Question

我正在创建一个应用程序，您可以在其中添加列表中的项目。还会保存列表。列表中的添加项目正在运行，我认为保存部分存在问题 import java.util.ArrayList;

    import java.util.ArrayList;

import android.os.Bundle;
import android.view.KeyEvent;
import android.view.View;
import android.view.View.OnKeyListener;
import android.widget.ArrayAdapter;
import android.widget.EditText;
import android.widget.ListView;
import android.app.Activity;
import android.content.SharedPreferences;

public class MainActivity extends Activity {
    public static String file = "file";
    SharedPreferences data= getSharedPreferences(file, 0);
    public ArrayList<String> ListItems = new ArrayList<String>();
    public int size = ListItems.size();
    public SharedPreferences.Editor editor = data.edit();

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

        ListView myListView = (ListView) findViewById(R.id.listView1);


        final EditText myEditText = (EditText) findViewById(R.id.editText1);

        final ArrayAdapter<String> aa;
        aa = new ArrayAdapter<String>(this,
                android.R.layout.simple_list_item_1, ListItems);
        myListView.setAdapter(aa);

        myEditText.setOnKeyListener(new OnKeyListener() {
            public boolean onKey(View v, int keyCode, KeyEvent event) {
                if (event.getAction() == KeyEvent.ACTION_DOWN)
                    if (keyCode == KeyEvent.KEYCODE_ENTER) {
                        ListItems.add(0, myEditText.getText().toString());
                        aa.notifyDataSetChanged();
                        myEditText.setText("");

                        editor.putInt("list_size", size);

                        for (int i = 0; i < size; i++) {
                            editor.putString("list_" + i,
                                    (String) ListItems.get(i));
                        }
                        editor.commit();

                        return true;
                    }
                return false;
            }
        });

    }

    @Override
    protected void onResume() {
        // TODO Auto-generated method stub
        super.onResume();
        int size1 = (data.getInt("list_size", 0));
        for (int i = 0; i < size1; i++) {
            ListItems.set(i,
                    (data.getString("list_" + i, "error loading this item")));

        }
        editor.commit();
    }

}

错误日志：

05-07 17:30:47.678: E/AndroidRuntime(2162): FATAL EXCEPTION: main
05-07 17:30:47.678: E/AndroidRuntime(2162): Process: com.paramdevelopment.pizzamaking, PID: 2162
05-07 17:30:47.678: E/AndroidRuntime(2162): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.paramdevelopment.pizzamaking/com.paramdevelopment.pizzamaking.MainActivity}: java.lang.NullPointerException
05-07 17:30:47.678: E/AndroidRuntime(2162):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2121)
05-07 17:30:47.678: E/AndroidRuntime(2162):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
05-07 17:30:47.678: E/AndroidRuntime(2162):     at android.app.ActivityThread.access$800(ActivityThread.java:135)
05-07 17:30:47.678: E/AndroidRuntime(2162):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
05-07 17:30:47.678: E/AndroidRuntime(2162):     at android.os.Handler.dispatchMessage(Handler.java:102)
05-07 17:30:47.678: E/AndroidRuntime(2162):     at android.os.Looper.loop(Looper.java:136)
05-07 17:30:47.678: E/AndroidRuntime(2162):     at android.app.ActivityThread.main(ActivityThread.java:5017)
05-07 17:30:47.678: E/AndroidRuntime(2162):     at java.lang.reflect.Method.invokeNative(Native Method)
05-07 17:30:47.678: E/AndroidRuntime(2162):     at java.lang.reflect.Method.invoke(Method.java:515)
05-07 17:30:47.678: E/AndroidRuntime(2162):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
05-07 17:30:47.678: E/AndroidRuntime(2162):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
05-07 17:30:47.678: E/AndroidRuntime(2162):     at dalvik.system.NativeStart.main(Native Method)
05-07 17:30:47.678: E/AndroidRuntime(2162): Caused by: java.lang.NullPointerException
05-07 17:30:47.678: E/AndroidRuntime(2162):     at android.content.ContextWrapper.getSharedPreferences(ContextWrapper.java:173)
05-07 17:30:47.678: E/AndroidRuntime(2162):     at com.paramdevelopment.pizzamaking.MainActivity.<init>(MainActivity.java:16)
05-07 17:30:47.678: E/AndroidRuntime(2162):     at java.lang.Class.newInstanceImpl(Native Method)
05-07 17:30:47.678: E/AndroidRuntime(2162):     at java.lang.Class.newInstance(Class.java:1208)
05-07 17:30:47.678: E/AndroidRuntime(2162):     at android.app.Instrumentation.newActivity(Instrumentation.java:1061)
05-07 17:30:47.678: E/AndroidRuntime(2162):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2112)
05-07 17:30:47.678: E/AndroidRuntime(2162):     ... 11 more

Answer 1

您正在尝试使用未初始化的SharedPrefferences数据，并在后续步骤中使用它们。

关键线是：

SharedPreferences data;

您已声明数据但数据未初始化，并且在您尝试使用它的代码中（数据值为空），这会导致NullPointerException。

此行产生错误：

public SharedPreferences.Editor editor = data.edit();

您必须在使用它之前初始化变量，就像在onCreate方法中一样。

编辑1 :(基于OP编辑和评论）

正如我之前所说，SharedPrefferences为空，你有问题。您必须正确初始化SharedPrefferences。

您正在尝试使用编辑器而未初始化SharedPrefferences。您可以从许多地方了解如何使用SharedPrefferences，但很快您就可以查看here。

Answer 2

您可以在将活动用作上下文之前过早访问SharedPreferences。将共享首选项init移至onCreate()。

这个java android代码中的错误是什么

2 个答案: