java help: user-defined classes

[dbbolton]

luckily, i have realized that i hate programming so i'm not studying CS anymore after this semester. however, for now i still have a programming assignment to do.

i need some help with user defined classes.

i am given the class Person, and i have to do the following:

-Check whether a given last name is the same name as the last name of this person
-Check whether a given first name is the same name as the first name of this person
-Check whether a given middle name is the same name as the middle name of this person
-Employ a method called equals that returns true if two objects contain the same first and last name
-Employ a method called makeCopy that copies the instance variables of a Person object into another Person object
-Employ a method called getCopy that creates and returns the address of the object, which is a copy of another Person object.

there were other tasks that i already added as well, so this is what we have:



public class Person
{
private String firstName; //store the first name
private String middleName; //store the middle name
private String lastName; //store the last name

//default constructorwrw
public Person()
{
firstName = "";
middleName = "";
lastName = "";
}

//copy constructor
public Person(Person otherPerson)
{
firstName = otherPerson.firstName;
middleName = otherPerson.middleName;
lastName = otherPerson.lastName;
}

//create a person with a given first and last name
public Person(String first, String last)
{
setName(first, last);
}

//create a person with a given first, middle, and last name
public Person (String first, String middle, String last)
{
setName(first, middle, last);
}

//turn the full name into a string
public String toString()
{
return (firstName + " " + middleName + " " + lastName);
}

//set the first name of a person
public void setFirstName(String first)
{
firstName = first;
}

//set the middle name of a person
public void setMiddleName(String middle)
{
middleName = middle;
}

//set the last name of a person
public void setLastName(String last)
{
lastName = last;
}

//set both the first and last names of a person
public void setName(String first, String last)
{
firstName = first;
lastName = last;
}

//set the first, middle, and last names of a person
public void setName(String first, String middle, String last)
{
firstName = first;
middleName = middle;
lastName = last;
}

//get the first name of a person
public String getFirstName()
{
return firstName;
}

//return the middle name of a person
public String getMiddleName()
{
return middleName;
}

//return the las name of a person
public String getLastName()
{
return lastName;
}

}


when i tried to create the "equals" method, i got a ton of errors. as for the copying tasks, i added the copy constructor but i'm not sure how to use it.

i don't want someone to simply do this for me, because i actually want to learn how to do it. so, some guidance to set me in the right direction would be greatly appreciated.

[CptPicard]

luckily, i have realized that i hate programming so i'm not studying CS anymore after this semester. however, for now i still have a programming assignment to do.

Oh! I hate programming too and I have my Master's! Programming is mostly Menial Labour, but as an undergrad you will have to learn it in order to use it as a tool later on when you must, but the most interesting CS stuff is perfectly doable with your mind, pencil and paper, unless you're into testing stuff with simulation. Even then, your code is allowed to be ugly, because it's an academic exercise... as long as it works :)

Trust me, the fun stuff is yet to come. Then again, if you're stuck already by this point...

First of all, get the equals method working, you're going to want it. I'd like to see your errors, but I suspect it is because equals has to be implemented to take Object. So inside the method, your first task is usually to check whether your parameter Object o is an "instanceof" Person (ok, your FIRST task REALLY is usually checking for reference equality). If it is, you can then start comparing field by field -- and because your fields are Strings, you'll be asking the String equals() method for String equality.

The copying methods are simple, you just instantiate a new Person and set the fields to match what you have in your original object, right?

[dbbolton]

I agree with you on that. I think certain parts of CS are interesting, like algorithm design. I just can't get past the programming, so I decided to be an English major :)

I think I figured part of it out. Here is what I tried as "equals" method:


public boolean equals(Person otherPerson)
{
return(firstName == otherPerson.firstName
&& lastName == otherPerson.lastName);
}


Does that seem right?

