Trying to teach myself Java

[whughesiii]

Hey all,
I have some experience with programming with Perl, PHP and a tiny bit of C++. I am trying to teach myself Java. Of course the book I am using doesn't have solutions to the exercises with explanations at all. On one exercise it says to take bits of code from the examples and make them into a program that compiles and runs. I believe I have it right, but as I am a beginner I probably am missing something. I have a file called Exercise4.java and the code is as follows:

//: Exercise4.java

public class Exercise4
{
 class DataOnly
  {
   int i;
   double d;
   boolean b;
  }
 DataOnly data = new DataOnly();

 data.i = 47;
 data.d = 1.1;
 data.b = false;
}

However when I compile it, I get these errors:

Exercise4.java:13: <identifier> expected
 data.i = 47;
        ^
Exercise4.java:14: <identifier> expected
 data.d = 1.1;
        ^
Exercise4.java:15: <identifier> expected
 data.b = false;
        ^
3 errors

What am I doing wrong?

 

[Diancecht]

Java variables are not accesed like struct members. I think you should read the basis first, getting concept like classes and methods.

You can find a lot ot tutorials like this

Cheers,
Dian

 

[mikrom]

To run your example you need a main() method.
I tried to do it at 2 ways:

1. With package-private class for the data structure

[COLOR=#0000ff]//: Exercise41.java[/color]

[COLOR=#0000ff]// package-private class[/color]
[COLOR=#2e8b57][b]class[/b][/color] DataOnly {
  [COLOR=#2e8b57][b]int[/b][/color] i;
  [COLOR=#2e8b57][b]double[/b][/color] d;
  [COLOR=#2e8b57][b]boolean[/b][/color] b;
}

[COLOR=#0000ff]// public class with main() method[/color]
[COLOR=#2e8b57][b]public[/b][/color] [COLOR=#2e8b57][b]class[/b][/color] Exercise41 {
  [COLOR=#2e8b57][b]public[/b][/color] [COLOR=#2e8b57][b]static[/b][/color] [COLOR=#2e8b57][b]void[/b][/color] main(String[] argv) {
    DataOnly data1  = [COLOR=#804040][b]new[/b][/color] DataOnly();
    DataOnly data2  = [COLOR=#804040][b]new[/b][/color] DataOnly();

    [COLOR=#0000ff]// set values[/color]
    data1.i = [COLOR=#ff00ff]47[/color];
    data1.d = [COLOR=#ff00ff]1.1[/color];
    data1.b = [COLOR=#ff00ff]false[/color];

    data2.i = [COLOR=#ff00ff]147[/color];
    data2.d = [COLOR=#ff00ff]11.1[/color];
    data2.b = [COLOR=#ff00ff]true[/color];    

    [COLOR=#0000ff]// prrint out[/color]
    System.out.println([COLOR=#ff00ff]"data1.i ='"[/color] + data1.i +[COLOR=#ff00ff]"'"[/color]);
    System.out.println([COLOR=#ff00ff]"data1.d ='"[/color] + data1.d +[COLOR=#ff00ff]"'"[/color]);
    System.out.println([COLOR=#ff00ff]"data1.b ='"[/color] + data1.b +[COLOR=#ff00ff]"'[/color][COLOR=#6a5acd]\n[/color][COLOR=#ff00ff]"[/color]); 

    System.out.println([COLOR=#ff00ff]"data2.i ='"[/color] + data2.i +[COLOR=#ff00ff]"'"[/color]);
    System.out.println([COLOR=#ff00ff]"data2.d ='"[/color] + data2.d +[COLOR=#ff00ff]"'"[/color]);
    System.out.println([COLOR=#ff00ff]"data2.b ='"[/color] + data2.b +[COLOR=#ff00ff]"'"[/color]); 
  }
}

C:\_mikrom\Work>javac -classpath . Exercise41.java

C:\_mikrom\Work>java -cp . Exercise41
data1.i ='47'
data1.d ='1.1'
data1.b ='false'

data2.i ='147'
data2.d ='11.1'
data2.b ='true'

2. With subclass for the data structure

[COLOR=#0000ff]//: Exercise42.java[/color]

[COLOR=#0000ff]// public class with main() method[/color]
[COLOR=#2e8b57][b]public[/b][/color] [COLOR=#2e8b57][b]class[/b][/color] Exercise42 {

  [COLOR=#0000ff]// static subclass[/color]
  [COLOR=#2e8b57][b]static[/b][/color] [COLOR=#2e8b57][b]class[/b][/color] DataOnly {
    [COLOR=#2e8b57][b]int[/b][/color] i;
    [COLOR=#2e8b57][b]double[/b][/color] d;
    [COLOR=#2e8b57][b]boolean[/b][/color] b;
  }

