如何解决对空对象引用调用虚拟方法'void android.widget.TextView.setText（java.lang.CharSequence）'的尝试

Question

I am getting this error while running my app. I can't figure out what to do.

Here's my MainActivity.java

    package com.example.user.sensor;

import android.content.Context;
import android.content.DialogInterface;
import android.hardware.Sensor;
import android.hardware.SensorEvent;
import android.hardware.SensorEventListener;
import android.hardware.SensorManager;
import android.os.Build;
import android.os.Bundle;
import android.os.Vibrator;
import android.support.v7.app.AlertDialog;
import android.support.v7.app.AppCompatActivity;
import android.widget.TextView;



public class MainActivity extends AppCompatActivity implements SensorEventListener {


    private float lastX = 0, lastY = 0, lastZ = 0;

    private SensorManager sensorManager;
    private Sensor accelerometer;

    private float deltaXMax = 0;
    private float deltaYMax = 0;
    private float deltaZMax = 0;

    private float deltaX = 0;
    private float deltaY = 0;
    private float deltaZ = 0;

    private float vibrateThreshold = 0;

    TextView currentX, currentY, currentZ;

    public Vibrator v;


    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        getSupportActionBar().getThemedContext();

        AlertDialog.Builder builder;
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) {
            builder = new AlertDialog.Builder(getSupportActionBar().getThemedContext(), android.R.style.Theme_Material_Dialog_Alert);
        } else {
            builder = new AlertDialog.Builder(getSupportActionBar().getThemedContext());
        }
        builder.setTitle("Alert Dialog")
                .setPositiveButton(android.R.string.yes, new DialogInterface.OnClickListener() {
                    public void onClick(DialogInterface dialog, int which) {

                    }
                })
                .setIcon(android.R.drawable.ic_dialog_alert)
                .show();

        sensorManager = (SensorManager) getSystemService(Context.SENSOR_SERVICE);
        if (sensorManager.getDefaultSensor(Sensor.TYPE_ACCELEROMETER) != null) {
            // success! we have an accelerometer

            accelerometer = sensorManager.getDefaultSensor(Sensor.TYPE_ACCELEROMETER);
            sensorManager.registerListener(this, accelerometer, SensorManager.SENSOR_DELAY_NORMAL);
            vibrateThreshold = accelerometer.getMaximumRange() / 2;
        } else {
            // fai! we dont have an accelerometer!
        }

        //initialize vibration
        v = (Vibrator) this.getSystemService(Context.VIBRATOR_SERVICE);

    }


    //onResume() register the accelerometer for listening the events
    protected void onResume() {
        super.onResume();
        sensorManager.registerListener(this, accelerometer, SensorManager.SENSOR_DELAY_NORMAL);
    }

    //onPause() unregister the accelerometer for stop listening the events
    protected void onPause() {
        super.onPause();
        sensorManager.unregisterListener(this);
    }

    @Override
    public void onAccuracyChanged(Sensor sensor, int accuracy) {

    }

    @Override
    public void onSensorChanged(SensorEvent event) {

        // clean current values
        displayCleanValues();
        // display the current x,y,z accelerometer values
        displayCurrentValues();
        // display the max x,y,z accelerometer values

        // get the change of the x,y,z values of the accelerometer
        deltaX = Math.abs(lastX - event.values[0]);
        deltaY = Math.abs(lastY - event.values[1]);
        deltaZ = Math.abs(lastZ - event.values[2]);

        // if the change is below 2, it is just plain noise
        if (deltaX < 2)
            deltaX = 0;
        if (deltaY < 2)
            deltaY = 0;
        if (deltaZ > (vibrateThreshold) || (deltaY > vibrateThreshold) || (deltaZ > vibrateThreshold)) {
            v.vibrate(50);
        }
    }

    public void displayCleanValues() {
        currentX.setText("0.0");
        currentY.setText("0.0");
        currentZ.setText("0.0");
    }

    // display the current x,y,z accelerometer values
    public void displayCurrentValues() {
        currentX.setText(Float.toString(deltaX));
        currentY.setText(Float.toString(deltaY));
        currentZ.setText(Float.toString(deltaZ));
    }
}

