Getting weird String crash

Question

So basically when I start my program, it works fine, but when I press the back button to return to the previous activity it crashes almost immediately. I've checked with the debugging tool and it makes it as far as completing my onActivity Result method.

Error Logcat

  2018-11-13 00:37:48.874 8349-8349/com.example.caesp.todolist E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.caesp.todolist, PID: 8349
    java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String java.lang.Object.toString()' on a null object reference
        at android.widget.ArrayAdapter.createViewFromResource(ArrayAdapter.java:445)
        at android.widget.ArrayAdapter.getView(ArrayAdapter.java:407)
        at android.widget.AbsListView.obtainView(AbsListView.java:3189)
        at android.widget.ListView.makeAndAddView(ListView.java:2197)
        at android.widget.ListView.fillDown(ListView.java:824)
        at android.widget.ListView.fillSpecific(ListView.java:1534)
        at android.widget.ListView.layoutChildren(ListView.java:1914)
        at android.widget.AbsListView.onLayout(AbsListView.java:2961)
        at android.view.View.layout(View.java:20836)
        at android.view.ViewGroup.layout(ViewGroup.java:6401)
        at android.support.constraint.ConstraintLayout.onLayout(ConstraintLayout.java:1915)
        at android.view.View.layout(View.java:20836)
        at android.view.ViewGroup.layout(ViewGroup.java:6401)
        at android.widget.FrameLayout.layoutChildren(FrameLayout.java:323)
        at android.widget.FrameLayout.onLayout(FrameLayout.java:261)
        at android.view.View.layout(View.java:20836)
        at android.view.ViewGroup.layout(ViewGroup.java:6401)
        at android.support.v7.widget.ActionBarOverlayLayout.onLayout(ActionBarOverlayLayout.java:444)
        at android.view.View.layout(View.java:20836)
        at android.view.ViewGroup.layout(ViewGroup.java:6401)
        at android.widget.FrameLayout.layoutChildren(FrameLayout.java:323)
        at android.widget.FrameLayout.onLayout(FrameLayout.java:261)
        at android.view.View.layout(View.java:20836)
        at android.view.ViewGroup.layout(ViewGroup.java:6401)
        at android.widget.LinearLayout.setChildFrame(LinearLayout.java:1791)
        at android.widget.LinearLayout.layoutVertical(LinearLayout.java:1635)
        at android.widget.LinearLayout.onLayout(LinearLayout.java:1544)
        at android.view.View.layout(View.java:20836)
        at android.view.ViewGroup.layout(ViewGroup.java:6401)
        at android.widget.FrameLayout.layoutChildren(FrameLayout.java:323)
        at android.widget.FrameLayout.onLayout(FrameLayout.java:261)
        at com.android.internal.policy.DecorView.onLayout(DecorView.java:944)
        at android.view.View.layout(View.java:20836)
        at android.view.ViewGroup.layout(ViewGroup.java:6401)
        at android.view.ViewRootImpl.performLayout(ViewRootImpl.java:2948)
        at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:2635)
        at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1779)
        at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:7810)
        at android.view.Choreographer$CallbackRecord.run(Choreographer.java:911)
        at android.view.Choreographer.doCallbacks(Choreographer.java:723)
        at android.view.Choreographer.doFrame(Choreographer.java:658)
        at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:897)
        at android.os.Handler.handleCallback(Handler.java:789)
        at android.os.Handler.dispatchMessage(Handler.java:98)
        at android.os.Looper.loop(Looper.java:164)
        at android.app.ActivityThread.main(ActivityThread.java:6938)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:327)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1374)

Right now, my goal is to just solve the crash.

