Question

I am doing a project for a CS 140 class at my university and I am trying to create a program that uses selection sort with an array of the 50 states of the US with fields for name, population, capital and region. Here is the array given:

private static void loadStates() {
    // Set the search key to be the state name.
    State.setKey("name");

    us = new Country("United States", 311591917, "Washington DC",
            new State[]{
                new State("Alabama", 4802740, "Montgomery", "Southeast"),
                new State("Alaska", 722718, "Juneau", "West"),
                new State("Arizona", 6482505, "Phoenix", "Southwest"),
                new State("Arkansas", 2937979, "Little Rock", "Southeast"),
                new State("California", 37691912, "Sacramento", "West"),
                new State("Colorado", 5116769, "Denver", "West"),
                new State("Connecticut", 3580709, "Hartford", "Northeast"),
                new State("Delaware", 907135, "Dover", "Northeast"),
                new State("Florida", 19057542, "Tallahassee", "Southeast"),
                new State("Georgia", 9815210, "Atlanta", "Southeast"),
                new State("Hawaii", 1374810, "Honolulu", "West"),
                new State("Idaho", 1584985, "Boise", "West"),
                new State("Illinois", 12869257, "Springfield", "Midwest"),
                new State("Indiana", 6516922, "Indianapolis", "Midwest"),
                new State("Iowa", 3062309, "Des Moines", "Midwest"),
                new State("Kansas", 2871238, "Topeka", "Midwest"),
                new State("Kentucky", 4369356, "Frankfurt", "Southeast"),
                new State("Louisiana", 4574836, "Baton Rouge", "Southeast"),
                new State("Maine", 1328188, "Augusta", "Northeast"),
                new State("Maryland", 5828289, "Annapolis", "Northeast"),
                new State("Massachusetts", 6587536, "Boston", "Northeast"),
                new State("Michigan", 9876187, "Lansing", "Midwest"),
                new State("Minnesota", 5344861, "St. Paul", "Midwest"),
                new State("Mississippi", 2978512, "Jackson", "Southeast"),
                new State("Missouri", 6010688, "Jefferson City", "Midwest"),
                new State("Montana", 998199, "Helena", "West"),
                new State("Nebraska", 1842641, "Lincoln", "Midwest"),
                new State("Nevada", 2723322, "Carson City", "West"),
                new State("New Hampshire", 1318194, "Concord", "Northeast"),
                new State("New Jersey", 8821155, "Trenton", "Northeast"),
                new State("New Mexico", 2082224, "Santa Fe", "Southwest"),
                new State("New York", 19465197, "Albany", "Northeast"),
                new State("North Carolina", 9656401, "Raleigh", "Southeast"),
                new State("North Dakota", 683932, "Bismarck", "Midwest"),
                new State("Ohio", 11544951, "Columbus", "Midwest"),
                new State("Oklahoma", 3791508, "Oklahoma City", "Southwest"),
                new State("Oregon", 3871859, "Salem", "West"),
                new State("Pennsylvania", 12742886, "Harrisburg", "Northeast"),
                new State("Rhode Island", 1051302, "Providence", "Northeast"),
                new State("South Carolina", 4679230, "Columbia", "Southeast"),
                new State("South Dakota", 824082, "Pierre", "Midwest"),
                new State("Tennessee", 6403353, "Nashville", "Southeast"),
                new State("Texas", 25674681, "Austin", "Southwest"),
                new State("Utah", 2817222, "Salt Lake City", "West"),
                new State("Vermont", 4802740, "Montpelier", "Northeast"),
                new State("Virginia", 8096604, "Richmond", "Southeast"),
                new State("Washington", 6830038, "Olympia", "West"),
                new State("West Virginia", 1855364, "Charleston", "Southeast"),
                new State("Wisconsin", 5711767, "Madison", "Midwest"),
                new State("Wyoming", 568158, "Cheyenne", "West")
            });
} // end loadStates()

And so on for every state. Here’s what I have for my selection sort. It’s split into three methods as what is required by the professor.

private static void selectionSort(State[] states, String onField) {
    for (int top = 0; top < states.length - 1; top++) {
        swap(states, top, indexOfMinValueInArray(states, top, onField));
    }
} // end selectionSort()

private static int indexOfMinValueInArray(State[] states, int startAt,
        String onField) {
    int minIndex = startAt;
    if ("name".equals(onField)) {
        for (int index = startAt; index < states.length; index++) {
            if (states[index].getName().compareTo(states[minIndex].getName()) < 0) {
                minIndex = index;
            }
        }
    } else if ("population".equals(onField)) {
        for (int index = startAt; index < states.length; index++) {
            if (states[index].getPopulation() < states[minIndex].getPopulation()) {
                minIndex = index;
            }
        }
    }
    return minIndex;
}// end indexOfMinValueInArray()

private static void swap(State[] states, int index1, int index2) {
    State temp = states[index1];
    states[index1] = states[index2];
    states[index2] = temp;
}

Basically what this is supposed to do is take a “key” or, as stated in the methods, the “onField” which is supposed to tell the selection sort what to sort the array by, either by name, population, capital, or region. That is what I’m lost at. Could someone provide some direction on how to correct my selection sort? Also, here is the beginning of the code so you see the direction I need to take this.

public static void main(String[] args) {
    loadStates();

    State[] sortedStates = new State[50];
    for (int i = 0; i <= 49; i++) {
        sortedStates[i] = us.getStateAtIndex(i);
    }

    // TODO: Sort the states by population.
    selectionSort(sortedStates, "population");

    // TODO: List the states by population.
    listStates(sortedStates);

Answer 1

I’d suggest defining four Comparator objects, one for each field as the key. For instance, for the name field:

public class NameComparator implements Comparator<State> {
    public int compare(State s1, State s2) {
        return s1.name.compareTo(s2.name);
    }
}

The other comparators can be defined similarly. Then you can define your selection sort to use a Comparator<State> to do the item comparisons and you can pick which comparator to use based on the key designation.