// display the max x,y,z accelerometer values

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
    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:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity"
    android:orientation="vertical"
    android:layout_marginLeft="15dp">


    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="GYROSCOPE"
        android:textColor="@android:color/black"
        android:layout_marginTop="30dp"

        />

    <LinearLayout
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:orientation="vertical"
        android:layout_marginTop="8dp">

        <LinearLayout
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:orientation="horizontal">

            <TextView
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:text="X  :"
                />

            <TextView
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:text="0"
                android:layout_marginLeft="10dp"
                />

        </LinearLayout>
        <LinearLayout
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:orientation="horizontal">


            <TextView
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:text="Y  :"
                android:layout_marginTop="4dp"
                />

            <TextView
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:text="0"
                android:layout_marginLeft="10dp"
                android:layout_marginTop="4dp"
                />

        </LinearLayout>
        <LinearLayout
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:orientation="horizontal">
            <TextView
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:text="Z  :"
                android:layout_marginTop="4dp"
                />
            <TextView
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:text="0"
                android:layout_marginLeft="10dp"
                android:layout_marginTop="4dp"
                />

        </LinearLayout>





        <TextView
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="ACCELEROMETER"
            android:textColor="@android:color/black"
            android:layout_marginTop="30dp"

            />

        <LinearLayout
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:orientation="vertical"
            android:layout_marginTop="8dp">

            <LinearLayout
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:orientation="horizontal">

                <TextView
                    android:layout_width="wrap_content"
                    android:layout_height="wrap_content"
                    android:text="X  :"
                    />

                <TextView
                    android:id="@+id/currentX"
                    android:layout_width="wrap_content"
                    android:layout_height="wrap_content"
                    android:text="0"
                    android:layout_marginLeft="10dp"
                    />

            </LinearLayout>
            <LinearLayout
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:orientation="horizontal">


                <TextView
                    android:layout_width="wrap_content"
                    android:layout_height="wrap_content"
                    android:text="Y  :"
                    android:layout_marginTop="4dp"
                    />

                <TextView
                    android:id="@+id/currentY"
                    android:layout_width="wrap_content"
                    android:layout_height="wrap_content"
                    android:text="0"
                    android:layout_marginLeft="10dp"
                    android:layout_marginTop="4dp"
                    />

            </LinearLayout>
            <LinearLayout
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:orientation="horizontal">
                <TextView
                    android:layout_width="wrap_content"
                    android:layout_height="wrap_content"
                    android:text="Z  :"
                    android:layout_marginTop="4dp"
                    />
                <TextView
                    android:id="@+id/currentZ"
                    android:layout_width="wrap_content"
                    android:layout_height="wrap_content"
                    android:text="0"
                    android:layout_marginLeft="10dp"
                    android:layout_marginTop="4dp"
                    />

            </LinearLayout>


        </LinearLayout>


    </LinearLayout>

</LinearLayout>

and here is the log file:

E/AndroidRuntime: FATAL EXCEPTION: main Process: com.example.user.sensor, PID: 8586 java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.TextView.setText(java.lang.CharSequence)' on a null object reference at com.example.user.sensor.MainActivity.displayCleanValues(MainActivity.java:121) at com.example.user.sensor.MainActivity.onSensorChanged(MainActivity.java:100) at android.hardware.SystemSensorManager$SensorEventQueue.dispatchSensorEvent(SystemSensorManager.java:830) at android.os.MessageQueue.nativePollOnce(Native Method) at android.os.MessageQueue.next(MessageQueue.java:325) at android.os.Looper.loop(Looper.java:142) at android.app.ActivityThread.main(ActivityThread.java:6494) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:807)

Answer 1

First, you need to provide an id of a TextView in Layout file

<TextView
        android:id="@id/txt1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="GYROSCOPE"
        android:textColor="@android:color/black"
        android:layout_marginTop="30dp"
        />

After that you need to declare the TextView in java file see below code:

TextView txt1, txt2;

after that in onCreate() , initialize the TextView:

txt1 = (TextView) findViewById(R.id.txt1);// R.id.txt1 is a layout textbox ID