Sorry I forgot to add the actual code.

package com.example.caesp.todolist;


import android.content.Intent;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.AdapterView;
import android.widget.ArrayAdapter;
import android.widget.Button;
import android.widget.ListView;
import java.util.ArrayList;

public class MainActivity extends AppCompatActivity {


    ListView ListofList;
    ArrayList<String> Lists;
    ArrayAdapter<String> ListAdapter;
    Button button;
    ArrayList<String> Subject = new ArrayList<>();
    ArrayList<String> tit = new ArrayList<>();
    int Size;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        ListofList = findViewById(R.id.LofL);
        Lists = new ArrayList<String>();
        ListAdapter = new ArrayAdapter<String>(this, android.R.layout.simple_list_item_1, Lists);
        ListofList.setAdapter(ListAdapter);

        button = findViewById(R.id.AddList);
        Lists.add("Test");

        ListofList.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
                Intent i = new Intent(getBaseContext(), ItemList.class);
                String s = (String) ListofList.getItemAtPosition(position);
                i.putExtra("Title", s);
                i.putExtra("size", Size);
                int a = 1;
                for (String t : Subject) {
                    i.putExtra("Item" + a, t);
                    a++;
                }
                a = 1;
                for (String u : tit) {
                    i.putExtra("title" + a, u);
                    a++;
                }
                startActivityForResult(i, 2);
            }
        });

        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Intent i = new Intent(getBaseContext(), CreateItem.class);
                String message = "Please enter the name of the list below.";
                i.putExtra("mess", message);
                startActivityForResult(i, 1);
            }
        });



    }

    @Override
    protected void onActivityResult (int requestCode, int resultCode, Intent data) {
        Bundle extras = getIntent().getExtras();

        if (data == null) {

        }
        if (data != null) {
            String item = (data.getStringExtra("item"));
            Lists.add(item);
            ListAdapter.notifyDataSetChanged();
        }

        if (requestCode == 2) {
            ArrayList<String> subject = new ArrayList<String>();
            ArrayList<String> title = new ArrayList<String>();
            int i = 1;
            assert data != null;
            int size = (data.getIntExtra("ListSize", 0));
            Size = size;
            while (i <= size) {
                String item = (data.getStringExtra("item" + i));
                subject.add(item);

                i++;
            }
            i = 1;
            while (i <= size) {
                String item = (data.getStringExtra("title" + i));
                title.add(item);

                i++;
            }
            Subject = subject;
            tit = title;
        }
    }


}

it would help a lot if you would post a snippet of your code. But based on the error, the object is null and you are trying to read that null string object. — JoM, Nov 13 '18 at 05:42
Error log is not enogh to decide what happend, please provide code snippet relevant to this error — Blasanka, Nov 13 '18 at 05:43
Possible duplicate of [What is a NullPointerException, and how do I fix it?](https://stackoverflow.com/questions/218384/what-is-a-nullpointerexception-and-how-do-i-fix-it) — Blasanka, Nov 13 '18 at 05:44
Possible duplicate of [java / android NullPointerException thrown on a String](https://stackoverflow.com/questions/11613060/java-android-nullpointerexception-thrown-on-a-string) — Sniffer, Nov 13 '18 at 05:44
@Cristian Espaillat I would be interested to know why you consider this crash "weird" ... you're for sure accessing a null reference and calling a method (toString()) on it, so the system will throw an NPE, as expected. — Alessio, Nov 13 '18 at 05:46
hey people we need to see the code before posting the possible duplicate comment! — Badran, Nov 13 '18 at 05:47
one of the items that you are passing to your adapter is null. I msee nothing weird here. — Vladyslav Matviienko, Nov 13 '18 at 06:02

score 0 · Answer 1 · answered Nov 13 '18 at 16:55

I found the problem with my code. On coming back from a separate activity, the onActivityResult created a null value at

if (data != null) {
            String item = (data.getStringExtra("item"));
            Lists.add(item);
            ListAdapter.notifyDataSetChanged();
        }

Since there was no Extra called "item," it created a null value and added it to ListAdapter. Sorry to have wasted your time with this if you did.

Getting weird String crash

1 Answers1