requestFocus（）给出空指针异常

Question

I have an edit text in my activity and want to request focus on it. However, it always crashes upon launching the activity.

Java code:

foodText = (EditText) findViewById(R.id.foodName);
prepText = (EditText) findViewById(R.id.prepText);
foodText.requestFocus();

Activity.xml:

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/activity_add_food"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="com.example.rcadit.foodgenie.AddFood">
    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
                android:orientation="vertical"
style="@style/AppTheme"
        >


        <ScrollView
            android:layout_width="match_parent"
            android:layout_height="match_parent">



            <LinearLayout
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:orientation="vertical">
            <LinearLayout
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:orientation="vertical"
                android:background="@color/colorAccent"
                android:transitionName="addFood">


                <EditText
                    android:id="@+id/foodName"
                    android:layout_width="wrap_content"
                    android:layout_height="wrap_content"
                    android:hint="Food name"
                    android:textSize="32sp"
android:inputType="textCapWords"
android:textColor="@color/colorIcons"
                    android:layout_marginLeft="64dp"
                    android:layout_marginRight="64dp"
                    android:layout_marginBottom="32dp"
                    android:layout_marginTop="32dp"
                    android:textColorHint="@color/colorIcons"/>
            </LinearLayout>

            </LinearLayout>


        </ScrollView>
    </LinearLayout>

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:orientation="vertical"
        android:paddingLeft="@dimen/activity_horizontal_margin">


        <TextView
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:padding="16dp"

            android:textSize="16sp"

            android:text="Hunger Level" />
        <Spinner
            android:id="@+id/hunger_spinner"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"


            android:padding="16dp"

            >

        </Spinner>
        <TextView
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:padding="16dp"
            android:textSize="16sp"
            android:text="Cuisine" />
        <Spinner
            android:id="@+id/cuisine_spinner"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"


            android:padding="16dp"

            >

        </Spinner>
        <TextView
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:padding="16dp"
            android:textSize="16sp"
            android:text="Preparation Time" />
        <EditText
            android:id="@+id/prepText"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:textSize="16sp"
            android:padding="16dp"
            android:hint="Enter Preparation Time"/>


        <TextView
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:padding="16dp"
            android:textSize="16sp"
            android:text="Course" />
        <Spinner
            android:id="@+id/course_spinner"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"


            android:padding="16dp"

            >

        </Spinner>

        <TextView
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="Salty"
            android:padding="16dp"
            android:textSize="16sp"
            />
        <TextView
            android:id="@+id/howSalty"
            android:hint="Salty"
            android:textSize="12sp"
            android:paddingBottom="8dp"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_gravity="center_horizontal"
            />

        <SeekBar
            android:id="@+id/salty"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:thumbTint="@color/colorAccent"
            android:progressTint="@color/colorAccent"
            android:max="2"
            android:progress="1"
            />
        <TextView
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="Sweet"
            android:padding="16dp"
            android:textSize="16sp"/>
        <TextView
            android:id="@+id/howSweet"
            android:hint="Sweet"
            android:textSize="12sp"
            android:paddingBottom="8dp"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_gravity="center_horizontal"/>
        <SeekBar
            android:thumbTint="@color/colorAccent"
            android:progressTint="@color/colorAccent"
            android:id="@+id/sweet"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:max="2"
            android:progress="1"
            />


        <TextView
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="Spicy"
            android:padding="16dp"
            android:textSize="16sp"/>

        <TextView
            android:id="@+id/howSpicy"
            android:hint="Spicy"
            android:textSize="12sp"
            android:paddingBottom="8dp"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_gravity="center_horizontal"
            />
        <SeekBar
            android:thumbTint="@color/colorAccent"
            android:progressTint="@color/colorAccent"
            android:id="@+id/spicy"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:max="2"
            android:progress="1"

            />


        <TextView
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"

            android:padding="16dp"
            android:text="Special Requirements"

            android:textSize="16sp" />

        <GridLayout
            android:layout_width="match_parent"
            android:layout_height="wrap_content"

            android:layout_margin="16dp"
            android:columnCount="2"
            android:rowCount="3">



            <CheckBox
                android:layout_width="wrap_content"
                android:id="@+id/dairyFree"
                android:buttonTint="@color/colorAccent"
                android:layout_height="wrap_content"
                android:text="Dairy Free"
                android:textSize="16sp"
                android:onClick="onCheckboxClicked"/>

