAME ); // calls B, below     }     public Example ( String name ) { // constructor B         this ( name, DEFAULT_AGE ); // calls C, below     }     public Example ( String name, int age ) { // constructor C         this.name = name;         this.age = age;     } }

This kind of method-chaining is pretty common when you want a shorthand for a function that has a lot of parameters. The only thing special happening here is that you say "this()" instead of "ClassName()" and that it must be the first statement in your constructor.

C++11 introduced a similar mechanic, where you can chain constructors within the same class by putting them in the initializer list.

[Panphobia]

I just finished my programming exam, it had 70 m/c and 1 programming question, for the programming question it basically told you to create a media abstract class an mp3 subclass and a dvd subclass, and add methods and variables to each, add chained constructors etc, but it also asked you to make an equals method. I did all this according to plan, one problem was to compare the last two elements in the array of media objects and if they are equal you print the contents of the array. I did not cast the object when I compared them, did I need to? what i mean is if I declare a media object as Media m = new DVD(); and I used equals would I need to explicitly cast m.equals()?

[Insectoid]

.equals() would have to be defined inside class Media, if you are comparing an MP3 to a DVD. If you want to see if two MP3s are equal, or two DVDs, you should cast to that type (in which case, you should define another .equals inside DVD/MP3).

But I could be wrong, since I didn't pay much attention in this class and haven't used it...ever.

[Panphobia]

ahhhh alright, I guess that is the only thing I did not do right then, thanks though, if the last two elements in the array are not the same type then return false, I had the equals method in my mp3 class and dvd class but not one in Media, I needed to cast it right? because it wasnt being overriden?