尝试为.SharedPreferences字符串获取.setText（）时获取空指针异常

Question

我对android编程还很陌生，因此感谢您的帮助。

单击按钮时，我正在尝试将布尔值和字符串值存储到SharedPreferences中。存储后，它将更新mainActivty.xml页面。但是，经过许多尝试和数小时的查找，当尝试对字符串值进行.setText（）时，我似乎找不到摆脱Null Pointer Exception的方法。任何帮助将不胜感激，谢谢

我尝试创建一个充气机，因为某些问题是由于视图没有充气而引起的。我也曾尝试为我的textView初始化重新分配位置，但无济于事。

这是设置页代码：

打包com.example.sleep;

import android.content.SharedPreferences;
import android.os.Bundle;
import android.preference.PreferenceManager;
import android.util.Log;

import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Switch;
import android.widget.TextView;
import android.widget.Toast;

import androidx.appcompat.app.AppCompatActivity;


public class SettingPage extends AppCompatActivity{

public EditText setMinBar;
public Button setTimeEnterButton;
public Switch musicYesSwitch, soundYesSwitch;

public String SP = "New SP", timeStr = "nothing", finalTimeStr = "nil";

public boolean musicYes, soundYes;

public TextView minLeftTV, timerEndsTV;

public SharedPreferences varAcrossApp;
public SharedPreferences.Editor varEditor;

@Override //Creating the setting page
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.settingpage);

    setMinBar = findViewById(R.id.numMinsBar);
    setTimeEnterButton = findViewById(R.id.setTimeButton);
    musicYesSwitch = findViewById(R.id.musicSwitch);
    soundYesSwitch = findViewById(R.id.soundSwitch);
    timerEndsTV = findViewById(R.id.timerEndsTV);
    minLeftTV = (TextView) findViewById(R.id.minsLeftDisplay);

    varAcrossApp = PreferenceManager.getDefaultSharedPreferences(this);

    setTimeEnterButton.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {

            saveData();
            loadData();
            updateView();
            }
        });

    }


public void saveData(){ //Declaring method saveData
    varEditor = varAcrossApp.edit();
    varEditor.putString(timeStr, setMinBar.getText().toString());
    varEditor.putBoolean("mYes", musicYesSwitch.isChecked());
    varEditor.putBoolean("sYes", soundYesSwitch.isChecked());
    varEditor.apply();


}

public void loadData(){ //Declaring method loadData
    finalTimeStr = varAcrossApp.getString(timeStr, "0");
    musicYes = varAcrossApp.getBoolean("mYes", false);
    soundYes = varAcrossApp.getBoolean("sYes", false);

    Toast.makeText(getApplicationContext(), "Duration set" + 
finalTimeStr, Toast.LENGTH_SHORT).show();

}


public void updateView() {
    Log.d(SP, "Time is " + finalTimeStr);
    minLeftTV.setText(finalTimeStr);

    if (musicYes && soundYes) {
        timerEndsTV.setText("-Turn off music /n-Change sound profile to 
'Sound'");
    } else if (soundYes = true) {
        timerEndsTV.setText("-Change sound profile to 'Sound'");
    } else if (musicYes = true) {
        timerEndsTV.setText("-Turn off music");
    } else {
        timerEndsTV.setText("-Do nothing");
    }

}
}

这是Homepage.xml页面

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.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/homepage"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".MainActivity"
>

<TextView
    android:id="@+id/timeLeft"
    android:layout_width="191dp"
    android:layout_height="49dp"
    android:layout_marginStart="146dp"
    android:layout_marginTop="70dp"
    android:layout_marginEnd="147dp"
    android:fontFamily="@font/bree_serif"
    android:text="@string/text_header"
    android:textAlignment="center"
    android:textSize="36sp"
    android:textStyle="bold"
    app:layout_constraintEnd_toEndOf="parent"
    app:layout_constraintStart_toStartOf="parent"
    app:layout_constraintTop_toTopOf="parent" />

<TextView
    android:id="@+id/timeLeftNum"
    android:layout_width="84dp"
    android:layout_height="52dp"
    android:layout_marginTop="143dp"
    android:layout_marginEnd="103dp"
    android:layout_marginBottom="138dp"
    android:fontFamily="@font/bree_serif"
    android:text="@string/text_timeUnit"
    android:textAlignment="center"
    android:textSize="36sp"
    android:textStyle="bold"
    app:layout_constraintBottom_toTopOf="@+id/timerEndsTV"
    app:layout_constraintEnd_toEndOf="parent"
    app:layout_constraintTop_toBottomOf="@+id/timeLeft" />

<TextView
    android:id="@+id/whenTimerEnd"
    android:layout_width="132dp"
    android:layout_height="20dp"
    android:layout_marginStart="54dp"
    android:layout_marginEnd="253dp"
    android:layout_marginBottom="12dp"
    android:fontFamily="@font/bree_serif"
    android:text="@string/text_whenTimerEnd"
    android:textAlignment="viewStart"
    app:layout_constraintBottom_toTopOf="@+id/timerEndsTV"
    app:layout_constraintEnd_toEndOf="parent"
    app:layout_constraintHorizontal_bias="0.0"
    app:layout_constraintStart_toStartOf="parent" />

<TextView
    android:id="@+id/timerEndsTV"
    android:layout_width="299dp"
    android:layout_height="54dp"
    android:layout_marginStart="54dp"
    android:layout_marginEnd="58dp"
    android:layout_marginBottom="114dp"
    android:fontFamily="@font/bree_serif"
    android:text="TextView"
    app:layout_constraintBottom_toTopOf="@+id/startButton"
    app:layout_constraintEnd_toEndOf="parent"
    app:layout_constraintStart_toStartOf="parent" />

