Android NullPointerException causing crash

Question

I am creating an android card game. I have created a playing card class and a card deck class. When i run sample code it fails to build the deck. Here is my code along with the logcat

PlayingCard.java

package com.michaelpeerman.hi_low;

public class PlayingCard {
public String[] CardSuit = {"SPADE","HEART","CLUB","DIAMOND"};
public String[] CardRank= {"ACE", "TWO", "THREE", "FOUR", "FIVE", "SIX", "SEVEN", "EIGHT", "NINE","JACK","QUEEN","KING"};
public String theSuit;
public int theRank;
public boolean theState;

public PlayingCard(){
    theSuit = "SPADE";
    theRank = 1;
    theState = false;
}
public PlayingCard(String mySuit, int myRank){
    theSuit = mySuit;
    theRank = myRank;

}
public int getRank(){
    return theRank;

}
public String getSuit(){
    return theSuit;

}
public boolean getState(){
    return theState;

}
public void setState(boolean myState){
    theState = myState;
}
public void setRank(int myRank){
    theRank = myRank;
}
public void setSuit(String mySuit){
    theSuit = mySuit;
}
public String getCardString(){
     String cardString = null;
        String theCardRanks[] = {"A","2","3","4","5","6","7","8","9","T","J","Q","K"};

        if (theSuit == "CLUB")
            cardString = "C"+theCardRanks[theRank-1];
        else if (theSuit == "SPADE")
            cardString = "S"+theCardRanks[theRank-1];
        else if (theSuit == "HEART")
            cardString = "H"+theCardRanks[theRank-1];
        else if (theSuit == "DIAMOND")
            cardString = "D"+theCardRanks[theRank-1];
        return cardString;
}
}

CardDeck.java

package com.michaelpeerman.hi_low;
import java.util.Random;

import com.michaelpeerman.hi_low.PlayingCard;
public class CardDeck {
public PlayingCard[] cardArray;
public int num_decks;
public int cards_left;
public CardDeck(int my_decks)
{
num_decks = my_decks;
cardArray = new PlayingCard[52*num_decks];
cards_left = 52*num_decks;
for (int i=0; i< num_decks; i++){
for (int j = 0; j < 13; j++)
{

        cardArray[(j+(52*i))].setSuit("SPADE");
        cardArray[j+13+(52*i)].setSuit("HEART");
        cardArray[j+26+(52*i)].setSuit("CLUB");
        cardArray[j+39+(52*i)].setSuit("DIAMOND");




    if (j==0){
        cardArray[j+(52*i)].setRank(1);
        cardArray[j+13+(52*i)].setRank(1);
        cardArray[j+26+(52*i)].setRank(1);
        cardArray[j+39+(52*i)].setRank(1);
        }
    else if (j==1){
        cardArray[j+(52*i)].setRank(2);
        cardArray[j+13+(52*i)].setRank(2);
        cardArray[j+26+(52*i)].setRank(2);
        cardArray[j+39+(52*i)].setRank(2);
        }
    else if (j==2){
        cardArray[j+(52*i)].setRank(3);
        cardArray[j+13+(52*i)].setRank(3);
        cardArray[j+26+(52*i)].setRank(3);
        cardArray[j+39+(52*i)].setRank(3);
        }
    else if (j==3){
        cardArray[j+(52*i)].setRank(4);
        cardArray[j+13+(52*i)].setRank(4);
        cardArray[j+26+(52*i)].setRank(4);
        cardArray[j+39+(52*i)].setRank(4);
        }
    else if (j==4){
        cardArray[j+(52*i)].setRank(5);
        cardArray[j+13+(52*i)].setRank(5);
        cardArray[j+26+(52*i)].setRank(5);
        cardArray[j+39+(52*i)].setRank(5);
        }
    else if (j==5){
        cardArray[j+(52*i)].setRank(6);
        cardArray[j+13+(52*i)].setRank(6);
        cardArray[j+26+(52*i)].setRank(6);
        cardArray[j+39+(52*i)].setRank(6);
        }
    else if (j==6){
        cardArray[j+(52*i)].setRank(7);
        cardArray[j+13+(52*i)].setRank(7);
        cardArray[j+26+(52*i)].setRank(7);
        cardArray[j+39+(52*i)].setRank(7);
        }
    else if (j==7){
        cardArray[j+(52*i)].setRank(8);
        cardArray[j+13+(52*i)].setRank(8);
        cardArray[j+26+(52*i)].setRank(8);
        cardArray[j+39+(52*i)].setRank(8);
        }
    else if (j==8){
        cardArray[j+(52*i)].setRank(9);
        cardArray[j+13+(52*i)].setRank(9);
        cardArray[j+26+(52*i)].setRank(9);
        cardArray[j+39+(52*i)].setRank(9);
        }
    else if (j==9){
        cardArray[j+(52*i)].setRank(10);
        cardArray[j+13+(52*i)].setRank(10);
        cardArray[j+26+(52*i)].setRank(10);
        cardArray[j+39+(52*i)].setRank(10);
        }
    else if (j==10){
        cardArray[j+(52*i)].setRank(11);
        cardArray[j+13+(52*i)].setRank(11);
        cardArray[j+26+(52*i)].setRank(11);
        cardArray[j+39+(52*i)].setRank(11);
        }
    else if (j==11){
        cardArray[j+(52*i)].setRank(12);
        cardArray[j+13+(52*i)].setRank(12);
        cardArray[j+26+(52*i)].setRank(12);
        cardArray[j+39+(52*i)].setRank(12);
        }
    else if (j==12){
        cardArray[j+(52*i)].setRank(13);
        cardArray[j+13+(52*i)].setRank(13);
        cardArray[j+26+(52*i)].setRank(13);
        cardArray[j+39+(52*i)].setRank(13);
        }

