Why we always use constructor?

Question

Dear community,

in the video "Building the Model", why we can´t declare and initialise the mSongs variable this way?

public class SongBook {
List<Song> mSongs = new ArrayList<Song>();
}

Why we have to declare the mSongs in the SongBook classs and initialise the same mSongs in the constructor of the SongBook class???

I hope you understand my question ...

:)

Answer 1 · 2015-07-17T02:44:18Z

July 17, 2015 2:44am

Hopefully my explanation isn't too hectic.

So, you just made a brand new class: SongBook. You need a SongBook constructor to even be able to instantiate it anywhere else. That's the rule. Think of a constructor like the instructions to build something.

Things like int, String, boolean don't need a constructor because these are primitive variables already built into Java. However, "SongBook" is clearly not built in. You created it ... so now you need a constructor in order to "make it exist." Without a constructor, you wouldn't be able to use SongBook in another class.

For example, if you created a SongMixer class (which remixes 2 songs together) you would use a constructor to instantiate SongBook in your SongMixer class.

Usually, you make a constructor method right after you declare your class.

public class MyCustomClass {
   public MyCustomClass() {
      //This is the constructor
      //Insert instructions on how to build MyCustomClass
      //This will allow you to instantiate MyCustomClass in other places
   }

   //Do other things here if you have to.
}

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Grigorij Schleifer

Grigorij Schleifer

Why we always use constructor?

1 Answer

Dennis Ping

Dennis Ping