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Sorting An Array of Objects

Greenhorn

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posted 24 years ago

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I've created a class with several string variables. I created an ArrayList of the class objects. I want to sort the list on one of the variables. I change the list to an array to sort it. When I try to sort the array I get a java.lang.ClassCastException error.

Do I need a comparator? What is a comparator and how do I use it here?
[ May 31, 2002: Message edited by: Duane Eddy ]

John Dale

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posted 24 years ago

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You need a comparitor or a compareTo() method. If you have a compareTo(), you should have an equals(). If you have an equals(), you must have a hashCode(). So providing a comparitor might be simpler. However, here is a demo with CompareTo.

import java.util.*;
class Test3 implements Comparable{
    String a;
    String b;
    Test3(String a, String b) {
        this.a = a;
        this.b = b;
    }
    
    public int compareTo(Object other) {
    Test3 o = (Test3)other;
    if (a.compareTo(o.a) != 0)
    return a.compareTo(o.a);
    return b.compareTo(o.b);
    }
    
    public boolean equals(Object other) {
    if (!(other instanceof Test3))
    return false;
    Test3 o = (Test3)other;
    return a.equals(o.a) && b.equals(o.b);
    }
    
    public int hashCode() {
    int results = 17;
    results = 37*results + a.hashCode();
    results = 37*results + b.hashCode();
    return results;
    }
    
    public String toString() {
    return super.toString() + "(a=" + a + ", b=" + b + ")";
    }
    
    public static void main(String[] args) {
        List list = new ArrayList();
        for (int i = 0; i < 10; i++) {
            list.add(new Test3(Integer.toString(99-i), Integer.toString(50-i)));
        }
        Test3[] a = (Test3[]) list.toArray(new Test3[0]);
        print(a);
        Arrays.sort(a);
        print(a);
    }
    
    public static void print(Object[] a) {
    StringBuffer b = new StringBuffer();
    for (int i = 0; i < a.length; ++i) {
    b.append(a[i]);
    b.append(" ");
    }
    System.out.println(b);
    }
}

tisham

Greenhorn

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posted 24 years ago

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Hello,
The dictionary class might be helpful in comparing and sorting strings!!Just a suggestion
Cheers

Ilja Preuss

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posted 24 years ago

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Originally posted by Duane Eddy:
I change the list to an array to sort it.

You could use one of the Collections.sort methods instead.

The soul is dyed the color of its thoughts. Think only on those things that are in line with your principles and can bear the light of day. The content of your character is your choice. Day by day, what you do is who you become. Your integrity is your destiny - it is the light that guides your way. - Heraclitus

Ilja Preuss

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posted 24 years ago

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Originally posted by tisham:

The dictionary class might be helpful in comparing and sorting strings!!

Would it? In which way?

John Dale

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posted 24 years ago

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As suggested, there is no need to convert to an array to sort.
But I would think that to do any sorting, one would need criteria for comparing the objects. Hence the need for Comparable (that is, with a compareTo()) or a Comparitor (with its compare() method).
Once you provide a way to compare -- a sorting order -- you might even find you want to use a SortedSet (TreeSet) instead of an unsorted List (ArrayList) -- if Set semantics match your requirements better than List semantics.
[ June 01, 2002: Message edited by: John Dale ]

Duane Eddy

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posted 24 years ago

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Does anyone have an example of using a comparitor?

Ilja Preuss

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posted 24 years ago

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Originally posted by Duane Eddy:
Does anyone have an example of using a comparitor?

Yes: http://java.sun.com/docs/books/tutorial/collections/interfaces/order.html
BTW, I found this by searching for "java comparator tutorial" at www.google.com