}
}
}
public void useCard(){
cards_left = cards_left - 1;
}
public int get_cards_left(){
return cards_left;
}
public void setCard(int card, PlayingCard myCard){
cardArray[card]=myCard;

}

public PlayingCard getCard(int card){

return cardArray[card];

}

public void printDeck(){
for (int j = 0; j < (52*num_decks); j++){
    // Print out to textview
    //String cardString = cardArray[j].getCardString();
}

}
public void suffleDeck(){

for (int i = 0; i < 100*num_decks; i++)
{
    int rand1 = 1 + new Random().nextInt(52*num_decks);
    int rand2 = 1 + new Random().nextInt(52*num_decks);
    PlayingCard tmpCard = cardArray[rand1];
    cardArray[rand1] = cardArray[rand2];
    cardArray[rand2] = tmpCard;
}
}
public boolean isDeckFinished(){
boolean isFinished =true;
for (int i = 0; i<52*num_decks; i++){
    PlayingCard myCard = cardArray[i];
    if (myCard.getState() == false){
        isFinished = false;
    }
}
return isFinished;

}

}

StartScreen.java

package com.michaelpeerman.hi_low;

import android.app.Activity;
import android.os.Bundle;
import android.widget.TextView;
import com.michaelpeerman.hi_low.R;
import com.michaelpeerman.hi_low.CardDeck;
import com.michaelpeerman.hi_low.PlayingCard;

public class StartScreen extends Activity {
/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);
    CardDeck myDeck = new CardDeck(1);
    int cards = myDeck.cards_left;
    String text = Integer.toString(cards);
   TextView t=new TextView(this); 

    t=(TextView)findViewById(R.id.main_text); 
    t.append(text);

}
}

main.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical"
android:layout_width="fill_parent"
android:layout_height="fill_parent" >
<ScrollView
android:layout_width="fill_parent"
android:layout_height="fill_parent">
<TextView
    android:id="@+id/main_text"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
     />
</ScrollView>
</LinearLayout>

AndroidManifest.xml

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.michaelpeerman.hi_low"
android:versionCode="1"
android:versionName="1.0" >

<uses-sdk android:minSdkVersion="13" />

<application
    android:icon="@drawable/ic_launcher"
    android:label="@string/app_name" >
    <activity
        android:name=".StartScreen"
        android:label="@string/app_name" >
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />

            <category android:name="android.intent.category.LAUNCHER" />
        </intent-filter>
    </activity>
</application>

