Android - how to use Spinner to hide/display View objects

Question

For an Android application, I'm trying to use a Spinner which, when an option is selected, will hide/display relevant View objects. For my application, these objects are an EditText and an associated TextView label for the field. Unfortunately, I can't seem to get the EditText to hide/display, and when I add it code to hide/display the TextView, I get a NullPointerException. I presume that since I am laying out the view objects in a RelativeLayout, by hiding one of the view objects, I am removing its relationship with other view objects, hence the NullPointer.

Can anyone figure out why this might be happening? Here's my code:

public class FormFields extends Activity {
    private Spinner mSpinner;
    private EditText mTextField;
    private TextView mLabel;

    private static final int SPINNER_OPTION_FIRST = 0;
    private static final int SPINNER_OPTION_SECOND = 1;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        setContentView(R.layout.form_fields);

        mTextField = (EditText) findViewById(R.id.text_field);
        mLabel = (TextView) findViewById(R.id.field_label)
        mSpinner = (Spinner) findViewById(R.id.spinner);

        ArrayAdapter adapter1 = ArrayAdapter.createFromResource(
            this, R.array.spinnerOptions, android.R.layout.simple_spinner_item);
        adapter1.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
        mSpinner.setAdapter(adapter1);

        mSpinner.setOnItemSelectedListener(new OnItemSelectedListener() {
            @Override
            public void onItemSelected(AdapterView<?> parentView, View selectedItemView, int position, long id) {
                switch(position) {
                    case SPINNER_OPTION_FIRST: {
                        mLabel.setVisibility(View.GONE);
                        mTextField.setVisibility(View.GONE);
                    }
                    case SPINNER_OPTION_SECOND: {
                        mLabel.setVisibility(View.VISIBLE);
                        mTextField.setVisibility(View.VISIBLE);
                    }
                }
            }

            @Override
            public void onNothingSelected(AdapterView<?> parentView) {
                // Do nothing
            }
        });
    }
}

form_fields.xml

<?xml version="1.0" encoding="utf-8"?>
<ScrollView xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent">

    <RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
        android:layout_width="fill_parent"
        android:layout_height="fill_parent"
        android:background="#104667">

        <TextView
            android:id="@+id/spinner_label"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_centerHorizontal="true"
            android:layout_marginTop="15dip"
            android:textStyle="bold"
            android:text="Please select an option" />

        <Spinner
            android:id="@+id/spinner"
            android:layout_width="fill_parent"
            android:layout_height="wrap_content"
            android:layout_below="@+id/spinner_label"
            android:layout_marginLeft="25dip"
            android:layout_marginRight="25dip"
            android:drawSelectorOnTop="true"
            android:prompt="@string/spinnerPrompt" />

        <TextView
            android:id="@+id/field_label"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_below="@+id/spinner"
            android:layout_centerHorizontal="true"
            android:layout_marginTop="15dip"
            android:textStyle="bold"
            android:text="Enter text here: "
            android:visibility="gone" />

        <EditText
            android:id="@+id/text_field"
            android:layout_width="fill_parent"
            android:layout_height="wrap_content"
            android:layout_marginLeft="25dip"
            android:layout_marginRight="25dip"
            android:layout_below="@+id/field_label" 
            android:visibility="gone" />
    </RelativeLayout>
</ScrollView>

Solution

There are a few small omissions in the code you posted. When I made the following changes, I was able to compile and run your code successfully.

1. You are missing a semicolon after

   mLabel = (TextView) findViewById(R.id.field_label)
   

2. Insert a break; statement in between your two case options.

3. You can remove unnecessary braces around your case statements.

   case SPINNER_OPTION_FIRST:
       mLabel.setVisibility(View.GONE);
       mTextField.setVisibility(View.GONE);
       break;        
   case SPINNER_OPTION_SECOND:
       mLabel.setVisibility(View.VISIBLE);
       mTextField.setVisibility(View.VISIBLE);
   

4. Although not necessary to get your program to run, it would be better to explicitly specify ArrayAdapter<CharSequence> when defining adapter1 to avoid type issues.

Comments

[Keeb13r]

Thanks a bunch. Since this code was slightly adjusted from my actual application, the semicolon wasn't the actual problem (otherwise, my application wouldn't have compiled). It was actually that I didn't set my mLabel member variable at all! Go figure. I'm curious - what is the purpose of the break statement?

[Brian]

Without break; statements separating each case statement, execution "falls through" to each case statement successively. There are reasons why you might want this to happen, but ordinarily a switch block is used to provide alternative options and each case should only run the code for that particular condition. For some examples and a detailed explanation of the switch statement, see: download.oracle.com/javase/tutorial/java/nutsandbolts/…