如何在单独的行上返回 ArrayList 的对象

Question

I have created a program that sorts trading cards and places them in a collection that is then compared to other collections to look for duplicates. I have had no problems up until my final toString method. My issue is that I cannot seem to get the return statement to separate the various cards onto their own separate lines. instead of Alan Turing, Grace Hopper, Ada Lovelace, I need:

Alan Turing

Grace Hopper

Ada Lovelace

Below is a copy of my code. I am fairly new to java so I apologize for any lack of knowledge pertaining to methods specific to this, but I have only found ones using System.out.println, and not mentioning return in any way. My problem lies in the method defined by **. I appreciate any and all help and am sorry if this question is not 100% clear. (I have tried my own research to no avail!)

// First Class
public class Card implements Comparable<Card> {
    private String name;
    private String nationality;
    private int yearBorn;
    private int yearDied;

    public Card(String name, String nationality, int yearBorn, int yearDied) {
        this.name=name;
        this.nationality=nationality;
        this.yearBorn=yearBorn;
        this.yearDied=yearDied;
    }
    public int compareTo(Card c) {
        if (this.name.equals(c.name)) return 0;
        else if (this.name.compareTo(c.name)>0) return 1;
        else return -1;
    }

    public String toString() {
        return String.format("%s (%d - %d) - %s", name, yearBorn, yearDied, nationality);
    }

}
// Second Class

import java.util.ArrayList;
import java.util.List;

public class CardCollection {
    private String owner;
    private List<Card> myCollection;

    public CardCollection(String owner) {
        this.owner = owner;
        this.myCollection = new ArrayList<>();

    }

    public boolean addCard(Card c) {
        int p = 0;
        while (p < myCollection.size()) {
            int q = myCollection.get(p).compareTo(c);
            if (q == 0) {
                return false;
            } else if (q > 0) {
                myCollection.add(p, c);
                return true;
            }
            p++;
        }
        myCollection.add(c);
        return true;
    }

    public void removeCard(int r) {
        myCollection.remove(r);
    }

    public int getSize() {
        return myCollection.size();
    }

    public ArrayList<Card> mergeCollections(CardCollection cc) {
        ArrayList<Card> dupes = new ArrayList<>();
        while (cc.getSize() > 0) {
            Card c = cc.myCollection.remove(0);
            if (myCollection.contains(c)) {
                dupes.add(c);
            }
            else myCollection.add(c);
        }
        return dupes;
    }
    **public String toString() {
            String s = "";
            for (int i = 0; i < owner.length(); i++) {
                s += "-";
            }
            return String.format("%s\n%s\n%s\n", owner, s, myCollection);**
    }
}
// Runner Class
import java.util.ArrayList;
public class CCRunner {
    public static void main(String[] args) {
        CardCollection c1 = new CardCollection("Alan");
        CardCollection c2 = new CardCollection("Grace");
        Card turing = new Card("Alan Turing","British",1912,1954);
        Card hopper = new Card("Grace Hopper","American",1906,1992);
        Card vonneumann = new Card("John Von Neumann","Hungarian",1903,1957);
        Card shannon = new Card("Claude Shannon","American",1916,2001);
        Card johnson = new Card("Katherine Johnson","American",1918,-1);
        Card lovelace = new Card("Ada Lovelace","British",1815,1852);
        Card cerf = new Card("Vint Cerf","American",1943,-1);
        Card brin = new Card("Sergey Brin","Russian",1973,-1);
        c1.addCard(turing);
        c1.addCard(vonneumann);
        c1.addCard(shannon);
        c1.addCard(johnson);
        c1.addCard(cerf);
        c1.addCard(brin);
        c2.addCard(cerf);
        c2.addCard(lovelace);
        c2.addCard(johnson);
        c2.addCard(vonneumann);
        c2.addCard(hopper);
        System.out.println(c1);
        System.out.println(c2);
        ArrayList<Card> dupes = c1.mergeCollections(c2);
        System.out.println(c1);
        System.out.println(c2);
        System.out.println("Duplicates:\n-----------");
        for (Card c : dupes) {
            System.out.println(c);
        }
    }
}

This is run in a separate class, but I don't think it will elucidate my problem to include it. Let me know if I am wrong.

Answer 1

You can either change your implementation of toString() of your CardCollection class. Examples:

Loop again, this time over the List :

public final String toString() {
  String s = "";
  for (int i = 0; i < owner.length(); i++) {
    s += "-";
  }
  for (int i = 0; i < myCollection.size(); i++) {
    s += "\n" + myCollection.get(i);
  }
  return String.format("%s\n%s\n", owner, s);
}

Using a stream (requires an additional import java.util.stream.Collectors; ):

public final String toString() {
  String s = "";
  for (int i = 0; i < owner.length(); i++) {
    s += "-";
  }
  return String.format("%s\n%s\n%s\n",
                       owner,
                       s,
                       myCollection.stream()
                                   .map(Card::toString)
                                   .collect(Collectors.joining("\n")));
}

---

Or you can @Override the toString() method of the ArrayList , also in your CardCollection class. Example:

this.myCollection = new ArrayList<>(){
                      @Override
                      public String toString(){
                        String s = "";
                        if (size() > 0) s = get(0).toString();
                        for (int i = 1; i < size(); i++) {
                          s += "\n" + get(i).toString(); 
                        }
                        return s;
                      }
                    };

---

All examples will result in this for Alan (before the merge):

Alan
----
Alan Turing (1912 - 1954) - British
Claude Shannon (1916 - 2001) - American
John Von Neumann (1903 - 1957) - Hungarian
Katherine Johnson (1918 - -1) - American
Sergey Brin (1973 - -1) - Russian
Vint Cerf (1943 - -1) - American

---

Note: I'd personally go with changing the implementation of toString() of the CardCollection class. I would also perfer the way tquadrat did it in their answer . The overriding of ArrayList looks to messy in my opinion, and keeping the stringrepresentation in the toString() method makes more sense to me.

Answer 2

Try this as a replacement to the marked toString() implementation:

…
public final String toString() 
{
    var s = "-".repeat( owner.length() );
    var joiner = new StringJoiner( "\n", String.format( "%s%n%s%n", owner, s ), "" );
    for( var c : myCollection ) joiner.add( c.toString() );
    var retValue = joiner.toString();

    return retValue;
}

Basically, the output would look like this:

<owner>
------- 
<collectible1> 
<collectible2> 
<…>

If you are using a Java version before Java 11, you may need to replace var with the respective types.