<Button
    android:id="@+id/startButton"
    android:layout_width="382dp"
    android:layout_height="52dp"
    android:layout_marginStart="13dp"
    android:layout_marginEnd="16dp"
    android:layout_marginBottom="16dp"
    android:fontFamily="@font/bree_serif"
    android:text="@string/text_startButton"
    app:layout_constraintBottom_toBottomOf="parent"
    app:layout_constraintEnd_toEndOf="parent"
    app:layout_constraintStart_toStartOf="parent" />

<TextView
    android:id="@+id/minsLeftDisplay"
    android:layout_width="155dp"
    android:layout_height="144dp"
    android:layout_marginStart="68dp"
    android:layout_marginTop="56dp"
    android:layout_marginEnd="1dp"
    android:layout_marginBottom="107dp"
    android:text="@string/numberInDisplay"
    android:textAlignment="textEnd"
    android:textSize="125sp"
    app:layout_constraintBottom_toTopOf="@+id/whenTimerEnd"
    app:layout_constraintEnd_toStartOf="@+id/timeLeftNum"
    app:layout_constraintStart_toStartOf="parent"
    app:layout_constraintTop_toBottomOf="@+id/timeLeft" />

</androidx.constraintlayout.widget.ConstraintLayout>

这是SettingPage.xml页面：

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.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/settingPageConstrainLayout"
android:layout_width="match_parent"
android:layout_height="match_parent">

<LinearLayout
    android:id="@+id/inputLayout"
    android:layout_width="299dp"
    android:layout_height="81dp"
    android:layout_marginStart="56dp"
    android:layout_marginEnd="56dp"
    android:orientation="horizontal"
    app:layout_constraintEnd_toEndOf="parent"
    app:layout_constraintStart_toStartOf="parent"
    app:layout_constraintTop_toBottomOf="@+id/TextboxDurationMins">

    <EditText
        android:id="@+id/numMinsBar"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:ems="10"
        android:inputType="number" />

    <Button
        android:id="@+id/setTimeButton"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:fontFamily="@font/bree_serif"
        android:text="@string/setTimeButton" />
</LinearLayout>

<LinearLayout
    android:id="@+id/switchesLayout"
    android:layout_width="64dp"
    android:layout_height="69dp"
    android:layout_marginEnd="32dp"
    android:layout_marginBottom="218dp"
    android:orientation="vertical"
    app:layout_constraintBottom_toBottomOf="parent"
    app:layout_constraintEnd_toEndOf="parent">

    <Switch
        android:id="@+id/musicSwitch"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content" />

    <Switch
        android:id="@+id/soundSwitch"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content" />
</LinearLayout>

<LinearLayout
    android:id="@+id/textBoxLayout"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_marginStart="56dp"
    android:layout_marginEnd="69dp"
    android:layout_marginBottom="230dp"
    android:orientation="vertical"
    app:layout_constraintBottom_toBottomOf="parent"
    app:layout_constraintEnd_toStartOf="@+id/switchesLayout"
    app:layout_constraintHorizontal_bias="0.0"
    app:layout_constraintStart_toStartOf="parent">

    <TextView
        android:id="@+id/turnOffMusics"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:fontFamily="@font/bree_serif"
        android:text="@string/offMusic"
        android:textAlignment="textStart"
        android:textSize="18sp" />

    <TextView
        android:id="@+id/onSound"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:fontFamily="@font/bree_serif"
        android:text="@string/onSound"
        android:textAlignment="textStart"
        android:textSize="18sp" />
</LinearLayout>

<TextView
    android:id="@+id/TextboxDurationMins"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_marginTop="176dp"
    android:layout_marginEnd="120dp"
    android:fontFamily="@font/bree_serif"
    android:text="@string/setDurationMins"
    android:textSize="18sp"
    app:layout_constraintEnd_toEndOf="parent"
    app:layout_constraintTop_toTopOf="parent" />

</androidx.constraintlayout.widget.ConstraintLayout>

这是错误代码：

E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.sleep, PID: 16835
java.lang.NullPointerException: Attempt to invoke virtual method 'void 
android.widget.TextView.setText(java.lang.CharSequence)' on a null object 
reference
    at com.example.sleep.SettingPage.updateView(SettingPage.java:82)
    at com.example.sleep.SettingPage$1.onClick(SettingPage.java:53)
    at android.view.View.performClick(View.java:7125)
    at android.view.View.performClickInternal(View.java:7102)
    at android.view.View.access$3400(View.java:801)
    at android.view.View$PerformClick.run(View.java:27301)
    at android.os.Handler.handleCallback(Handler.java:883)
    at android.os.Handler.dispatchMessage(Handler.java:100)
    at android.os.Looper.loop(Looper.java:214)
    at android.app.ActivityThread.main(ActivityThread.java:7319)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:934)

Answer 1

问题与minLeftTV.setText(finalTimeStr);

有关

您正在通过调用

创建实例

minLeftTV = (TextView) findViewById(R.id.minsLeftDisplay);

但是您的xml布局没有具有该ID的任何视图-@+id/minsLeftDisplay，因此minLeftTV为null，并且您将获得null指针异常。

您有一个ID => @+id/TextboxDurationMins的TextView，这是您要将文本设置为的视图（我假设）。更改布局中的ID或您的活动代码。