Programming Forums > Application Development > Java

s�kki · Nov 25th, 2005, 2:00 PM

Here's the problem little simplified:

I have made two classes Person and CardIndex.

I created Objects from both
Person myP = new Person("FirstName", "LastName");
CardIndex cInd = new CardIndex();

CardIndex is basically a vector where I dump those Persons.

Classes have some other stuff in them too, and Person is a super-class for two other Classes, so in CardIndex-vector can be at least three kind of classes or something like that.
Problem:
I have to sort the cInd by LastName using sort()-method. I'm lost

//Collections.sort(cInd**!*"*#) brainMalfunction. Thanks for any help.

Arevos · Nov 25th, 2005, 2:22 PM

You need a custom Comparator to tell Java how to sort. Comparator objects have a "compare" method that takes two Objects as arguments and returns an integer. If the integer is negative, the first object is less than the second; if the integer is positive, the first object is greater than the second; if the integer is 0, the objects are equal. The String.compareTo method can compare two strings in this way.

You can use an anonymous class to save cluttering up your code with a class that will only be used once. Maybe something like:

(Toggle Plain Text)

Collections.sort(cInd, new Comparator() {
   int compare(Person a, Person b) {
      return a.getLastName().compareTo(b.getLastName());
   }
});

xavier · Nov 25th, 2005, 2:30 PM

Shouldn't that be :

(Toggle Plain Text)

Collections.sort(cInd, new Comparator() {
   int compare(Object o1, Object o2) {
      Person a = (Person)o1;
      Person b = (Person)o2;
      return a.getLastName().compareTo(b.getLastName());
   }
});

Arevos · Nov 25th, 2005, 2:50 PM

Yes, you're right. Though, if you have Java 1.5:

(Toggle Plain Text)

Collections.sort(cInd, new Comparator<Person>() {
   int compare(Person a, Person b) {
      return a.getLastName().compareTo(b.getLastName());
   }
});

(Assuming cInd is descended from List<Person>)

s�kki · Nov 25th, 2005, 3:17 PM

Quote:

Originally Posted by Arevos

Yes, you're right. Though, if you have Java 1.5:

(Toggle Plain Text)

Collections.sort(cInd, new Comparator<Person>() {
   int compare(Person a, Person b) {
      return a.getLastName().compareTo(b.getLastName());
   }
});

(Assuming cInd is descended from List<Person>)

well it actually isn't descended from List<Person>, if I understand what you mean. cInd contains three kind of "persons" could be like this:
cInd {Person("FN", "LN"), Student("FN", "LN"), Worker("FN", "LN")} <- not meant to be anykind of syntax

just to clarify things.
Student and Worker are descended from Person.

Thanks for these tips allready, I'll try to get forward in this but I really am miserable in this.

Arevos · Nov 25th, 2005, 4:02 PM

Quote:

Originally Posted by s�kki

well it actually isn't descended from List<Person>, if I understand what you mean. cInd contains three kind of "persons" could be like this:
cInd {Person("FN", "LN"), Student("FN", "LN"), Worker("FN", "LN")} <- not meant to be anykind of syntax

just to clarify things.
Student and Worker are descended from Person.

Java 1.5 has a new feature called generics, which, amongst other things, allows you you to tie a type to a container

Before Java 1.5, you had to cast an element as the correct type when removing it from a container:

(Toggle Plain Text)

Vector list = new Vector();
list.add("Hello");
list.add("World");
String lastElement = (String)list.get(0);

In Java 1.5, you can use generics to tell the JVM which type to expect from a container:

(Toggle Plain Text)

Vector<String> list = new Vector<String>();
list.add("Hello");
list.add("World");
String lastElement = list.get(0);

The class Vector<Person> will take Persons, and descedants of Persons (ie. Workers and Students).

(Toggle Plain Text)

Vector<Person> persons = new Vector<Person>();

persons.add(new Person("A", "B"));
persons.add(new Worker("A", "B"));
persons.add(new Student("A", "B"));

for (Person person : persons) {
   System.out.println(person);
}

Of course, if you're not using Java 1.5, you can safely ignore all this talk about generics.

Quote:

Originally Posted by s�kki

Thanks for these tips allready, I'll try to get forward in this but I really am miserable in this.

You seem to be doing okay so far

s�kki · Nov 27th, 2005, 3:01 PM

OK thank you guys, finally got it working. Had some problems with the whole idea (what to do in which class etc). Post my solution here if it may help someone else.

