Android：NullPointerException使用SharedPreferences

Question

使用SharedPreferences，此活動在啟動時崩潰。 首先，我將發布活動代碼，然后將我的LogCat發布。 非常感謝你們，你們總是有很大幫助！ :-)

活動代碼；

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"));
        }
    }

}

這是我的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

Answer 1

您必須在調用onCreate之后訪問共享首選項。 否則上下文將為null :)這就是為什么您會得到null指針異常的原因

移動這條線：

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

在onCreate（）中

Answer 2

將settings實例變量的實例化到您的onCreate方法中。 在super.onCreate調用之后。

您現在的操作方式是將其設置為null。

Answer 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 ...
}