[CptPicard]

Check this out for a fast Googled reference: http://www.javapractices.com/Topic17.cjp


No, it does not seem right, because you're making the fundamental reference comparison vs. equals-equality mistake. This is why the equals() method really exists... mind you, in your case, what you are doing might actually seem like working for a while, but only for as long as your entire system has a single instance of each literal string.

The point is that if you can actually have separate objects, say x and y, of String that all contain, say, "ABC". For them, the reference comparison x == y will actually return false, as they are separate objects in memory. However, x.equals(y) and y.equals(x) is supposed to return true, as they are the same string by their contents!

What you want to be returning for an equals value is thus a delegation to the String equals method:



return firstName.equals(otherPerson.firstName) && lastName.equals(otherPerson.lastName);

[dbbolton]

that's funny, because i copied that idea from an example in my textbook, which was an "equals" method for two members of a predefined "Clock" class:



public boolean equals(Clock otherClock)
{
return(hr == otherClock.hr
&& min == otherClock.min
&& sec == otherClock.sec);
}


so, did the book just give a poor example? (i wouldn't be surprised. i've found errors in it before.)

[CptPicard]

The Clock's members probably aren't objects, but primitives such as int's. They can be compared directly like that. Objects can't, because the value of an "object variable" is actually a reference... essentially a memory address (a pointer, in C terms) that is used to access the object at that location.

An object is anything you initialize with a "new", in general... with String being a nasty little exception.... So... if you've got a



String a = "Hello world!"; // could easily be written explicitly as new String("Hello world!");
String b = "Hello world!";


Then a == b will be false (they are two separate objects, references a and b point to separate memory locations) but a.equals(b) will be true (they are the same string by value) but



int x = 5;
int y = 5;

x == y // This is true!

String c = a;

c == a // this is true, as c now "points to" the same object as a
c.equals(a); c.equals(b); // these are, of course, true as values are the same

[dbbolton]

ah! now that you mention it, i (vaguely) remember talking about it in class. so, now i understand why the first idea doesn't work.

another part of the assignment was to write a small program to test the class Person. here is what i wrote:



import java.util.*;

class TestPerson
{
public static void main(String [] args)
{
Scanner in = new Scanner(System.in);

Person person1 = new Person(Daniel, Barrett, Bolton);
Person person2 = new Person();

String first, middle, last;

System.out.print("Person1: ");
person1.printName();
System.out.println();

person2.setName(Wolfgang, Amadeus, Mozart);

System.out.print("After setting the name, person2: ");
person2.printName();
System.out.println();

if (person1.equals(person2))
{
System.out.println("The two persons are equal.");
}
else
System.out.println("The two names are not equal.");

System.out.println("Enter a first name.");
first = in.next();

System.out.println("Enter a middle name.");
middle = in.next();

System.out.println("Enter a last name.");
last = in.next();

person1.setName(first, middle, last);

System.out.print("New name of person1: ");
person1.printName();
System.out.println();

person2.makeCopy(person1);
System.out.println("After copying person1 into person2," +
" person2: ");
person2.printName();
System.out.print("\n\n The program is finished.");
}//end main

}//end class


when i tried to compile it (i'n using Geany) it said that virtually all of the variables can't be resolved. any idea why?

[CptPicard]

Wonder what you mean by "virtually all variables"; seeing the error dump might enlighten. It does sound like some sort of a classpath issue, but nothing in your code seems obviously wrong, really.

Btw, a good analogy with your W.A. Mozart "Person" and reference-comparing == and value-comparing equals() is that == compares for the same person (there are just distinctly separate individual people), while equals() tells whether some two people have the same name (you could conceivably have some other guy called W.A. Mozart).

[dbbolton]

i would have posted the errors, but for some reason geany only allows one to copy one line at a time-- you can't just highlight all of the output at once. you just have to right click on a line and click copy.

here is the output from the compiler:

http://i103.photobucket.com/albums/m128/envyouraudience/err1.png
http://i103.photobucket.com/albums/m128/envyouraudience/err2.png

[CptPicard]

I get the vague impression that Geany doesn't run javac with any kind of a classpath set. Can you compile anything from there? What about from command line... like "javac PersonTest.java", and then "java PersonTest"...

