我必须为实习开发一个应用程序，我正在使用 AndroidStudio。
现在我只是在尝试一些东西，我正在开发一个打屁股计数器应用程序(是的，我知道)。
我知道那里已经有回应了: What is a NullPointerException, and how do I fix it? 以及许多其他与我类似的问题。 事实上，我理解错误，但我无法在我的代码中找到它。
(我没有放 xml 文件，因为我认为它没有用，如果你想看的话，请告诉我。)
如果这篇文章难以阅读，请见谅。第一次在论坛上问问题。 提前感谢那些有勇气阅读它并帮助我的人。
所以这是错误:
--------- beginning of crash
06-02 18:05:31.515 2680-2680/com.example.florian.fessee E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.florian.fessee, PID: 2680
java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.florian.fessee/com.example.florian.fessee.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String android.content.Context.getPackageName()' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2236)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2387)
at android.app.ActivityThread.access$800(ActivityThread.java:151)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:135)
at android.app.ActivityThread.main(ActivityThread.java:5254)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String android.content.Context.getPackageName()' on a null object reference
at android.content.ContextWrapper.getPackageName(ContextWrapper.java:132)
at android.preference.PreferenceManager.getDefaultSharedPreferencesName(PreferenceManager.java:374)
at android.preference.PreferenceManager.getDefaultSharedPreferences(PreferenceManager.java:369)
at com.example.florian.fessee.MainActivity.<init>(MainActivity.java:23)
at java.lang.reflect.Constructor.newInstance(Native Method)
at java.lang.Class.newInstance(Class.java:1606)
at android.app.Instrumentation.newActivity(Instrumentation.java:1066)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2226)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2387)Â
at android.app.ActivityThread.access$800(ActivityThread.java:151)Â
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303)Â
at android.os.Handler.dispatchMessage(Handler.java:102)Â
at android.os.Looper.loop(Looper.java:135)Â
at android.app.ActivityThread.main(ActivityThread.java:5254)Â
at java.lang.reflect.Method.invoke(Native Method)Â
at java.lang.reflect.Method.invoke(Method.java:372)Â
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)Â
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)Â
06-02 18:11:28.569 3101-3101/com.example.florian.fessee D/AndroidRuntime: Shutting down VM
06-02 18:11:28.570 3101-3101/com.example.florian.fessee E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.florian.fessee, PID: 3101
java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.florian.fessee/com.example.florian.fessee.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String android.content.Context.getPackageName()' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2236)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2387)
at android.app.ActivityThread.access$800(ActivityThread.java:151)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:135)
at android.app.ActivityThread.main(ActivityThread.java:5254)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String android.content.Context.getPackageName()' on a null object reference
at android.content.ContextWrapper.getPackageName(ContextWrapper.java:132)
at android.preference.PreferenceManager.getDefaultSharedPreferencesName(PreferenceManager.java:374)
at android.preference.PreferenceManager.getDefaultSharedPreferences(PreferenceManager.java:369)
at com.example.florian.fessee.MainActivity.<init>(MainActivity.java:23)
at java.lang.reflect.Constructor.newInstance(Native Method)
at java.lang.Class.newInstance(Class.java:1606)
at android.app.Instrumentation.newActivity(Instrumentation.java:1066)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2226)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2387)Â
at android.app.ActivityThread.access$800(ActivityThread.java:151)Â
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303)Â
at android.os.Handler.dispatchMessage(Handler.java:102)Â
at android.os.Looper.loop(Looper.java:135)Â
at android.app.ActivityThread.main(ActivityThread.java:5254)Â
at java.lang.reflect.Method.invoke(Native Method)Â
at java.lang.reflect.Method.invoke(Method.java:372)Â
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)Â
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)Â
06-02 18:16:53.409 3339-3339/com.example.florian.fessee D/AndroidRuntime: Shutting down VM
06-02 18:16:53.435 3339-3339/com.example.florian.fessee E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.florian.fessee, PID: 3339
java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.florian.fessee/com.example.florian.fessee.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String android.content.Context.getPackageName()' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2236)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2387)
at android.app.ActivityThread.access$800(ActivityThread.java:151)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:135)
at android.app.ActivityThread.main(ActivityThread.java:5254)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String android.content.Context.getPackageName()' on a null object reference
at android.content.ContextWrapper.getPackageName(ContextWrapper.java:132)
at android.preference.PreferenceManager.getDefaultSharedPreferencesName(PreferenceManager.java:374)
at android.preference.PreferenceManager.getDefaultSharedPreferences(PreferenceManager.java:369)
at com.example.florian.fessee.MainActivity.<init>(MainActivity.java:23)
at java.lang.reflect.Constructor.newInstance(Native Method)
at java.lang.Class.newInstance(Class.java:1606)
at android.app.Instrumentation.newActivity(Instrumentation.java:1066)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2226)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2387)Â
at android.app.ActivityThread.access$800(ActivityThread.java:151)Â
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303)Â
at android.os.Handler.dispatchMessage(Handler.java:102)Â
at android.os.Looper.loop(Looper.java:135)Â
