java - 尝试在空对象引用上调用 ScrollView.removeAllViews()

Question

我正在制作一个简单的提醒应用程序。

当我关闭第二个使用 SharedPreferences 添加新任务的 Activity 时，就会出现问题，并且在此操作之前没有任何任务 (count_of_tasks = 0)。

异常说 findViewById(R.id.scroll_view) 返回 null :

ScrollView scrollView = findViewById(R.id.scroll_view);

但是前面的代码行不返回 null :

LinearLayout parent_layout = findViewById(R.id.parent_layout);

为什么？

如果我重新启动应用程序，布局就不会出现任何问题。

错误:

java.lang.RuntimeException: Unable to resume activity {com.example.memorytasks/com.example.memorytasks.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ScrollView.removeAllViews()' on a null object reference
        at android.app.ActivityThread.performResumeActivity(ActivityThread.java:4014)
        at android.app.ActivityThread.handleResumeActivity(ActivityThread.java:4046)
        at android.app.servertransaction.ResumeActivityItem.execute(ResumeActivityItem.java:51)
        at android.app.servertransaction.TransactionExecutor.executeLifecycleState(TransactionExecutor.java:145)
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:70)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1948)
        at android.os.Handler.dispatchMessage(Handler.java:106)
        at android.os.Looper.loop(Looper.java:214)
        at android.app.ActivityThread.main(ActivityThread.java:7050)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:965)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ScrollView.removeAllViews()' on a null object reference
        at com.example.memorytasks.MainActivity.loadTasks(MainActivity.java:52)
        at com.example.memorytasks.MainActivity.onResume(MainActivity.java:40)
        at android.app.Instrumentation.callActivityOnResume(Instrumentation.java:1412)
        at android.app.Activity.performResume(Activity.java:7572)
        at android.app.ActivityThread.performResumeActivity(ActivityThread.java:4006)
        at android.app.ActivityThread.handleResumeActivity(ActivityThread.java:4046) 
        at android.app.servertransaction.ResumeActivityItem.execute(ResumeActivityItem.java:51) 
        at android.app.servertransaction.TransactionExecutor.executeLifecycleState(TransactionExecutor.java:145) 
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:70) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1948) 
        at android.os.Handler.dispatchMessage(Handler.java:106) 
        at android.os.Looper.loop(Looper.java:214) 
        at android.app.ActivityThread.main(ActivityThread.java:7050) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:965) 

公共(public)类 MainActivity 扩展 AppCompatActivity {

    Toolbar toolbar;
    FloatingActionButton fab;

    @Override
    protected void onCreate (Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        toolbar = findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);
        fab = findViewById(R.id.fab);
    }
    @Override
    protected void onResume () {

        super.onResume();

        loadTasks();
    }
    private void loadTasks () {

        LinearLayout parent_layout = findViewById(R.id.parent_layout);
        ScrollView scrollView = findViewById(R.id.scroll_view);
        LinearLayout tasks_layout = findViewById(R.id.tasks_layout);

        SharedPreferences sPref = getSharedPreferences("tasks", MODE_PRIVATE);
        int count_of_tasks = sPref.getInt("count_of_tasks", 0);

        parent_layout.removeAllViews();
        scrollView.removeAllViews();
        tasks_layout.removeAllViews();

        if (count_of_tasks > 0) {
            parent_layout.addView(scrollView);
            scrollView.addView(tasks_layout);
            for (int i = count_of_tasks; i > 0; i--) {
                /** Task name */
                {
                    TextView t = new TextView(this);
                    t.setId(i);
                    t.setText(sPref.getString("taskName" + i,""));
                    t.setTextSize(TypedValue.COMPLEX_UNIT_SP,32);
                    t.setBackgroundColor(Color.argb(50,56,142,60));
                    t.setPadding(20, 20, 20, 20);
                    LinearLayout.LayoutParams params = new LinearLayout.LayoutParams(LinearLayout.LayoutParams.MATCH_PARENT, LinearLayout.LayoutParams.WRAP_CONTENT);
                    params.setMargins(20,10,20,10);
                    t.setLayoutParams(params);
                    tasks_layout.addView(t);
                }
                /** Extra information */
                {
                    TextView t = new TextView(this);
                    t.setId((i + 1) * i);