</manifest>

LogCat Output

05-03 18:03:55.067: E/AndroidRuntime(9805): FATAL EXCEPTION: main
05-03 18:03:55.067: E/AndroidRuntime(9805): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.michaelpeerman.hi_low/com.michaelpeerman.hi_low.StartScreen}: java.lang.NullPointerException
05-03 18:03:55.067: E/AndroidRuntime(9805):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1956)
05-03 18:03:55.067: E/AndroidRuntime(9805):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1981)
05-03 18:03:55.067: E/AndroidRuntime(9805):     at android.app.ActivityThread.access$600(ActivityThread.java:123)
05-03 18:03:55.067: E/AndroidRuntime(9805):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1147)
05-03 18:03:55.067: E/AndroidRuntime(9805):     at android.os.Handler.dispatchMessage(Handler.java:99)
05-03 18:03:55.067: E/AndroidRuntime(9805):     at android.os.Looper.loop(Looper.java:137)
05-03 18:03:55.067: E/AndroidRuntime(9805):     at android.app.ActivityThread.main(ActivityThread.java:4424)
05-03 18:03:55.067: E/AndroidRuntime(9805):     at java.lang.reflect.Method.invokeNative(Native Method)
05-03 18:03:55.067: E/AndroidRuntime(9805):     at java.lang.reflect.Method.invoke(Method.java:511)
05-03 18:03:55.067: E/AndroidRuntime(9805):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784)
05-03 18:03:55.067: E/AndroidRuntime(9805):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:551)
05-03 18:03:55.067: E/AndroidRuntime(9805):     at dalvik.system.NativeStart.main(Native Method)
05-03 18:03:55.067: E/AndroidRuntime(9805): Caused by: java.lang.NullPointerException
05-03 18:03:55.067: E/AndroidRuntime(9805):     at com.michaelpeerman.hi_low.CardDeck.<init>(CardDeck.java:18)
05-03 18:03:55.067: E/AndroidRuntime(9805):     at com.michaelpeerman.hi_low.StartScreen.onCreate(StartScreen.java:16)
05-03 18:03:55.067: E/AndroidRuntime(9805):     at android.app.Activity.performCreate(Activity.java:4465)
05-03 18:03:55.067: E/AndroidRuntime(9805):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1049)
05-03 18:03:55.067: E/AndroidRuntime(9805):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1920)
05-03 18:03:55.067: E/AndroidRuntime(9805):     ... 11 more

score 1 · Accepted Answer · answered May 03 '12 at 22:25

1

You're creating the cardArray properly, but you're not filling it with any references to PlayingCard. They're all nulls until you set them to something.

answered May 03 '12 at 22:25

DeeV

35,865
9
108
95

i thought using cardArray = new PlayingCard[52*num_decks]; in the constructor of the CardDeck class does this? – mpeerman May 03 '12 at 22:27
so i need to run cardArray[j] = new PlayingCard; is that right? – mpeerman May 03 '12 at 22:29
1

No. It just creates and allocates memory for `52*num_decks` amount of references. The objects aren't created until you explicitly create them. `for(int k = 0; k < cardArray.length; i++){cardArray[k] = new PlayingCard();}` will do this. – DeeV May 03 '12 at 22:29
Hmm, looks like my answer was too late :) But no, all you've done there is to define an array that will hold PlayingCard objects and that is currently empty. You'll need to instantiate and add the object yourself. – stephendnicholas May 03 '12 at 22:30
Or at the beginning of your loop, create the object before you use it. In this case it looks like you need to create four per cycle. – DeeV May 03 '12 at 22:31
1

An on an unrelated note, maybe you could do something like this in your `public CardDeck(int my_decks)` function: `cardArray[j+(52*i)].setRank(j+1);` instead of doing this giant if/else thing... Would be a bit more readable. – epichorns May 03 '12 at 22:37
Ok one last question can i pass a textview to a class? Such as passing a textview to my printdeck function? – mpeerman May 04 '12 at 00:35
It's an object just like any other Java object so you can pass it to a class just like any Java object. Just keep in mind that UI elements like `TextView` hold references to your activity, so they can cause memory leaks if one of your classes holds a reference to the UI element when the Activity is destroyed. – DeeV May 04 '12 at 15:30

score 0 · Answer 2 · answered May 03 '12 at 22:28

0

According to the stack trace, the line at issue is:

cardArray[(j+(52*i))].setSuit("SPADE");

In CardDeck.java.

And I'm guessing it's because the content of that element of the array is null. While you have defined the array, you don't appear to put any objects into it before you try to call .setSuit(...) on an element of it - so you are calling .setSuit(...) on null - hence NPE.

answered May 03 '12 at 22:28

stephendnicholas

1,810
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what is the easiest way to populate the array with objects of type PlayingCard? – mpeerman May 03 '12 at 22:29
Forgive me i originally wrote this program in c++ and trying to convert it to android – mpeerman May 03 '12 at 22:30
No worries. The simplest way would be just to loop through adding PlayingCard objects. You could do: (apologies for lack of formatting) for(int i=0; i – stephendnicholas May 03 '12 at 22:34

Android NullPointerException causing crash

2 Answers2