            <CheckBox
                android:layout_width="wrap_content"
                android:id="@+id/nutFree"
                android:buttonTint="@color/colorAccent"
                android:layout_height="wrap_content"
                android:text="Nut Free"
                android:textSize="16sp"
                android:onClick="onCheckboxClicked"/>

            <CheckBox
                android:id="@+id/eggFree"
                android:layout_width="wrap_content"
                android:buttonTint="@color/colorAccent"
                android:layout_height="wrap_content"
                android:text="Egg Free"
                android:textSize="16sp"
                android:onClick="onCheckboxClicked"
                />

            <CheckBox
                android:id="@+id/vegetarian"
                android:layout_width="wrap_content"
                android:buttonTint="@color/colorAccent"
                android:layout_height="wrap_content"
                android:text="Vegetarian"
                android:textSize="16sp"
                android:onClick="onCheckboxClicked"/>
        </GridLayout>
        <Button
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"

            android:layout_margin="16dp"
            android:onClick="add"

            android:text="Add" />
    </LinearLayout>

</android.support.constraint.ConstraintLayout>

I have looked at other questions but I can't see why my code doesn't work as the ID's are correct and I can't find the fault in the code.

Log file:

E/AndroidRuntime: FATAL EXCEPTION: main
                  Process: com.example.rcadit.foodgenie, PID: 3221
                  java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.rcadit.foodgenie/com.example.rcadit.foodgenie.AddFood}: java.lang.NullPointerException: Attempt to invoke virtual method 'boolean android.widget.EditText.requestFocus()' on a null object reference
                      at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2416)
                      at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476)
                      at android.app.ActivityThread.-wrap11(ActivityThread.java)
                      at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344)
                      at android.os.Handler.dispatchMessage(Handler.java:102)
                      at android.os.Looper.loop(Looper.java:148)
                      at android.app.ActivityThread.main(ActivityThread.java:5417)
                      at java.lang.reflect.Method.invoke(Native Method)
                      at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
                      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
                   Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'boolean android.widget.EditText.requestFocus()' on a null object reference
                      at com.example.rcadit.foodgenie.AddFood.onCreate(AddFood.java:60)
                      at android.app.Activity.performCreate(Activity.java:6237)
                      at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1107)
                      at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2369)
                      at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476) 
                      at android.app.ActivityThread.-wrap11(ActivityThread.java) 
                      at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344) 
                      at android.os.Handler.dispatchMessage(Handler.java:102) 
                      at android.os.Looper.loop(Looper.java:148) 
                      at android.app.ActivityThread.main(ActivityThread.java:5417) 
                      at java.lang.reflect.Method.invoke(Native Method) 
                      at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726) 
                      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616) 
I/Process: Sending signal. PID: 3221 SIG: 9
Application terminated.

What's wrong?

Answer 1

So I was able to reproduce your error by not calling setContentView() before using your code. So the correct way to do it is to do:

protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.your_activity);
        foodText = (EditText) findViewById(R.id.foodName);
        prepText = (EditText) findViewById(R.id.prepText);
        foodText.requestFocus();
}

your_activity is basically the name of your xml file without the xml extension. For more documentation on Views you can check this link or this link ifwhat you're doing is creating a second Activity. (both refer to the official documentation and give examples and explanations)

Also I suppose that you would get the same error if you don't use the right name of file for the setContentView() method.

Answer 2

I have tried at my end its working fine with exact your params.

MainActivity:

public class MainActivity extends AppCompatActivity {

    Context context;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        context = this;

        EditText number = (EditText) findViewById(R.id.foodName);
        number.requestFocus();

    }
}

activity_main.xml:

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/base"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_margin="10dp"
    android:background="@color/white"
    android:orientation="horizontal"
    android:weightSum="10">


    <EditText
        android:id="@+id/foodName"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginBottom="32dp"
        android:layout_marginLeft="64dp"
        android:layout_marginRight="64dp"
        android:layout_marginTop="32dp"
        android:hint="Food name"
        android:inputType="textCapWords"
        android:textColor="@color/colorPrimaryDark"
        android:textColorHint="@color/colorPrimaryDark"
        android:textSize="32sp" />

</RelativeLayout>

Answer 3

我猜你在setContentView()方法之前调用了findViewById() setContentView()方法。