Re: Comparable loses its interface powers.

"John B. Matthews" <nospam@nospam.invalid>
Wed, 24 Feb 2010 16:28:11 -0500
In article <>,
 Roedy Green <> wrote:

But generics butt in and it seems to be impossible to keep around
arrays of Comparables for sorting columns.

I came up with this scheme for a type-safe Comparator<Record>. It lets
the compiler catch a Field that isn't Comparable:

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;

/** @author John B. Matthews */
public class ComparatorTest {

    public static void main(String[] args) {
        List<Record> list = new ArrayList<Record>(Arrays.asList(
            new Record(99, "Xxx"),
            new Record(56, "Yyy"),
            new Record(42, "Zzz")));
        print(list, Sort.ASCENDING, Field.I);
        print(list, Sort.DESCENDING, Field.S);

    private static void print(List<Record> list, Sort s, Field f) {
        RecordComparator rc = new RecordComparator(s, f);
        Collections.sort(list, rc);
        for (Record r : list) {

class Record {

    private Integer i;
    private String s;

    public Record(Integer i, String s) {
        this.i = i;
        this.s = s;

    public String toString() {
        return i + " " + s;

    public int compareTo(Field field, Record record) {
        switch (field) {
            case I: return this.i.compareTo(record.i);
            case S: return this.s.compareTo(record.s);
            default: throw new IllegalArgumentException(
                "Unable to sort Records by " + field.getType());


enum Field {

    I(Integer.class), S(String.class);

    private Class type;
    Field(Class<? extends Comparable> type) {
        this.type = type;

    public Class getType() {
        return type;

class RecordComparator implements Comparator<Record> {

    private Field field;
    private Sort sort;

    public RecordComparator(Sort sort, Field field) {
        this.sort = sort;
        this.field = field;

    public final int compare(Record a, Record b) {
        int result = a.compareTo(field, b);
        if (sort == Sort.ASCENDING) return result;
        else return -result;

John B. Matthews
trashgod at gmail dot com

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