Sorting ArrayList of Objects Java

Question

I have an ArrayList of Objects that I'd like to sort by their value. Basically I have 9 different mathematical functions (i.e., f1(n) = log n, f2(n) = n, f3(n) = n log n, etc). I plugged the value of 1 into all 9 functions and I placed their results in an ArrayList of Objects with their label attached to them as shown in the code below. I'd like to sort the entire list of results.

ArrayList<Object>val1 = new ArrayList<Object>();
        val1.add("f\u2081(1) = " + log(funcValues[0]));
        val1.add("f\u2082(1) = " + funcValues[0]);
        val1.add("f\u2083(1) = " + exponent(funcValues[0]));
        val1.add("f\u2084(1) = " + f4(funcValues[0]));
        val1.add("f\u2085(1) = " + squared(funcValues[0]));
        val1.add("f\u2086(1) = " + cubed(funcValues[0]));
        val1.add("f\u2087(1) = " + twoN(funcValues[0]));
        val1.add("f\u2088(1) = " + factorial(funcValues[0]));
        val1.add("f\u2089(1) = " + f9(funcValues[0]));

Basically, wherever you see log, funcValues, exponent, f4, squared, etc those are all functions that compute the answers to the mathematical functions. The output of this ArrayList is:

f₁(1) = 0.0
f₂(1) = 1
f₃(1) = 1.0
f₄(1) = 0.0
f₅(1) = 1
f₆(1) = 1.0
f₇(1) = 2.0
f₈(1) = 1
f₉(1) = 0.0

I'd like to sort only the numbers. I was trying to do it this way:

class ValuesSorted implements Comparator<Object> {

        @Override
        public int compare(Object v1, Object v2) {
            if ()
            return 0;
        }
    }

I am stuck on the if statement because I can't do something like if (v1.getValue > v2.getValue) because I am using 9 different function calls to pull each of those values.

I'd suggest creating your own custom class that contains both the function and value. Override the equals, hashCode, and toString methods and implement a Comparator for that class that sorts based on the value field. — Marc Baumbach
– Marc Baumbach, Commented Sep 16, 2014 at 4:52

Key_coder · Accepted Answer · 2014-09-16 04:51:57Z

2

Simply try

Collections.sort(testList);
Collections.reverse(testList);

http://docs.oracle.com/javase/7/docs/api/java/util/Collections.html

answered Sep 16, 2014 at 4:51

Key_coder

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4 Comments

x4nd3r Over a year ago

This is more of a comment than an answer.

Key_coder Over a year ago

@voidHead comment requires 50 reputation

Giovanni Over a year ago

@Gaurav This would not work because you cannot use Collections.sort on an Arraylist of objects.

skuntsel Over a year ago

@Gaurav This is not an exam. See the answer as something that is all-embracing on the question posed. That way you'll be able to write a good answer.

user949300 · Accepted Answer · 2014-09-16 06:01:00Z

1

You are trying to track two things, the value and a label. When you are tracking two associated things, use a Map. In this case, a sorted map, i.e. a TreeMap.

TreeMap map = new TreeMap<Double, String>();
// for each function, map.put(value, label), e.g.
map.put(log(funcValues[0]), "f\u2081(1)");
...
map.put(f4(funcValues[0]),"f\u2084(1)");
...
map.put(f9(funcValues[0]), "f\u2089(1)");

And result will be sorted by numerical value. map.values() will have the labels in sorted order.

edited Sep 16, 2014 at 6:01

answered Sep 16, 2014 at 5:55

user949300

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2 Comments

Giovanni Over a year ago

it seems as though when I do this, only three results output and the value does not output. For example, here is the output: [f₉(1) = , f₈(1) = , f₇(1) = ]

user949300 Over a year ago

Hmm, that's because in this example you only have 3 distinct values, 0.0, 1.0, and 2.0. In a "real" example would there be more variety? I.e. don't use 1 as the argument to each function, as 1 squared = 1 cubed = twoToThe(1). If you really have duplicate values this answer doesn't work.

indika · Accepted Answer · 2014-09-16 17:37:28Z

class Pair<X,Y>{

    private X first;
    private Y second;

    Pair(X first,Y second){
        this.first=first;
        this.second=second;
    }

    public X getX() {
        return first;
    }

    public void setX(X first) {
        this.first = first;
    }

    public Y getY() {
        return second;
    }

    public void setY(Y second) {
        this.second = second;
    }


    public Comparator<Pair<X, Y>> getComparator(){
        return new Comparator<Pair<X, Y>>() {

            @Override
            public int compare(Pair<X, Y> o1, Pair<X, Y> o2) {
                double a=(Double) o1.getY();
                double b=(Double) o2.getY();
                if(a==b){
                    return 0;
                }else if(a>b){
                    return 1;
                }else{
                    return -1;
                }
            }
        };
    }

}

public class Main{

    public static void main(String[] arg){

        List<Pair<String,Double>> val1 = new ArrayList<Pair<String,Double>>();

        val1.add(new Pair<String,Double>("f\u2081(1) = ", 0.1));
        val1.add(new Pair<String,Double>("f\u2082(1) = ", 0.2));
        val1.add(new Pair<String,Double>("f\u2083(1) = ", 0.1));
        val1.add(new Pair<String,Double>("f\u2084(1) = ", 1.1));
        val1.add(new Pair<String,Double>("f\u2085(1) = ", 1.2));
        val1.add(new Pair<String,Double>("f\u2086(1) = ", 2.0));
        val1.add(new Pair<String,Double>("f\u2087(1) = ", 2.1));
        val1.add(new Pair<String,Double>("f\u2088(1) = ", 0.3));

        Collections.sort(val1,new Pair<String,Double>("",0.0).getComparator());

        for (Pair<String, Double> pair : val1) {
            System.out.println(pair.getX()+" "+pair.getY());
        }
    }
}

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