And which Java do you have? gcj or Sun? The gcj one is a PITA...

[dbbolton]

it's odd. on my other computer, i used geany to write a program that compiled without anys errors, and the .class worked fine. when i tried to compile it with geany on this computer, it gave a lot of errors. so i'll try compiling TestPerson on my other as soon as i get a chance.

i have Sun (i think i have both jre and jdk 6).

[Ramses de Norre]

What's the output of java -version? It should be at least java 5 to use Scanner. Use sudo update-java-alternatives with the -l and -s switches to change otherwise, read the man page for more info.

About your code, you might want to add tests for null-pointers in your setters and mind that person1.equals(person2) gives a NullPointerException if person1==null but not if person2==null while person1!=null, so mind the sequence. You should also use your getters instead of calling fields explicitly, instead of person.firstName you should invoke person.getFirstName(). This makes your code adaptable, if you'd ever change something about the implementation of the firstName field, you'd just need to change the getter and the setter and all other methods can be left unchanged.

This is what I'd use as equals() method, just for reference, I've written it fast so minor bugs are possible.


/**
* Equals method, tests on first and last name only.
*/
@Override
public boolean equals(Object otherPerson)
{
if(otherPerson!=null &&
otherPerson.getClass().equals(this.getClass()))
{
return (getFirstName().equals(((Person)otherPerson).getFi rstName())
&& getLastName().equals(((Person)otherPerson).getLast Name()));
}
return false;
}

[dbbolton]

'java -version' gives:



java version "1.6.0"
Java(TM) SE Runtime Environment (build 1.6.0-b105)
Java HotSpot(TM) Client VM (build 1.6.0-b105, mixed mode, sharing)


yet i have the package 'sun-java6-jre' installed...

[CptPicard]

It's the "jdk" one you want as you're trying to compile...

[dbbolton]

It's the "jdk" one you want as you're trying to compile...
yes; i also have 'sun-java6-jdk' installed.

[CptPicard]

You actually need just either, not both :)

[Ramses de Norre]

What's the output of echo $CLASSPATH? It should point to your java installation lib directories, I get this here: (java installed in /opt/)

pts/1 $ echo $CLASSPATH
:/opt/java/lib:/opt/java/jre/lib

You can set it correctly otherwise in your ~/.bashrc (by exporting CLASSPATH directly or by specifying JAVA_HOME).

[dbbolton]

there was no output (other than a blank line)



daniel@slimline:~$ echo $CLASSPATH

daniel@slimline:~$ echo $CLASSPATH

[Ramses de Norre]

Set it to the correct value by exporting it in your ~/.bashrc then. You need to specify the lib directories as in my example but you have to search where ubuntu installs java.

[CptPicard]

That may help but with newer Javas that is no longer really neccessary. For example my $CLASSPATH is not set, and Java works fine.

[jespdj]

What's the output of echo $CLASSPATH? It should point to your java installation lib directories, I get this here: (java installed in /opt/)

pts/1 $ echo $CLASSPATH
:/opt/java/lib:/opt/java/jre/lib

You can set it correctly otherwise in your ~/.bashrc (by exporting CLASSPATH directly or by specifying JAVA_HOME).
No, this is wrong. You do not need the lib directories in your CLASSPATH. You do not need to set a CLASSPATH at all. If you leave the CLASSPATH unset, Java will look for class files in the current directory. It knows how to find the JRE's classes automatically; these never have to be included in the CLASSPATH.

[geirha]

This is a typical gotcha. Ubuntu ships by default with gcj, a gnu-alternative to java, it's probably still using this, even though you have sun's java installed. Run
update-java-alternatives --list to see what java packages are installed, and set the sun-java to be the one to use. I have java 1.5.0 so I would do:
sudo update-java-alternatives --set java-1.5.0-sun to achieve this.