Java异常：是否需要默认构造函数？ 已经有一个

Question

I am writing code to try to create a poker playing AI. I have been provided with .jar files containing classes that will handle playing the game, as long as I get the AI code working.

With that said, I am encountering the following rather annoying error. For clarity, the following code block contains the error, the section of code that calls my player in PokerGame.class , the PokerPlayer interface, and my class that extends the PokerPlayer interface.

This is the exception:

Exception in thread "main" java.lang.RuntimeException: Class blake.MyPokerPlayer must have a default constructor
    at pokergame.PokerGame.loadPlayer(PokerGame.java:84)
    at pokergame.PokerGame.<init>(PokerGame.java:100)
    at pokergame.PokerGame.main(PokerGame.java:590)
    at blake.MyPokerPlayer.main(MyPokerPlayer.java:18)
Caused by: java.lang.NoSuchMethodException: blake.MyPokerPlayer.<init>()
    at java.lang.Class.getConstructor0(Class.java:3082)
    at java.lang.Class.getConstructor(Class.java:1825)
    at pokergame.PokerGame.loadPlayer(PokerGame.java:78)
    ... 3 more

And this is the code:

package pokergame;

import java.util.*;
import java.lang.reflect.*;

public class PokerGame implements PokerListener {
    // Other methods above
    private PokerPlayer loadPlayer(String classname) {
        PokerPlayer ret = null;
        try {
            Class<?> cls = Class.forName(classname);
            Constructor<?> ctor = cls.getConstructor();
            Object o = ctor.newInstance();
            ret = (PokerPlayer)o;
        } catch (ClassNotFoundException e) {
            throw new RuntimeException("Cannot find player class: " + classname, e);
        } catch (NoSuchMethodException e) {
            throw new RuntimeException("Class " + classname + " must have a default constructor", e);
        } catch (ClassCastException e) {
            throw new RuntimeException("Player class does not implement PokerPlayer interface: " + classname, e);
        } catch (Exception e) {
            throw new RuntimeException("Error trying to instantiate player class: " + classname, e);
        }
        return ret;
    }
    // Other methods below
}

package pokergame;

public interface PokerPlayer extends PokerListener {
    int numChips();
    void collectChips(int numChips);
    void acceptChips(int numChips);
    void setId(String id);  
    PokerDecision decide(PokerGameDetails game, int betRequiredToCall);
    PokerHand bestHand();
}

package player;

import java.util.*;
import pokergame.*;

public class MyPokerPlayer implements PokerPlayer {
    private String id;
    private ArrayList<PokerCard> handCards = new ArrayList<>();
    private ArrayList<PokerCard> tableCards = new ArrayList<>();
    private ArrayList<PokerCard> deck = new ArrayList<>();
    private int numChips;

    public static void main(String[] args) {
        PokerGame.main(new String[]{"pokerstooges.Larry", "player.MyPokerPlayer"});
    }

    MyPokerPlayer() { }

    // ***************
    // THESE NEED WORK
    // ***************

    public void onEvent(PokerEvent e) {
        if (e instanceof PokerNewHandEvent) { System.out.println(e); }
        else if (e instanceof PokerPlayerEliminatedEvent) { System.out.println(e); }
        else if (e instanceof PokerPotDistributionEvent) { System.out.println(e); }
        else if (e instanceof PokerCollectingBlindEvent) { System.out.println(e); }
        else if (e instanceof PokerNewCardEvent) { System.out.println(e); }
        else if (e instanceof PokerDecisionEvent) { System.out.println(e); }
        else if (e instanceof PokerHandRevealEvent) { System.out.println(e); }
    }

    public PokerDecision decide(PokerGameDetails game, int betRequiredToCall) {
        PokerDecision decision = new PokerDecision(PokerDecision.TYPE.FOLD);
        return decision;
    }

    public String toString() {
        // Something here...
        return "";
    }

    // *****************
    // THESE SHOULD WORK
    // *****************

    public int numChips() { return this.numChips; }
    public void collectChips(int numChips) { this.numChips -= numChips; }
    public void acceptChips(int numChips) { this.numChips += numChips; }
    public void setId(String id) { this.id = id; }
    public String getId() { return this.id; }

    public PokerHand bestHand() {
        ArrayList<String> hands = new ArrayList<>();
        String handCardString = handCards.get(0) + " " + handCards.get(1);
        if (tableCards.size() == 0) {
            hands.add(handCardString);
        }
        else if (tableCards.size() == 3) {
            hands.add(handCardString + " " + tableCards.get(0) + " " + tableCards.get(1) + " " + tableCards.get(2));
        }
        else if (tableCards.size() > 3) {
            hands.add(handCardString + " " + tableCards.get(0) + " " + tableCards.get(1) + " " + tableCards.get(2));
            hands.add(handCardString + " " + tableCards.get(0) + " " + tableCards.get(1) + " " + tableCards.get(3));
            hands.add(handCardString + " " + tableCards.get(0) + " " + tableCards.get(2) + " " + tableCards.get(3));
            hands.add(handCardString + " " + tableCards.get(1) + " " + tableCards.get(2) + " " + tableCards.get(3));
            if (tableCards.size() == 5) {
                hands.add(handCardString + " " + tableCards.get(0) + " " + tableCards.get(1) + " " + tableCards.get(4));
                hands.add(handCardString + " " + tableCards.get(0) + " " + tableCards.get(2) + " " + tableCards.get(4));
                hands.add(handCardString + " " + tableCards.get(0) + " " + tableCards.get(3) + " " + tableCards.get(4));
                hands.add(handCardString + " " + tableCards.get(1) + " " + tableCards.get(2) + " " + tableCards.get(4));
                hands.add(handCardString + " " + tableCards.get(2) + " " + tableCards.get(3) + " " + tableCards.get(4));
            }
        }

        PokerHand bestHand = compareHands(hands);
        return bestHand;
    }
    public PokerHand compareHands(ArrayList<String> hands) {
        PokerHand handA = new PokerHand(hands.remove(0));
        PokerHand handB;
        if (hands.size() == 1) { return new PokerHand(hands.remove(0)); }
        else if (hands.size() > 2) { handB = compareHands(hands); }
        else { handB = new PokerHand(hands.remove(0)); }

        if (handA.compareTo(handB) > 0) { return handA; }
        else { return handB; }
    }
    private void addCards(List<String> cardStrings) {
        for (String cardString : cardStrings) { deck.add(new PokerCard(cardString)); }
    }
}

As MyPokerPlayer shows, there is a default constructor:

MyPokerPlayer() { }

I have tried adding public in front of it, just in case, but that doesn't help either.

Answer 1

There seems to be a mismatch between the class in the exception ( Class blake.MyPokerPlayer must have a default constructor ) and the class in your code snippet ( player.MyPokerPlayer )

Can you check the class name you're passing to loadPlayer(String)?

Additionally, the constructor MyPokerPlayer is package-scoped, which could cause the runtime to fail reflective invocation (please make the constructor public). This would typically be an issue as the pokergame package is not the one defining the class.