Android：使用 SharedPreferences 时出现 NullPointerException答案

【问题标题】：Android: NullPointerException Working With SharedPreferencesAndroid：使用 SharedPreferences 时出现 NullPointerException
【发布时间】：2012-09-28 21:28:09
【问题描述】：

使用 SharedPreferences，此活动在启动时崩溃。首先我会发布活动代码，然后我会发布我的 LogCat。非常感谢你们，你们总是这么大的帮助！ :-)

活动代码；

package com.creativecoders.gymbuddy;

import com.creativecoders.gymbuddy.R;

import android.app.Activity;
import android.content.Context;
import android.content.SharedPreferences;
import android.os.Bundle;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.EditText;
import android.widget.TextView;

public class Benchmark extends Activity {

public static final String GB_PREFERENCES_BENCH = "Bench";
public static final String GB_PREFERENCES_FLIES = "Flies";

SharedPreferences settings = getSharedPreferences("gBValues", 
     Context.MODE_PRIVATE);

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_benchmark);
    }

public void onStart() {
    super.onStart();

    findViewById(R.id.button5).setOnClickListener(new handleButton5());

}

class handleButton5 implements OnClickListener {
    public void onClick(View v) {

        EditText editText1 = (EditText)findViewById(R.id.editText1);
        String sWeight = editText1.getText().toString();
        final double dWeight = Double.parseDouble(sWeight);

        EditText editText2 = (EditText)findViewById(R.id.editText2);
        String sPush = editText2.getText().toString();
        final double dPush = Double.parseDouble(sPush);

        EditText editText3 = (EditText)findViewById(R.id.editText3);
        String sSit = editText3.getText().toString();
        final double dSit = Double.parseDouble(sSit);

        EditText editText4 = (EditText)findViewById(R.id.editText4);
        String sPull = editText4.getText().toString();
        final double dPull = Double.parseDouble(sPull);

        double dBench = (((Math.floor(dWeight*.0664))*10)-10)+dPush;
        double dFlies = (Math.floor(((Math.floor(dBench*.6)/10)*10)));

        int iBench = (int)dBench;
        int iFlies = (int)dFlies;

        String sBench = "" + iBench;
        String sFlies = "" + iFlies;

        SharedPreferences.Editor editor1 = settings.edit();
        editor1.putString(GB_PREFERENCES_BENCH, sBench);
        editor1.commit();

        SharedPreferences.Editor editor2 = settings.edit();
        editor2.putString(GB_PREFERENCES_FLIES, sFlies);
        editor2.commit();

        TextView TextView1 = (TextView)findViewById(R.id.textView1);
        TextView1.setText(String.valueOf("Bench Press "+ iBench +" lbs"));

        TextView TextView2 = (TextView)findViewById(R.id.textView2);
        TextView2.setText(String.valueOf("Bar Curls "+ iCurls +" lbs"));
        }
    }

}

这是我的 LogCat；

09-28 21:18:52.636: E/AndroidRuntime(686): FATAL EXCEPTION: main
09-28 21:18:52.636: E/AndroidRuntime(686): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.creativecoders.gymbuddy/com.creativecoders.gymbuddy.Benchmark}: java.lang.NullPointerException
09-28 21:18:52.636: E/AndroidRuntime(686):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1879)
09-28 21:18:52.636: E/AndroidRuntime(686):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1980)
09-28 21:18:52.636: E/AndroidRuntime(686):  at android.app.ActivityThread.access$600(ActivityThread.java:122)
09-28 21:18:52.636: E/AndroidRuntime(686):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1146)
09-28 21:18:52.636: E/AndroidRuntime(686):  at android.os.Handler.dispatchMessage(Handler.java:99)
09-28 21:18:52.636: E/AndroidRuntime(686):  at android.os.Looper.loop(Looper.java:137)
09-28 21:18:52.636: E/AndroidRuntime(686):  at android.app.ActivityThread.main(ActivityThread.java:4340)
09-28 21:18:52.636: E/AndroidRuntime(686):  at java.lang.reflect.Method.invokeNative(Native Method)
09-28 21:18:52.636: E/AndroidRuntime(686):  at java.lang.reflect.Method.invoke(Method.java:511)
09-28 21:18:52.636: E/AndroidRuntime(686):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784)
09-28 21:18:52.636: E/AndroidRuntime(686):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:551)
09-28 21:18:52.636: E/AndroidRuntime(686):  at dalvik.system.NativeStart.main(Native Method)
09-28 21:18:52.636: E/AndroidRuntime(686): Caused by: java.lang.NullPointerException
09-28 21:18:52.636: E/AndroidRuntime(686):  at android.content.ContextWrapper.getSharedPreferences(ContextWrapper.java:153)
09-28 21:18:52.636: E/AndroidRuntime(686):  at com.creativecoders.gymbuddy.Benchmark.<init>(Benchmark.java:35)
09-28 21:18:52.636: E/AndroidRuntime(686):  at java.lang.Class.newInstanceImpl(Native Method)
09-28 21:18:52.636: E/AndroidRuntime(686):  at java.lang.Class.newInstance(Class.java:1319)
09-28 21:18:52.636: E/AndroidRuntime(686):  at android.app.Instrumentation.newActivity(Instrumentation.java:1023)
09-28 21:18:52.636: E/AndroidRuntime(686):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1870)
09-28 21:18:52.636: E/AndroidRuntime(686):  ... 11 more

【问题讨论】：

标签： java android sharedpreferences

【解决方案1】：

您必须在调用 onCreate 后访问共享首选项。否则上下文将为 null :) 这就是为什么你得到一个空指针异常

移动这一行：

SharedPreferences settings = getSharedPreferences("gBValues", 
     Context.MODE_PRIVATE);

在 onCreate() 中

【讨论】：

你先生.. 值得奖励；接受的答案听起来如何？ :)（当然需要 6 分钟）

【解决方案2】：

将设置实例变量的实例化移动到您的 onCreate 方法中。在 super.onCreate 调用之后。

你现在的做法是设置为 null；

【讨论】：

【解决方案3】：

public class MainActivity extends Activity{

SharedPreferences sharedpreferences;    

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

sharedpreferences = getSharedPreferences("preferences", Context.MODE_PRIVATE);  

            editor.putBoolean("sharedpreferences", true);
            editor.commit();

// your code ...

}

// your code ...
}

【讨论】：