App crash when onClick on 2nd item and so on in listview

Question

I am just a beginner in programming. I am having issue with my code. Whenever I try to click on the 2nd item in a list, the app suddenly crashes.

This is my code:

public class MainActivity extends AppCompatActivity {

    ListView lvCategories;
    String[] categories = {
        "School", "Work", "Family Outings", "Friends Outings", "Groceries" , "Appointments", "Indoor Activities",
            "Outdoor Activities", "Games","Overseas Travelling"
    };

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        lvCategories = (ListView)findViewById(R.id.listViewCategories);

        final ArrayAdapter<String> adapter = new ArrayAdapter(this, android.R.layout.simple_list_item_1, categories);
        lvCategories.setAdapter(adapter);

        lvCategories.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
                String category = (String) parent.getItemAtPosition(position);

                if (category == "School") {
                    Intent school = new Intent(view.getContext(), school_activity.class);
                    school.putExtra("school_category", "");
                    startActivity(school);
                }
                if (category == "Work") {
                    Intent work = new Intent(view.getContext(), work_activity.class);
                    work.putExtra("work_category", "");
                    startActivity(work);
                }
            }
        });
    }
}

May I know what is the issue?

Here is the error I found from LogCat.

Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.EditText.setText(java.lang.CharSequence)' on a null object reference

And here's my work_activity

public class work_activity extends AppCompatActivity {

    Button btnSave;
    EditText etContent;
    TextView tvLastUpdated;

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

        btnSave = (Button)findViewById(R.id.buttonSave);
        etContent = (EditText)findViewById(R.id.editTextContent);
        tvLastUpdated = (TextView)findViewById(R.id.textViewLastUpdated);

        Intent intentReceived = getIntent();
        final String strContent = intentReceived.getStringExtra("work_category");
        etContent.setText(strContent);
        Calendar now = Calendar.getInstance();
        final String datetime = now.get(Calendar.DAY_OF_MONTH) + "/" +
                (now.get(Calendar.MONTH) + 1) + "/" +
                now.get(Calendar.YEAR) + " " +
                now.get(Calendar.HOUR_OF_DAY) + ":" +
                now.get(Calendar.MINUTE);
        tvLastUpdated.setText(datetime);

        btnSave.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                String strContentSaved = etContent.getText().toString();
                SharedPreferences preferences = getSharedPreferences("category_work_saved", MODE_PRIVATE);
                SharedPreferences.Editor prefEdit = preferences.edit();
                prefEdit.putString("contentSaved", strContentSaved);
                prefEdit.putString("date", datetime);
                prefEdit.commit();
                Toast.makeText(work_activity.this, "Content Saved Successfully", Toast.LENGTH_SHORT).show();
                finish();
            }
        });
    }

    @Override
    protected void onResume() {
        super.onResume();
        SharedPreferences preferences = getSharedPreferences("category_work_saved", MODE_PRIVATE);
        String contentSaved = preferences.getString("contentSaved", "");
        etContent.setText(contentSaved);
        String dateTime = preferences.getString("date","");
        tvLastUpdated.setText(dateTime);
    }
}

Answer 1

It looks like you may be trying to set your text

etContent.setText(strContent);

without a strContent value...is that the case? You're sending "work_category", but it may be a null value ""

Answer 2

You've some critical problems in your code. Let me point out these issues here.

In your MainActivity you're comparing the String values wrongly. You need to use equals() method instead of == operator comparison to compare String.

And yes, try sending something to the work_activity or school_activity.

lvCategories.setOnItemClickListener(new AdapterView.OnItemClickListener() {
    @Override
    public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
        String category = (String) parent.getItemAtPosition(position);

        if (category.equals("School")) {
            Intent school = new Intent(view.getContext(), school_activity.class);
            school.putExtra("school_category", "send something here");
            startActivity(school);
        }
        if (category.equals("Work")) {
            Intent work = new Intent(view.getContext(), work_activity.class);
            work.putExtra("work_category", "send something here");
            startActivity(work);
        }
    }
});

Now in your work_activity, you need to check for null value for received intent.

String strContent = null;
Intent intentReceived = getIntent();

// Check if the intent received is null
if (intentReceived != null) 
    strContent = intentReceived.getStringExtra("work_category");

// Now check if the string content is null 
if(strContent != null) 
    etContent.setText(strContent);

Another thing to check if the id of the EditText in your activity_work_activity layout is editTextContent. The id needs to match to find the EditText which might cause the crash here.

Answer 3

The error says,

Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.EditText.setText(java.lang.CharSequence)' on a null object reference

So that means findViewById(R.id.editTextContent); is returning null.

I think you forgot to add the id editTextContent to the EditText or you don't have the Editetext in layout activity_work_activity.xml.

Also as cricket007 suggested in the comments, use equals to compare Strings as follows,

if (category.equals("School")){
    ...
}