android - NullPointer 异常将 Intent 字符串保存为实例状态 Android

Question

我遇到了一些 saveInstanceState 问题。

一个 Activity 通过 Intent 向另一个 Activity 发送一个字符串值，这非常有效。现在在第二个 Activity 中，我选择一个行项目以进入第三个 Activity ，这也有效。但是如果我使用操作栏后退按钮(到第二个 Activity )，我会得到一个 NullPointerException。

原因当然是无法加载第一个 Activity 的 Intent 字符串。因此，在第二个 Activity 中，我尝试将此 Intent 字符串值保存为实例状态，并再次在 oncreate 方法中获取此值。我谷歌搜索了很多，我尝试了很多。但是空指针不会消失。

如果有一点提示就太好了。

我的第二个activity代码:

    package de.cityknight.app;

import android.app.Activity;
import android.content.Context;
import android.content.Intent;
import android.content.SharedPreferences;
import android.support.v7.app.ActionBarActivity;
import android.os.Bundle;
import android.util.Log;
import android.view.Gravity;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.AdapterView;
import android.widget.AdapterView.OnItemClickListener;
import android.widget.ListView;
import android.widget.TextView;
import android.widget.Toast;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;


public class RouteChooseActivity extends ActionBarActivity implements OnItemClickListener {

    String citySave;
    String[] titelRoute;
    String[] descRoute;
    Integer[] bildRoute;

    ListView listView;
    List<RowItem> rowItems;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_route_choose);
        Bundle b = getIntent().getExtras();
        if (savedInstanceState != null) {
                savedInstanceState.getString("stadt");
         }
         else {
            b.getString("stadt");
         }
            TextView headingRoute = (TextView) findViewById(R.id.routeTitel);
            headingRoute.setText(String.format(getResources().getString(R.string.route_text)) + " " + b.getString("stadt") + " aus:");
            if (b.getString("stadt").equals("Passau")) {
                titelRoute = new String[]{"Die Altstadt", "Universität", "Essen und Trinken"};
                descRoute = new String[]{"Die Altstadt Passaus bietet viele Attraktion: Rathaus, Dom und die vielen kleinen Gassen.",
                        "Deutschlands schönster Campus.", "Kulinarische Köstlichkeiten in Passau"};
                bildRoute = new Integer[]{R.drawable.augsburg, R.drawable.bamberg, R.drawable.passau};
            } else if (b.getString("stadt").equals("Augsburg")) {
                titelRoute = new String[]{"Augsburg1", "Augsburg2", "Augsburg3"};
                descRoute = new String[]{"Bla bla", "Mehr Bla", "Blubb"};
                bildRoute = new Integer[]{R.drawable.augsburg, R.drawable.bamberg, R.drawable.passau};
            } else if (b.getString("stadt").equals("Bamberg")) {
                titelRoute = new String[]{"Bamberg", "Bamberg2", "Bamberg3"};
                descRoute = new String[]{"Bla bla", "Mehr Bla", "Blubb"};
                bildRoute = new Integer[]{R.drawable.augsburg, R.drawable.bamberg, R.drawable.passau};
            } else {
                try {
                    TextView errorMessage = (TextView) findViewById(R.id.routeTitel);
                    errorMessage.setText(R.string.errorroute);
                } catch (NullPointerException e) {
                    Context context = getApplicationContext();
                    CharSequence text = getResources().getString(R.string.errorroute);
                    int duration = Toast.LENGTH_LONG;
                    Toast toast = Toast.makeText(context, text, duration);
                    toast.setGravity(Gravity.BOTTOM | Gravity.CENTER_HORIZONTAL, 0, 0);
                    toast.show();
                }

            }

            try {
                rowItems = new ArrayList<RowItem>();
                for (int i = 0; i < titelRoute.length; i++) {
                    RowItem item = new RowItem(bildRoute[i], titelRoute[i], descRoute[i]);
                    rowItems.add(item);
                }
                Comparator<RowItem> sortRoutes = new Comparator<RowItem>() {
                    @Override
                    public int compare(RowItem rowItem1, RowItem rowItem2) {
                        return rowItem1.getTitle().compareTo(rowItem2.getTitle());
                    }
                };
                Collections.sort(rowItems, sortRoutes);
                listView = (ListView) findViewById(R.id.listRoute);
                CityListViewAdapter adapter = new CityListViewAdapter(this, R.layout.row_items, rowItems);
                listView.setAdapter(adapter);
                listView.setOnItemClickListener(this);
            } catch (ArrayIndexOutOfBoundsException rowItems) {
                Context context = getApplicationContext();
                CharSequence text = getResources().getString(R.string.ressourcefailed);
                int duration = Toast.LENGTH_LONG;
                Toast toast = Toast.makeText(context, text, duration);
                toast.setGravity(Gravity.BOTTOM | Gravity.CENTER_HORIZONTAL, 0, 0);
                toast.show();
            }

    }

    @Override
    public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
        Intent intent = new Intent();
        intent.setClass(RouteChooseActivity.this, RouteView.class);
        RowItem item = (RowItem) parent.getItemAtPosition(position);
        Bundle b = new Bundle();
        b.putString("route", item.getTitle().toString());
        intent.putExtras(b);
        startActivity(intent);
    }

    protected void onSaveInstanceState(Bundle savedInstanceState) {
        Bundle b = getIntent().getExtras();
        b.getString("stadt");
        savedInstanceState.putString("stadt", citySave);
        super.onSaveInstanceState(savedInstanceState);
    }

    @Override
    public void onRestoreInstanceState(Bundle savedInstanceState) {
        super.onRestoreInstanceState(savedInstanceState);
        // Restore UI state from the savedInstanceState.
        // This bundle has also been passed to onCreate.
        savedInstanceState.getString(citySave);
        Log.i("debug", "saved data: " + citySave);
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.route, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        switch (item.getItemId()) {
            case R.id.action_stadt:
                setContentView(R.layout.activity_stadt);
                Intent stadt = new Intent(RouteChooseActivity.this, StadtActivity.class);
                startActivity(stadt);
                return true;
            case R.id.action_help:
                setContentView(R.layout.activity_help);
                Intent help = new Intent(RouteChooseActivity.this, Help.class);
                startActivity(help);
                return true;
            case R.id.action_exit:
                moveTaskToBack(true);
                System.exit(0);
                return true;
            default:
                return super.onOptionsItemSelected(item);
        }
    }
    }

Answer 1

1. 这将引发 NullPointerException :

   savedInstanceState.getString(citySave);
   Log.i("debug", "saved data: " + citySave);
   

   应该是:

   citySave = savedInstanceState.getString("stadt");
   Log.i("debug", "saved data: " + citySave);
   

2. 在 onSaveInstanceState 中，您应该在向 savedInstanceState 添加字符串之前调用 super.onSaveInstanceState(savedInstanceState);(否则它将被删除出...)

3. 整个 savedInstanceState 并不真正适合这个目的。 Android 可以在您进行第 3 个 Activity 时将其删除! (这取决于系统是否认为它需要内存并因此完全销毁第二个 Activity 及其 savedInstanceState)。

   做事的好方法是覆盖你的第三个 Activity 的 onBackPressed() 方法来触发一个 Intent 来启动你的第二个 Activity ，其值放在 Intent 的“额外”包中.

   通过这种方法，您的问题应该得到解决。