                    String task = sPref.getString("task" + i,"");
                    if (task.equals("")) {
                        task += "\n\n";
                    }
                    task += sPref.getString("date" + i,"");

                    t.setText(task);
                    t.setTextSize(TypedValue.COMPLEX_UNIT_SP,20);
                    t.setBackgroundColor(Color.argb(20,56,142,60));
                    t.setPadding(20, 20, 20, 20);
                    LinearLayout.LayoutParams params = new LinearLayout.LayoutParams(LinearLayout.LayoutParams.MATCH_PARENT, LinearLayout.LayoutParams.WRAP_CONTENT);
                    params.setMargins(20,-10,20,10);
                    t.setLayoutParams(params);
                    tasks_layout.addView(t);
                }}}
        else {
            TextView task1 = new TextView(this);
            task1.setId(R.id.task1);
            task1.setText("Здесь пока ничего нет.");
            task1.setTextSize(TypedValue.COMPLEX_UNIT_SP,32);
            task1.setGravity(Gravity.CENTER);
            task1.setLayoutParams(new LinearLayout.LayoutParams(LinearLayout.LayoutParams.MATCH_PARENT, LinearLayout.LayoutParams.MATCH_PARENT));

            parent_layout.addView(task1);
        }
    }
    public void fabOnClick (View v) {

        startActivity(new Intent(MainActivity.this, NewTaskActivity.class));
    }
    @Override
    public boolean onCreateOptionsMenu (Menu menu) {

        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.menu_main, menu);
        return true;
    }
    @SuppressLint("ApplySharedPref")
    @Override
    public boolean onOptionsItemSelected (MenuItem item) {

        if (item.getItemId() == R.id.action_settings) {
            getSharedPreferences("tasks", MODE_PRIVATE).edit().clear().commit();
            onResume();
            return true;
        }
        return super.onOptionsItemSelected(item);
    }
}

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<android.support.design.widget.CoordinatorLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <android.support.design.widget.AppBarLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:theme="@style/AppTheme.AppBarOverlay">

        <android.support.v7.widget.Toolbar
            android:id="@+id/toolbar"
            android:layout_width="match_parent"
            android:layout_height="?attr/actionBarSize"
            android:background="#388E3C"
            app:popupTheme="@style/AppTheme.PopupOverlay"
            app:title="Memory Tasks" />

    </android.support.design.widget.AppBarLayout>

    <include layout="@layout/content_main" />

    <android.support.design.widget.FloatingActionButton
        android:id="@+id/fab"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="bottom|end"
        android:layout_margin="@dimen/fab_margin"
        android:onClick="fabOnClick"
        app:backgroundTint="#205020"
        app:srcCompat="@android:drawable/ic_input_add" />

</android.support.design.widget.CoordinatorLayout>

context_main.xml

?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/parent_layout"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    app:layout_behavior="@string/appbar_scrolling_view_behavior"
    tools:context=".MainActivity"
    tools:layout_editor_absoluteY="56dp"
    tools:showIn="@layout/activity_main">

    <ScrollView
        android:id="@+id/scroll_view"
        android:layout_width="match_parent"
        android:layout_height="match_parent">

        <LinearLayout
            android:id="@+id/tasks_layout"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:orientation="vertical" >
        </LinearLayout>
    </ScrollView>

</LinearLayout>

Answer 1

问题是，如果您的 count_of_tasks 为 0，则 ScrollView 将从内容 View 中删除并且永远不会放回原处，因此下一个 findViewById 调用无法找到它。您应该将该 findViewById 调用移至 onCreate 方法并将其存储为成员变量，类似于您对 fab 所做的操作。

这就是为什么将 setContentView 移动到 onResume 似乎可以修复它 - 它每次都会将其恢复。存储它将使您能够在 onCreate 中保留 setContentView 调用。