I was developing my first app and I keep getting this error when i run it on my phone.
I am trying to make an app for tic tac toe.
I am a complete beginner to programming, please help me out.

**Main\_activity.java** :

```
    package com.example.test3;

import android.app.Activity;
import android.os.Bundle;
import android.view.Menu;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;

public class MainActivity extends Activity {

    char[][] a;
        char b;
        int j,k,o=0;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        for(j=0;j<3;j++)
        {for(k=0;k<3;k++)
        {
            a[j][k]=' ';
        }
        }
        Button btn1 = (Button)findViewById(R.id.Btn1);
        Button btn2 = (Button)findViewById(R.id.Btn2);
        Button btn3 = (Button)findViewById(R.id.Btn3);
        Button btn4 = (Button)findViewById(R.id.Btn4);
        Button btn5 = (Button)findViewById(R.id.Btn5);
        Button btn6 = (Button)findViewById(R.id.Btn6);
        Button btn7 = (Button)findViewById(R.id.Btn7);
        Button btn8 = (Button)findViewById(R.id.Btn8);
        Button btn9 = (Button)findViewById(R.id.Btn9);

        for(int i=0;i<9;i++)
        {
            if(i%2==0)  {b='X';}
            else        {b='O';}

        btn1.setOnClickListener(new View.OnClickListener()
        {
            public void onClick(View v)
            {TextView op1 = (TextView)findViewById(R.id.Btn1);
             op1.setText(b);a[0][0]=b;j=0;k=0;
             check(a,j,k,o);
        }});
        btn2.setOnClickListener(new View.OnClickListener()
        {
            public void onClick(View v)
            {TextView op2 = (TextView)findViewById(R.id.Btn2);
             op2.setText(b);a[0][1]=b;j=0;k=1;
             check(a,j,k,o);
        }});
        btn3.setOnClickListener(new View.OnClickListener()
        {
            public void onClick(View v)
            {TextView op3 = (TextView)findViewById(R.id.Btn3);
             op3.setText(b);a[0][2]=b;j=0;k=2;
             check(a,j,k,o);
        }});
        btn4.setOnClickListener(new View.OnClickListener()
        {
            public void onClick(View v)
            {TextView op4 = (TextView)findViewById(R.id.Btn4);
             op4.setText(b);a[1][0]=b;j=1;k=0;
             check(a,j,k,o);
        }});
        btn5.setOnClickListener(new View.OnClickListener()
        {
            public void onClick(View v)
            {TextView op5 = (TextView)findViewById(R.id.Btn5);
             op5.setText(b);a[1][1]=b;j=1;k=1;
             check(a,j,k,o);
        }});
        btn6.setOnClickListener(new View.OnClickListener()
        {
            public void onClick(View v)
            {TextView op6 = (TextView)findViewById(R.id.Btn6);
             op6.setText(b);a[1][2]=b;j=1;k=2;
             check(a,j,k,o);
        }});
        btn7.setOnClickListener(new View.OnClickListener()
        {
            public void onClick(View v)
            {TextView op7 = (TextView)findViewById(R.id.Btn7);
             op7.setText(b);a[2][0]=b;j=2;k=0;
             check(a,j,k,o);
        }});
        btn8.setOnClickListener(new View.OnClickListener()
        {
            public void onClick(View v)
            {TextView op8 = (TextView)findViewById(R.id.Btn8);
             op8.setText(b);a[2][1]=b;j=2;k=1;
             check(a,j,k,o);
        }});
        btn9.setOnClickListener(new View.OnClickListener()
        {
            public void onClick(View v)
            {TextView op9 = (TextView)findViewById(R.id.Btn9);
             op9.setText(b);a[2][2]=b;j=2;k=2;
             check(a,j,k,o);
        }});

        if(o!=0) {i=9;}
        }

    }

    public void check(char[][] a,int j,int k,int o)
    {
        if(a[j][0]==a[j][1]&&a[j][1]==a[j][2]&&a[j][0]!=' ') o+=1;
        if(a[0][k]==a[1][k]&&a[1][k]==a[2][k]&&a[0][k]!=' ') o+=1;
        if(((a[0][0]==a[1][1]&&a[1][1]==a[2][2])||(a[0][2]==a[1][1]&&a[1][1]==a[2][0]))&&a[1][1]!=' ') o+=1;
    }
    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.main, menu);
        return true;
    }

}

