Question

我正在努力学习如何制作应用;当我去另一个活动时，我的应用程序崩溃，然后尝试回到我原来的活动。这是我的oncreate：

Bundle bundle;

        if (savedInstanceState != null){
            bundle = savedInstanceState;
        } else {
            bundle = getIntent().getExtras();
        }

        course = bundle.getString("com.naomi.classAlert.course");
        number = bundle.getString("com.naomi.classAlert.number");

我知道当我从活动中点回时，savedInstanceState不为null;但是，当我尝试从捆绑中获取字符串时，为什么会崩溃？我怎样才能解决这个问题？当我继续活动时，我希望课程和数字的价值相同。

这是我的完整代码：

public class classList extends AppCompatActivity {
    ListView listView;
    SimpleCursorAdapter mAdapter;
    static private String[] classes = {"class1","class2"};

    String course;
    String number;

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

        Bundle bundle;

        if (savedInstanceState != null){
            bundle = savedInstanceState;
        } else {
            bundle = getIntent().getExtras();
        }
        course = bundle.getString("com.naomi.classAlert.course");
        number = bundle.getString("com.naomi.classAlert.number");

        Toast.makeText(getApplicationContext(), course+number,
                Toast.LENGTH_LONG).show();
        setContentView(R.layout.activity_class_list);

        listView = (ListView) findViewById(R.id.list);


        // ALL THE DIFFERENT SECTIONS
        String[] values = new String[] {
                course+number+" 001", course+number+" 002", course+number+" 003"
        };

        ArrayAdapter<String> adapter = new ArrayAdapter<String>(this,
                android.R.layout.simple_list_item_2, android.R.id.text1, values);
        listView.setAdapter(adapter);

        // ListView Item Click Listener
        listView.setOnItemClickListener(new AdapterView.OnItemClickListener() {

            @Override
            public void onItemClick(AdapterView<?> parent, View view,
                                    int position, long id) {

                // ListView Clicked item index
                int itemPosition     = position;

                // ListView Clicked item value
                String  itemValue    = (String) listView.getItemAtPosition(position);

                // Show Alert
                Toast.makeText(getApplicationContext(),
                        "Position :"+ itemPosition+"  ListItem : " +itemValue , Toast.LENGTH_LONG)
                        .show();

                Intent intent1 = new Intent(classList.this, pickSection.class);
                intent1.putExtra("com.naomi.classAlert.classList.course",course);
                intent1.putExtra("com.naomi.classAlert.classList.number", number);
                intent1.putExtra("com.naomi.classAlert.classList.section",position+1);
                startActivity(intent1);

            }

        });
    }

    @Override
    public void onSaveInstanceState(Bundle outState){
        super.onSaveInstanceState(outState);
        outState.putString("com.naomi.classAlert.course", course);
        outState.putString("com.naomi.classAlert.number", number);
    }
}

日志：

10-14 00:16:08.411  19529-19529/com.example.naomikoo.classalert D/OpenGLRenderer﹕ Enabling debug mode 0
10-14 00:16:13.325  19529-19532/com.example.naomikoo.classalert D/dalvikvm﹕ GC_CONCURRENT freed 125K, 2% free 9350K/9504K, paused 4ms+3ms, total 29ms
10-14 00:16:15.254  19529-19529/com.example.naomikoo.classalert D/AndroidRuntime﹕ Shutting down VM
10-14 00:16:15.254  19529-19529/com.example.naomikoo.classalert W/dalvikvm﹕ threadid=1: thread exiting with uncaught exception (group=0x41d3c930)
10-14 00:16:15.262  19529-19529/com.example.naomikoo.classalert E/AndroidRuntime﹕ FATAL EXCEPTION: main
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.naomikoo.classalert/com.example.naomikoo.classalert.classList}: java.lang.NullPointerException
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2180)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2230)
            at android.app.ActivityThread.access$600(ActivityThread.java:141)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1234)
            at android.os.Handler.dispatchMessage(Handler.java:99)
            at android.os.Looper.loop(Looper.java:137)
            at android.app.ActivityThread.main(ActivityThread.java:5039)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:511)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
            at dalvik.system.NativeStart.main(Native Method)
     Caused by: java.lang.NullPointerException
            at com.example.naomikoo.classalert.classList.onCreate(classList.java:39)
            at android.app.Activity.performCreate(Activity.java:5104)
            at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1080)
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2144)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2230)
            at android.app.ActivityThread.access$600(ActivityThread.java:141)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1234)
            at android.os.Handler.dispatchMessage(Handler.java:99)
            at android.os.Looper.loop(Looper.java:137)
            at android.app.ActivityThread.main(ActivityThread.java:5039)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:511)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
            at dalvik.system.NativeStart.main(Native Method)

注意：这个问题不重复，我不是试图将活动的结果传递给上一个活动;我正试图保留这个状态，这样当我回去时就会一样。

Answer 1

将您的onSaveInstanceState更改为以下内容：

@Override
public void onSaveInstanceState(Bundle outState){

    outState.putString("com.naomi.classAlert.course", course);
    outState.putString("com.naomi.classAlert.number", number);

    // Always call the superclass so it can save the view hierarchy state
    super.onSaveInstanceState(outState);

}

Answer 2

您的代码中是否有onRestoreInstanceState？不确定你是否完整地发布了它。

@Override
public void onRestoreInstanceState(Bundle savedInstanceState) {
  super.onRestoreInstanceState(savedInstanceState);
course = savedInstanceState.getString("com.naomi.classAlert.course");
number = savedInstanceState.getString("com.naomi.classAlert.number");
Toast.makeText(getApplicationContext(), "Course: " + course + " ", number: " + number, Toast.LENGTH_LONG).show();
}

我还建议定义几个常量并使用它们

public static final String COURSENAME = "com.naomi.classAlert.course"
public static final String COURSENUMBER = "com.naomi.classAlert.number"

尝试测试课程和数字变量的初始化：

String course = "No Class";
String number = "99999";

也试试这个

if (savedInstanceState != null){
        bundle = savedInstanceState;
        course = bundle.getString("com.naomi.classAlert.course");
        number = bundle.getString("com.naomi.classAlert.number");
        }

从您的代码中，我不确定您的课程和编号应该从哪里分配（listview等），但这些需要在某个时刻分配 - 代码当前放置当前值（即使为null进入额外的。

当我尝试从savedInstanceState获取额外内容时，应用程序崩溃时它不为空

2 个答案: