[SOLVED] Sorting an ArrayList

[jakie presllie]

Hey, guys. I have a homework assignment due later tonight and am having a bit of trouble. I have to sort an array list as part of the assignment. My understanding is that must use Collections.sort, but I can't get that to work. I am not sure if that is due to my use of generics or what. Here is my current code.

import java.util.ArrayList;
import java.util.Collections;

//create generic MyList class with type parameter T
final class MyList<T extends Number>{

    //create ArrayList of type T
    ArrayList<T> list = new ArrayList<T>();

    //create an add method that adds argument of type T to ArrayList
    public void add( T new_element ){
	list.add( new_element );
    }

    //create method 'largest' that returns largest value of 'list'
    public T largest(){
	Collections.sort( list );
        //must return largest value after sorting
    }

    //create method 'smallest' that returns smallest value of 'list'
    public T smallest(){
	Collections.sort( list );
	//must return smallest value after sorting
    }
}

public final class GenericsHW{

    public static void main(final String[] args) {
	MyList<Number> list = new MyList<Number>();

	list.add( new Integer( 10  ) );
	list.add( new Double( 1.2 ) );
    }

}

and the error messages I am getting are:

/MyDev/java/HW3/GenericsHW.java
/MyDev/java/HW3/GenericsHW.java:30: cannot find symbol
symbol  : method sort(java.util.ArrayList<T>)
location: class java.util.Collections
	Collections.sort( list );
	                ^
MyDev/java/HW3/GenericsHW.java:36: cannot find symbol
symbol  : method sort(java.util.ArrayList<T>)
location: class java.util.Collections
	Collections.sort( list );
	                ^

I've looked at quite a few explanations of sorting arraylists, and they all achieve sorting with Collections.sort( ArrayList );, but this isn't working for me. They all used string values for sorting and I am using numbers, but it seems like Collections.sort would be able to sort values of the Number type. Any help would be great. Let me know if I left out something you need to know.

[Henry Wong]

If you look at the JavaDoc for the Collections class, you will see that the sort() method that takes one parameter, takes a generic that is an upper bound of Comparable. The generic that you defined doesn't fulfill that requirement.

You have a few options... (1) you can create a new list that takes a comparable generic, and copy your elements over, (2) you can add an upper bound to your class to require that the type be comparable, or (3) you can use a comparator instead, as that sort method doesn't have that requirement.

The easiest of these options, is option 2... which means that you have to change your class as follows...

final class MyList<T extends Number & Comparable<? super T>> {  

Henry

[Henry Wong]

And further down your code, where you create a MyList<Number>.... Well, you can't do that. The Number class doesn't implement Comparable.

Henry

[jakie presllie]

Ah, thank you. That seems to have worked. Believe it or not, I did look at the javadoc; it's just that I don't really know how to read the documentation, or how to learn how to read and understand it. I saw

sort(List<T> list)
          Sorts the specified list into ascending order, according to the natural ordering of its elements.

and thought I just inserted a list (I assumed arraylist would work because I saw it in other peoples' code) of type T. Now that I've read your post I notice the

static
<T extends Comparable<? super T>>
void

beside it, but I didn't know what that meant, nor that I could do <T extends Number & Comparable<? super T>>.

I see that if you click "sort" it takes you to a more detailed explanation, but when I looked at it earlier I saw

public static <T extends Comparable<? super T>> void sort(List<T> list)

and didn't understand that I could do the same thing to a class, where as here it was a method.

Thanks for you help. I think that fixes it.

[Ben Logan]

For resources on generics, there were a few good links in this thread, particularly the Angelika Langer site:

https://coderanch.com/t/480743/Beginning-Java/java/About-Generics#2158324

[jakie presllie]

Ah, great. I will take a look. Thank you very much for your help.