java map toString() override

Question

System.out.println(map);

so result is {A=2, B=4, C=5..} but i want print

A 2
B 4
C 5
...

So, my processor's hint is map toSting() override, but i don't understand

this is my code

class MapManager2 {
public static Map<String, Double> readData(String fileName) {
    Map<String, Double> mapOfData;
    Map<String, Double> sortedByValue = null;

    try {
        Scanner file = new Scanner(new File(fileName)); // read file
        mapOfData = new TreeMap<>(); // create Map to store the values

        while (file.hasNextLine()) {
            String line = file.nextLine(); // read the line
            String[] words = line.split("\\s+"); // split the item and price by space
            Double price = Double.parseDouble(words[1]); // parse the double price
            mapOfData.put(words[0], price); // put the data in map
        }

        /* Sort the map on basis of value*/
        sortedByValue = mapOfData.entrySet()
                .stream().sorted((Map.Entry.<String, Double>comparingByValue()))
                .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue, (e1, e2) -> e1, LinkedHashMap::new));
    } catch (FileNotFoundException e) {
        System.out.println("Input file not found");
    }
    return sortedByValue;
}

}
public class Problem21 {
public static void main(String[] args) {
    Map<String, Double> map = MapManager2.readData("input.txt");
    if (map == null) {
        System.out.println("Input file not found.");
        return;
    }
    System.out.println(map);
}

}

Answer 1

Your prof is telling you to override the toString method of the Map class. I think the best idea is an anonymous override.

Map<String, String> test = new HashMap<>() {
    @Override
    public String toString() {
        StringBuilder stb = new StringBuilder();
        for (Map.Entry<String, String> entry : this.entrySet()) {
            stb.append(entry.getKey()).append(" ")
                    .append(entry.getValue()).append("\n");
        }
        return stb.toString();
    }
};

Note that I don't have any way to chexk this code for errors right now, so you may have to fix some names.

Answer 2

I'm not sure what you mean by "processor's hint" but you don't really need to override toString . Simpler is to just convert the map to the string you want:

map.entrySet().stream()
    .map(e -> e.getKey() + " " + e.getValue())
    .collect(Collectors.joining("\n"));

Or if you just want to print the map directly:

map.forEach((k, v) -> System.out.println(k + " " + v));

Answer 3

So, my processor's hint is map toSting() override, but i don't understand

I don't know who your 'processor' is (professor?), but that's a dumb hint.

overriding things makes sense when you're subclassing something. Don't subclass map, that's... not what you want.

instead of passing 'map' to System.out.println , which will result in println invoking .toString() on that object and printing the resulting string, make your own string:

String print = map.entrySet().stream()
    // convert each element in the map to the string "KEY VALUE"
    .map(x -> x.getKey() + " " + x.getValue())
    // collect em by joining em, separating each "KEY VALUE" string with a newline
    .collect(Collectors.joining("\n"));
System.out.println(print);

Answer 4

To override the toString() method, extend TreeMap class and replace your TreeMap instance with new class.

import java.util.Map;
import java.util.TreeMap;

public class MyTreeMap extends TreeMap<String,Double> {
    @Override
    public String toString() {
        StringBuilder result = new StringBuilder();
        this.entrySet().forEach(me -> {
            System.out.printf("%s %s%n", me.getKey(), me.getValue());
        });
        return result.toString();
    }
}