Question

I am getting a fatal error in an application that I have been working on. The log file and code is as follows. From what I can tell it is not even getting to the main activity.

Log File:

Shutting down VM
threadid=1: thread exiting with uncaught exception (group=0x401db760)
FATAL EXCEPTION: main
java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{A3.cal/A3.cal.A3Activity}: java.lang.NullPointerException
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1739)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1831)
    at android.app.ActivityThread.access$500(ActivityThread.java:122)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1024)
    at android.os.Handler.dispatchMessage(Handler.java:99)
    at android.os.Looper.loop(Looper.java:132)
    at android.app.ActivityThread.main(ActivityThread.java:4123)
    at java.lang.reflect.Method.invokeNative(Native Method)
    at java.lang.reflect.Method.invoke(Method.java:491)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:841)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:599)
    at dalvik.system.NativeStart.main(Native Method)
Caused by: java.lang.NullPointerException
    at android.content.ContextWrapper.getResources(ContextWrapper.java:81)
    at android.view.View.<init>(View.java:2366)
    at android.view.View.<init>(View.java:2411)
    at android.widget.TextView.<init>(TextView.java:371)
    at android.widget.Button.<init>(Button.java:108)
    at android.widget.Button.<init>(Button.java:104)
    at android.widget.Button.<init>(Button.java:100)
    at A3.cal.A3Activity.<init>(A3Activity.java:32)
    at java.lang.Class.newInstanceImpl(Native Method)
    at java.lang.Class.newInstance(Class.java:1301)
    at android.app.Instrumentation.newActivity(Instrumentation.java:1022)
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1730)
    ... 11 more

Code:

package A3.cal;

import java.util.Calendar;
import android.app.Activity;
import android.graphics.Color;
import android.os.Bundle;
import android.view.Display;
import android.view.Gravity;
import android.view.View;
import android.view.ViewGroup.LayoutParams;
import android.widget.Button;
import android.widget.DatePicker;
import android.widget.EditText;
import android.widget.LinearLayout;
import android.widget.PopupWindow;
import android.widget.Spinner;
import android.widget.TextView;
import android.widget.TimePicker;

public class A3Activity extends Activity {
    public int currentMonth = 0;
    private TextView[] dayTableTV = new TextView[6*7]; 
    private Integer[] dayTable = null;
    public Integer currentYear = 0;
    public int firstDayOfWeek = 0;
    public boolean leapYear = false;
    private PopupWindow createEventPopup = null;
    boolean click = false;
    private Button closeAddEventButton = new Button(this);
    private Button submitAddEventButton = new Button(this);

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        makeTable();

        Button addEvent = (Button) findViewById(R.id.addEvent);
        addEvent.setOnClickListener(new View.OnClickListener() {
            public void onClick(View v) {
                if(click == false){
                    addEventPopup();
                    click = true;
                }else{
                    createEventPopup.dismiss();
                    click = false;
                }
            }
        });

        Button nextMonthButton = (Button) findViewById(R.id.nextMonthButton);
        nextMonthButton.setOnClickListener(new View.OnClickListener() {
            public void onClick(View v) {
               changeMonthNext(getMonth(currentMonth, 1));
            }
        });

        Button previousMonthButton = (Button) findViewById(R.id.previousMonthButton);
        previousMonthButton.setOnClickListener(new View.OnClickListener() {
            public void onClick(View v) {
               changeMonthBack(getMonth(currentMonth, -1));
            }
        });

        closeAddEventButton.setOnClickListener(new View.OnClickListener() {
            public void onClick(View v) {
                createEventPopup.dismiss();
            }
        });

        submitAddEventButton.setOnClickListener(new View.OnClickListener() {
            public void onClick(View v) {
               submitEvent();
            }

        });

    }

There is more code after this, but for saving characters I am not posting it. If you need it please let me know in the comments.

Answer 1

Initialize your buttons elsewhere, like in onCreate.

---

The problem is this‘s (the A3Activity) state when the Button instances are instantiated in the instance variable initializers (closeAddEventButton and submitAddEventButton).

Instance variable initializers run before constructors, but an object is not completely initialized until its constructor(s) are executed. Objects referenced prior to that will be in an indeterminate state.

(The state could be determined; I just mean the instance may not be fully-usable yet.)

Something in the Button initialization flow uses an uninitialized value from the A3Activity and throws an NPE. (JLS 12.5: Creation of New Class Instances details instance initialization process.)

Noting that initializers run in textual order (JLS 12.4.2), before constructors, here’s a simple demo:

public class ThisHere {
    public String aww;
    public String notYet = this.aww;

    GedankenExperiment wat = new GedankenExperiment(this);

    // public class GedankenExperiment {
    //     public GedankenExperiment(thatThere) {
    //         thatThere.aww = ???
    //     }
    // }

    public ThisHere() {
        this.aww = "Hi there";
    }

    public static void main(String[] args) {
        ThisHere thisHere = new ThisHere();
        System.out.println("aww=" + thisHere.aww);
        // > aww=Hi there (initialized in ctor)
        System.out.println("notYet=" + thisHere.notYet);
        // > notYet=null (initialized before ctor)
    }
}

---

FYI, the tell-tale sign was this log line:

at A3.cal.A3Activity.<init>(A3Activity.java:32)

This is the only log line I really saw, because my first step was looking for a reference to anything outside of Android and Java packages. The <init> part means it wasn’t in a method, so it had something to do with the instance’s initializers.

The stack trace included Button initializer entries. That plus the use of this in their constructors meant that was the only real possibility, hence the quick turn-around without much digging.