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Java Java Data Structures Efficiency! Design the UI

Gustavo Winter
PLUS
Gustavo Winter
Courses Plus Student 27,382 Points

Don't get the syntax.

Hi, i don't understand why and how he can use the name of the class instead of String or Object to declare the type.

public KaraokeMachine(SongBook songBook){ // Why he use SongBook to declare a variable songBook
    mSongBook = songBook;
    new InputStreamReader(System.in)  
  }

If you can explain the purpose and how it's available, i'll appreciate.

package com.teamtreehouse.model;

import java.util.ArrayList;
import java.util.List;

public class SongBook{
  private List<Song> mSongs;

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

  public void addSong(Song song){
    mSongs.add(song);
  }

  public int getSongCount(){
    return mSongs.size();
  }

}

3 Answers

No, SongBook (singular and Capital S) is the class, a class is the blueprint of how to make an object. When we use the new keyword followed by any class name we get a single instance/object (object and instance are interchangeable). Like:

Object object = new Object();

The capital Object is the class (the blueprint for how to make a single object) and the lower case object is the variable that hold the value - new Object()

//Class Datatype                 //assignment operator = 
Object                   object    =             new Object();
                      //variable name                //new keyword used to create a object of type Object

The same would be for any other object: Object object = new Object();

SongBook songBook = new SongBook();
SongBook songBook1 = new SongBook();
SongBook songBook2 = new SongBook();

Above I made 3 instances of SongBook, each would have difference variable values

The 3 instance currently share the same values but if you changed the value of one variable it wouldn't the value of the other like:

To make this easier to understand I have to modify the SongBook Class a little:

public class SongBook{

 public String songBookName; //I added this just for the example below

 private List<Song> mSongs;

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

  public void addSong(Song song){
    mSongs.add(song);
  }

  public int getSongCount(){
    return mSongs.size();
  }

}

I gave the SongBook class another instance variable called songBookName, and its datatype is String This will allow me to:

SongBook songBook = new SongBook();
SongBook songBook1 = new SongBook();
SongBook songBook2 = new SongBook();

songBook.songBookName = "Book 0";
songBook1.songBookName = "Book 1";
songBook2.songBookName = "Book 2";

System.out.println( songBook.songBookName); //prints Book 0 to console
System.out.println( songBook1.songBookName); //prints Book 1 to console
System.out.println( songBook2.songBookName); //prints Book 2 to console

3 different objects of the Class SongBook, all have different variable values, changing one WON'T change the others

public KaraokeMachine(SongBook songBook)

The SongBook with the capital S is the datatype, while the songBook is the variable.

Similar would be: public KaraokeMachine(String songBook) but now the datatype of songBook is String not a SongBook object meaning developers could only pass in String Object arguments.

Sidenote: Java is a strongly type aka strictly typed language, meaning once you declare the datatype of a variable e.g.

String phoneNumber;
phoneNumber = "123";

You can only assign Strings to that variable. I can't do:

phoneNumber = 123;

This would give me a compilation error as we previously told the compiler that the datatype is String.

Verse other languages like JavaScript that is LooselyTyped meaning you can change the datatype of a variable anytime like:

var age = 10; //datatype = Number
age = "10; //datatype now = String