(Toggle Plain Text)

In class Person:
//implements Comparable
-----------------------------
public int compareTo(Object next) {
	String bLastN = ((Person)next.getLastName();
	int comp = ((Person)this).getLastName().compareTo(bLastN);   	
	if (comp == 0) return 0;    //guess this is the only return statement that is 
	if (comp < 0) return -1;  //needed, but the 2 other doesn't seem to do
	else return 1;  	//any harm either
}
------------------------------------------

In class CardIndex:
public void sortItorScrewIt(){
   	Collections.sort(cInd);
}

Nov 25th, 2005, 2:00 PM	#1
s�kki Newbie Join Date: Feb 2005 Posts: 9 Rep Power: 0	about sort()-method Here's the problem little simplified: I have made two classes Person and CardIndex. I created Objects from both Person myP = new Person("FirstName", "LastName"); CardIndex cInd = new CardIndex(); CardIndex is basically a vector where I dump those Persons. Classes have some other stuff in them too, and Person is a super-class for two other Classes, so in CardIndex-vector can be at least three kind of classes or something like that. Problem: I have to sort the cInd by LastName using sort()-method. I'm lost //Collections.sort(cInd*!"*#) brainMalfunction. Thanks for any help.

Nov 25th, 2005, 2:22 PM	#2
Arevos Programming Guru Join Date: Aug 2005 Location: England Posts: 1,499 Rep Power: 4	You need a custom Comparator to tell Java how to sort. Comparator objects have a "compare" method that takes two Objects as arguments and returns an integer. If the integer is negative, the first object is less than the second; if the integer is positive, the first object is greater than the second; if the integer is 0, the objects are equal. The String.compareTo method can compare two strings in this way. You can use an anonymous class to save cluttering up your code with a class that will only be used once. Maybe something like: (Toggle Plain Text) Collections.sort(cInd, new Comparator() { int compare(Person a, Person b) { return a.getLastName().compareTo(b.getLastName()); } }); Collections.sort(cInd, new Comparator() { int compare(Person a, Person b) { return a.getLastName().compareTo(b.getLastName()); } });

Nov 25th, 2005, 2:30 PM	#3
xavier Professional Programmer Join Date: Oct 2004 Location: .ro Posts: 373 Rep Power: 4	Shouldn't that be : (Toggle Plain Text) Collections.sort(cInd, new Comparator() { int compare(Object o1, Object o2) { Person a = (Person)o1; Person b = (Person)o2; return a.getLastName().compareTo(b.getLastName()); } }); Collections.sort(cInd, new Comparator() { int compare(Object o1, Object o2) { Person a = (Person)o1; Person b = (Person)o2; return a.getLastName().compareTo(b.getLastName()); } }); __________________ Don't take life too seriously, it's not permanent !

Nov 25th, 2005, 2:50 PM	#4
Arevos Programming Guru Join Date: Aug 2005 Location: England Posts: 1,499 Rep Power: 4	Yes, you're right. Though, if you have Java 1.5: (Toggle Plain Text) Collections.sort(cInd, new Comparator<Person>() { int compare(Person a, Person b) { return a.getLastName().compareTo(b.getLastName()); } }); Collections.sort(cInd, new Comparator<Person>() { int compare(Person a, Person b) { return a.getLastName().compareTo(b.getLastName()); } }); (Assuming cInd is descended from List<Person>)

Nov 27th, 2005, 3:01 PM	#7
s�kki Newbie Join Date: Feb 2005 Posts: 9 Rep Power: 0	OK thank you guys, finally got it working. Had some problems with the whole idea (what to do in which class etc). Post my solution here if it may help someone else. (Toggle Plain Text) In class Person: //implements Comparable ----------------------------- public int compareTo(Object next) { String bLastN = ((Person)next.getLastName(); int comp = ((Person)this).getLastName().compareTo(bLastN); if (comp == 0) return 0; //guess this is the only return statement that is if (comp < 0) return -1; //needed, but the 2 other doesn't seem to do else return 1; //any harm either } ------------------------------------------ In class CardIndex: public void sortItorScrewIt(){ Collections.sort(cInd); } In class Person: //implements Comparable ----------------------------- public int compareTo(Object next) { String bLastN = ((Person)next.getLastName(); int comp = ((Person)this).getLastName().compareTo(bLastN); if (comp == 0) return 0; //guess this is the only return statement that is if (comp < 0) return -1; //needed, but the 2 other doesn't seem to do else return 1; //any harm either } ------------------------------------------ In class CardIndex: public void sortItorScrewIt(){ Collections.sort(cInd); }

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