  [COLOR=#2e8b57][b]public[/b][/color] [COLOR=#2e8b57][b]static[/b][/color] [COLOR=#2e8b57][b]void[/b][/color] main(String[] argv) {
    DataOnly data1  = [COLOR=#804040][b]new[/b][/color] DataOnly();
    DataOnly data2  = [COLOR=#804040][b]new[/b][/color] DataOnly();

    [COLOR=#0000ff]// set values[/color]
    data1.i = [COLOR=#ff00ff]47[/color];
    data1.d = [COLOR=#ff00ff]1.1[/color];
    data1.b = [COLOR=#ff00ff]false[/color];

    data2.i = [COLOR=#ff00ff]147[/color];
    data2.d = [COLOR=#ff00ff]11.1[/color];
    data2.b = [COLOR=#ff00ff]true[/color];    

    [COLOR=#0000ff]// prrint out[/color]
    System.out.println([COLOR=#ff00ff]"data1.i ='"[/color] + data1.i +[COLOR=#ff00ff]"'"[/color]);
    System.out.println([COLOR=#ff00ff]"data1.d ='"[/color] + data1.d +[COLOR=#ff00ff]"'"[/color]);
    System.out.println([COLOR=#ff00ff]"data1.b ='"[/color] + data1.b +[COLOR=#ff00ff]"'[/color][COLOR=#6a5acd]\n[/color][COLOR=#ff00ff]"[/color]); 

    System.out.println([COLOR=#ff00ff]"data2.i ='"[/color] + data2.i +[COLOR=#ff00ff]"'"[/color]);
    System.out.println([COLOR=#ff00ff]"data2.d ='"[/color] + data2.d +[COLOR=#ff00ff]"'"[/color]);
    System.out.println([COLOR=#ff00ff]"data2.b ='"[/color] + data2.b +[COLOR=#ff00ff]"'"[/color]);      
  }
}

C:\_mikrom\Work>javac -classpath . Exercise42.java

C:\_mikrom\Work>java -cp . Exercise42
data1.i ='47'
data1.d ='1.1'
data1.b ='false'

data2.i ='147'
data2.d ='11.1'
data2.b ='true'

Both examples above seems to work, but because I'm not experienced Java programmer, there is surely better solution.

 

[whughesiii]

Thanks for the tips! I added a main() method and compile worked. I always struggle learning something new at first especially when I haven't used classes in about 6 years.

@Diancecht I am trying to learn the basics and understand classes and methods and such. That is why I asked for help when I couldn't figure out what the problem was.

The book I am using is probably not the best, I didn't see anything about needing a method to make it run. I plan to re-read this First chapter again to see if I missed anything.

 

[feherke]

Hi

Personally I would think about adding a constructor to DataOnly, so its properties can be set on instantiation :

[b]public[/b] [b]class[/b] [COLOR=mediumorchid]Exercise4[/color] [teal]{[/teal]
  [b]class[/b] [COLOR=mediumorchid]DataOnly[/color] [teal]{[/teal]
    [maroon]int[/maroon] i[teal];[/teal]
    [maroon]double[/maroon] d[teal];[/teal]
    [maroon]boolean[/maroon] b[teal];[/teal]
    [COLOR=darkgoldenrod]DataOnly[/color][teal]([/teal][maroon]int[/maroon] i[teal],[/teal] [maroon]double[/maroon] d[teal],[/teal] [maroon]boolean[/maroon] b[teal])[/teal]
    [teal]{[/teal]
      [b]this[/b][teal].[/teal]i [teal]=[/teal] i[teal];[/teal]
      [b]this[/b][teal].[/teal]d [teal]=[/teal] d[teal];[/teal]
      [b]this[/b][teal].[/teal]b [teal]=[/teal] b[teal];[/teal]
    [teal]}[/teal]
  [teal]}[/teal]
  [COLOR=orchid]DataOnly[/color] data [teal]=[/teal] [b]new[/b] [COLOR=darkgoldenrod]DataOnly[/color][teal]([/teal][purple]47[/purple][teal],[/teal] [purple]1.1[/purple][teal],[/teal] [b]false[/b][teal]);[/teal]
[teal]}[/teal]

( Note that I used BlueJ to test the class on its object bench without adding [tt]main()[/tt] method or anything else. )

Feherke.

http://free.rootshell.be/~feherke/

 

[Diancecht]

I still think it is not a good practice to access class variables that way.

Cheers,
Dian

 

[mikrom]

I agree with feherke and Diancecht: Use constructors and rather access attributes through getters/setters.

 

[whughesiii]

Thanks for your help all. I got it figured out. This is going to be a slow learning process. I am starting to remember the basics of C++ that I learned many years ago and it will help make Java easier.