at android.app.ActivityThread.main(ActivityThread.java:5254)Â
at java.lang.reflect.Method.invoke(Native Method)Â
at java.lang.reflect.Method.invoke(Method.java:372)Â
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)Â
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)Â
06-02 18:19:42.932 3515-3515/com.example.florian.fessee D/AndroidRuntime: Shutting down VM
06-02 18:19:42.935 3515-3515/com.example.florian.fessee E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.florian.fessee, PID: 3515
java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.florian.fessee/com.example.florian.fessee.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String android.content.Context.getPackageName()' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2236)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2387)
at android.app.ActivityThread.access$800(ActivityThread.java:151)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:135)
at android.app.ActivityThread.main(ActivityThread.java:5254)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String android.content.Context.getPackageName()' on a null object reference
at android.content.ContextWrapper.getPackageName(ContextWrapper.java:132)
at android.preference.PreferenceManager.getDefaultSharedPreferencesName(PreferenceManager.java:374)
at android.preference.PreferenceManager.getDefaultSharedPreferences(PreferenceManager.java:369)
at com.example.florian.fessee.MainActivity.<init>(MainActivity.java:23)
at java.lang.reflect.Constructor.newInstance(Native Method)
at java.lang.Class.newInstance(Class.java:1606)
at android.app.Instrumentation.newActivity(Instrumentation.java:1066)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2226)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2387)Â
at android.app.ActivityThread.access$800(ActivityThread.java:151)Â
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303)Â
at android.os.Handler.dispatchMessage(Handler.java:102)Â
at android.os.Looper.loop(Looper.java:135)Â
at android.app.ActivityThread.main(ActivityThread.java:5254)Â
at java.lang.reflect.Method.invoke(Native Method)Â
at java.lang.reflect.Method.invoke(Method.java:372)Â
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)Â
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)Â
这是我的代码:
package com.example.florian.fessee;
import android.content.SharedPreferences;
import android.preference.PreferenceManager;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;
import android.widget.Toast;
public class MainActivity extends AppCompatActivity {
private int compt = 0;
private Button b1 = null;
private Button b5 = null;
private Button b01 = null;
private Button b05 = null;
private Button braz = null;
private TextView t = null;
private String COMPT = "comptfess";
SharedPreferences preferences = PreferenceManager.getDefaultSharedPreferences(this);
SharedPreferences.Editor editor = preferences.edit();
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
b1 = (Button) findViewById(R.id.button1);
b5 = (Button) findViewById(R.id.button5);
b01 = (Button) findViewById(R.id.button01);
b05 = (Button) findViewById(R.id.button05);
braz = (Button) findViewById(R.id.buttonraz);
t = (TextView) findViewById(R.id.textView);
compt = preferences.getInt(COMPT, 0);
b1.setOnClickListener(b1Listener);
b5.setOnClickListener(b5Listener);
b01.setOnClickListener(b01Listener);
b05.setOnClickListener(b05Listener);
braz.setOnClickListener(brazListener);
if (compt == 1) {
t.setText("Je dois 1 fessée à Sophie.");
}
else if (compt!=0){
t.setText("Je dois " + compt + "fessées à Sophie.");
}
}
private View.OnClickListener b1Listener = new View.OnClickListener() {
@Override
public void onClick(View v) {
compt +=1;
editor.putInt(COMPT,compt);
editor.commit();
if (compt == 1) {
t.setText("Je dois 1 fessée à Sophie.");
}
else if (compt!=0){
t.setText("Je dois " + compt + "fessées à Sophie.");
}
}
};
private View.OnClickListener b5Listener = new View.OnClickListener() {
@Override
public void onClick(View v) {
compt +=5;
editor.putInt(COMPT,compt);
editor.commit();
if (compt == 1) {
t.setText("Je dois 1 fessée à Sophie.");
}
else if (compt!=0){
t.setText("Je dois " + compt + "fessées à Sophie.");
}
}
};
private View.OnClickListener b01Listener = new View.OnClickListener() {
@Override
public void onClick(View v) {
if(compt!=0){
compt -=1;
editor.putInt(COMPT,compt);
editor.commit();
if (compt==0){
t.setText("Je ne dois plus de fessées å Sophie");
}
else if (compt == 1) {
t.setText("Je dois 1 fessée à Sophie.");
}
else{
t.setText("Je dois " + compt + "fessées à Sophie.");
}
}
else {
Toast.makeText(MainActivity.this, "Un nombre de fessées doit être positif.", Toast.LENGTH_SHORT).show();
}
}
};
private View.OnClickListener b05Listener = new View.OnClickListener() {
@Override
public void onClick(View v) {
if(compt>4){
compt -=5;
editor.putInt(COMPT,compt);
editor.commit();
if (compt==0){
t.setText("Je ne dois plus de fessées å Sophie.");
}
else if (compt == 1) {
t.setText("Je dois 1 fessée à Sophie.");
}
else{
t.setText("Je dois " + compt + "fessées à Sophie.");
}
}
else {
Toast.makeText(MainActivity.this, "Un nombre de fessées doit être positif.", Toast.LENGTH_SHORT).show();
}
}
};
private View.OnClickListener brazListener = new View.OnClickListener() {
@Override
public void onClick(View v) {
compt = 0;
editor.putInt(COMPT,compt);
editor.commit();
t.setText("Je ne dois plus de fessées à Sophie.");
}
};
}
最佳答案
这是你的问题:
SharedPreferences preferences = PreferenceManager.getDefaultSharedPreferences(this);
SharedPreferences.Editor editor = preferences.edit();
您不能在创建 Activity 之前初始化 SharedPreferences。像这样分开你的声明和实例化:
...
SharedPreferences preferences;
SharedPreferences.Editor editor;
...
@Override
public void onCreate(Bundle savedInstanceState){
...
preferences = PreferenceManager.getDefaultSharedPreferences(this);
editor = preferences.edit();
...
}
关于java - 尝试获取 Activity 的 DefaultSharedPreferences 时出现 NullPointerException，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/37598663/