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Trying to teach myself Java

Trying to teach myself Java

(OP)
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:

CODE

//: 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:

CODE

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?

RE: Trying to teach myself Java

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

RE: Trying to teach myself Java

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

CODE

//: Exercise41.java

// package-private class
class DataOnly {
  int i;
  double d;
  boolean b;
}

// public class with main() method
public class Exercise41 {
  public static void main(String[] argv) {
    DataOnly data1  = new DataOnly();
    DataOnly data2  = new DataOnly();

    // set values
    data1.i = 47;
    data1.d = 1.1;
    data1.b = false;

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

    // prrint out
    System.out.println("data1.i ='" + data1.i +"'");
    System.out.println("data1.d ='" + data1.d +"'");
    System.out.println("data1.b ='" + data1.b +"'\n");

    System.out.println("data2.i ='" + data2.i +"'");
    System.out.println("data2.d ='" + data2.d +"'");
    System.out.println("data2.b ='" + data2.b +"'");
  }
}

CODE

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

CODE

//: Exercise42.java

// public class with main() method
public class Exercise42 {

  // static subclass
  static class DataOnly {
    int i;
    double d;
    boolean b;
  }

  public static void main(String[] argv) {
    DataOnly data1  = new DataOnly();
    DataOnly data2  = new DataOnly();

    // set values
    data1.i = 47;
    data1.d = 1.1;
    data1.b = false;

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

    // prrint out
    System.out.println("data1.i ='" + data1.i +"'");
    System.out.println("data1.d ='" + data1.d +"'");
    System.out.println("data1.b ='" + data1.b +"'\n");

    System.out.println("data2.i ='" + data2.i +"'");
    System.out.println("data2.d ='" + data2.d +"'");
    System.out.println("data2.b ='" + data2.b +"'");      
  }
}

CODE

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.
smile
 

RE: Trying to teach myself Java

(OP)
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.

RE: Trying to teach myself Java

Hi

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

CODE --> Java

public class Exercise4 {
  class DataOnly {
    int i;
    double d;
    boolean b;
    DataOnly(int i, double d, boolean b)
    {
      this.i = i;
      this.d = d;
      this.b = b;
    }
  }
  DataOnly data = new DataOnly(47, 1.1, false);
}
( Note that I used BlueJ to test the class on its object bench without adding main() method or anything else. )

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

RE: Trying to teach myself Java

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

Cheers,
Dian

RE: Trying to teach myself Java

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

RE: Trying to teach myself Java

(OP)
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.

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