```

**activity\_main.xml**:

```
<ScrollView xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent">

<TableLayout
    android:id="@+id/TableLayout01"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:layout_gravity="center">

        <TableRow
            android:id="@+id/TableRow1"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_weight="1">
            <Button

                android:text="@string/Btn1"
                android:id="@+id/Btn1"
                android:layout_weight="1"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content">
            </Button>
            <Button

                android:text="@string/Btn2"
                android:id="@+id/Btn2"
                android:layout_weight="1"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content">
            </Button>
            <Button

                android:text="@string/Btn3"
                android:id="@+id/Btn3"
                android:layout_weight="1"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content">
            </Button>
        </TableRow>
        <TableRow
            android:id="@+id/TableRow2"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_weight="1">
            <Button

                android:text="@string/Btn4"
                android:id="@+id/Btn4"
                android:layout_weight="1"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content">
            </Button>
            <Button

                android:text="@string/Btn5"
                android:id="@+id/Btn5"
                android:layout_weight="1"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content">
            </Button>
            <Button

                android:text="@string/Btn6"
                android:id="@+id/Btn6"
                android:layout_weight="1"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content">
                </Button>
                </TableRow>
    <TableRow
            android:id="@+id/TableRow3"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_weight="1">
            <Button

                android:text="@string/Btn7"
                android:id="@+id/Btn7"
                android:layout_weight="1"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content">
            </Button>
            <Button

                android:text="@string/Btn8"
                android:id="@+id/Btn8"
                android:layout_weight="1"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content">
            </Button>
            <Button

                android:text="@string/Btn9"
                android:id="@+id/Btn9"
                android:layout_weight="1"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content">
                </Button>
                </TableRow>
    </TableLayout>
    </ScrollView>

```

**LOGCAT**:

```
03-18 00:25:39.138: I/Process(17623): Sending signal. PID: 17623 SIG: 9
03-18 00:25:51.378: D/AndroidRuntime(17794): Shutting down VM
03-18 00:25:51.378: W/dalvikvm(17794): threadid=1: thread exiting with uncaught exception (group=0x41db9ba8)
03-18 00:25:51.378: E/AndroidRuntime(17794): FATAL EXCEPTION: main
03-18 00:25:51.378: E/AndroidRuntime(17794): Process: com.example.test3, PID: 17794
03-18 00:25:51.378: E/AndroidRuntime(17794): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.test3/com.example.test3.MainActivity}: java.lang.NullPointerException
03-18 00:25:51.378: E/AndroidRuntime(17794):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2195)
03-18 00:25:51.378: E/AndroidRuntime(17794):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
03-18 00:25:51.378: E/AndroidRuntime(17794):    at android.app.ActivityThread.access$800(ActivityThread.java:135)
03-18 00:25:51.378: E/AndroidRuntime(17794):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
03-18 00:25:51.378: E/AndroidRuntime(17794):    at android.os.Handler.dispatchMessage(Handler.java:102)
03-18 00:25:51.378: E/AndroidRuntime(17794):    at android.os.Looper.loop(Looper.java:136)
03-18 00:25:51.378: E/AndroidRuntime(17794):    at android.app.ActivityThread.main(ActivityThread.java:5017)
03-18 00:25:51.378: E/AndroidRuntime(17794):    at java.lang.reflect.Method.invokeNative(Native Method)
03-18 00:25:51.378: E/AndroidRuntime(17794):    at java.lang.reflect.Method.invoke(Method.java:515)
03-18 00:25:51.378: E/AndroidRuntime(17794):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
03-18 00:25:51.378: E/AndroidRuntime(17794):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
03-18 00:25:51.378: E/AndroidRuntime(17794):    at dalvik.system.NativeStart.main(Native Method)
03-18 00:25:51.378: E/AndroidRuntime(17794): Caused by: java.lang.NullPointerException
03-18 00:25:51.378: E/AndroidRuntime(17794):    at com.example.test3.MainActivity.onCreate(MainActivity.java:22)
03-18 00:25:51.378: E/AndroidRuntime(17794):    at android.app.Activity.performCreate(Activity.java:5231)
03-18 00:25:51.378: E/AndroidRuntime(17794):    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
03-18 00:25:51.378: E/AndroidRuntime(17794):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2159)
03-18 00:25:51.378: E/AndroidRuntime(17794):    ... 11 more
03-18 00:25:56.948: I/Process(17794): Sending signal. PID: 17794 SIG: 9

```