如果我尝试进行特定活动，则应用程序崩溃

Question

So I am making a simple app. It's just basically making a list of win with a custom list view at the end.

It starts off on the main screen where there are two buttons, one is an "Add" button which takes you to the Add activity. If you push this it'll take you to a page where you type in the name,price, description of the wine and then hit a submit button to the list. The other button on the main screen is a "Go to List" button which just takes you directly to the list activity.

If I go through the Add button, add a wine, and then go to the list, it works fine. I can see the list. It even works if I don't add anything to the list. I can see the empty list activity.

When I push the "Go to List" button on the main screen though, it crashes and says "The application has stopped".

I don't get why I can go through the Add button to get to the list fine, but this button doesn't work at all.

Could I get some help?

Here are the three relevant activities, the AddActivity, the ListActivity, and the MainActivity.

AddActivity:

public class AddActivity extends AppCompatActivity {


EditText editWineName;
EditText editWinePrice;
EditText editWineDescription;
Button btnSubmit;
Button btnGoToList;
String stringWineName;
String stringWinePrice;
String stringWineDescription;
ArrayList<String> listWineName = new ArrayList<>();
ArrayList<String> listPrice = new ArrayList<>();
ArrayList<String> listWineDescription = new ArrayList<>();

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_add);
    setVariables();



    btnSubmit.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            setVariables();
            listWineName.add(stringWineName);
            listPrice.add(stringWinePrice);
            listWineDescription.add(stringWineDescription);
            Toast.makeText(AddActivity.this, stringWineName + " was added to the list.", Toast.LENGTH_SHORT).show();
            clearEditText();

        }
    });

    btnGoToList.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            Intent intentGoToList = new Intent(AddActivity.this,ListActivity.class);
            intentGoToList.putStringArrayListExtra("WINENAME", listWineName);
            intentGoToList.putStringArrayListExtra("WINEPRICE", listPrice);
            intentGoToList.putStringArrayListExtra("WINEDESCRIPTION", listWineDescription);
            startActivity(intentGoToList);
        }
    });

}

private void setVariables(){
    editWineName = (EditText) findViewById(R.id.editName);
    editWinePrice = (EditText) findViewById(R.id.editPrice);
    editWineDescription = (EditText) findViewById(R.id.editDescription);
    btnSubmit = (Button) findViewById(R.id.btnSubmit);
    btnGoToList = (Button) findViewById(R.id.btnGoToList);
    stringWineName = editWineName.getText().toString();
    stringWinePrice = "$" + editWinePrice.getText().toString();
    stringWineDescription = editWineDescription.getText().toString();
}

private void clearEditText() {
    editWineName.getText().clear();
    editWinePrice.getText().clear();
    editWineDescription.getText().clear();
}
}

ListActivity:

public class ListActivity extends AppCompatActivity {

ListView wineList;
ArrayAdapter adapter;
Button btnBacktoMain;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_list);
    setVariables();



    ArrayList<String> listWineName = getIntent().getStringArrayListExtra("WINENAME");
    ArrayList<String > listWinePrice = getIntent().getStringArrayListExtra("WINEPRICE");
    ArrayList<String> listWineDescription = getIntent().getStringArrayListExtra("WINEDESCRIPTION");

    adapter = new CustomAdapter(this, listWineName, listWinePrice, listWineDescription);
    wineList.setAdapter(adapter);


    btnBacktoMain.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            Intent intentBackToMain = new Intent(ListActivity.this,MainActivity.class);
            startActivity(intentBackToMain);
        }
    });

}

private void setVariables (){
    btnBacktoMain = (Button) findViewById(R.id.btnBackToMain);
    wineList = (ListView) findViewById(R.id.listWine);

}

}

MainActivity:

public class MainActivity extends AppCompatActivity {

Button btnAdd;
Button btnList;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    setVariables();

    btnAdd.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {                                                //Goes to the add activity
            Intent intentAdd = new Intent(MainActivity.this, AddActivity.class);
            startActivity(intentAdd);
        }
    });

    btnList.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {                                              //Goes to the list activity
            Intent intentList = new Intent(MainActivity.this, ListActivity.class);
            startActivity(intentList);
        }
    });
}

private void setVariables(){
    btnAdd = (Button) findViewById(R.id.btnAddWine);
    btnList = (Button) findViewById(R.id.btnViewList);
}

}

Answer 1

Caused by: java.lang.NullPointerException: Attempt to invoke interface method 'int java.util.List.size()' on a null object reference
    at android.widget.ArrayAdapter.getCount(ArrayAdapter.java:344)
    at android.widget.ListView.setAdapter(ListView.java:493)
    at com.example.jeremy.mywine.ListActivity.onCreate(ListActivity.java:33)

Your crash indicates that the data in the adapter given to the ListView in ListActivity is null. Make it not null. Start at ListActivity.java at line 33 and go backwards to find where you are (or this case are not) initializing the data in the list adapter.

In you case, you are expecting data in your intent. OK, where is your intent set up? In your MainActivity click. Well, there you just launch the activity without passing any data in the intent extras, hence there is nothing to pull out from the intent in ListActivity , hence your crash.

So you need to initialize the data in MainActivity and set it as extras in the Intent you use to launch ListActivity .

Since the ListActivity is expecting this:

ArrayList<String> listWineName = getIntent().getStringArrayListExtra("WINENAME");
ArrayList<String > listWinePrice = getIntent().getStringArrayListExtra("WINEPRICE");
ArrayList<String> listWineDescription = getIntent().getStringArrayListExtra("WINEDESCRIPTION");

You would update your MainActivity to do something like this (where getDescriptions() is a fictitious method you would create to return a list of strings)

btnList.setOnClickListener(new View.OnClickListener() {
    @Override
    public void onClick(View view) {
        //Goes to the list activity
        Intent intentList = new Intent(MainActivity.this, ListActivity.class);
        intentList.putExtra("WINENAME", new ArrayList<String>());     // Empty list
        intentList.putExtra("WINEPRICE", Arrays.asList("Foo", "Bar")); // Explicit list named items
        intentList.putExtra("WINEDESCRIPTION", getDescriptions());    // Get list from private method
        startActivity(intentList);
    }
});

Also check this post my be useful for learning how to read and understand a crash log.

And check the documentation on how to start activities .

Hope that helps!