将 TextView 添加到 FrameLayout 中的位置

Question

创建 FrameLayout 后，我​​想将 TextView 添加到特定位置。但是当我尝试添加查看应用程序崩溃。怎么了？

我的代码：

float metrics;
TextView text, text2;
FrameLayout screen;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        screen = (FrameLayout)findViewById(R.id.screen);
        DisplayMetrics metrics = getResources().getDisplayMetrics();
        text = (TextView)findViewById(R.id.text);
        text.setText(""+metrics + "\n"+metrics.widthPixels + "\n"+ metrics.heightPixels);
        int width = metrics.widthPixels;
        int height = metrics.heightPixels;

        FrameLayout.LayoutParams params = new FrameLayout.LayoutParams(100, 100);
        text.setText("XXX");
        screen.addView(text2, 100, 100);

这里是我的 XML 代码：

<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/screen"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="@drawable/a"
    android:orientation="vertical"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context=".MainActivity" >

    <TextView
        android:id="@+id/text"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Large Text"
        android:textAppearance="?android:attr/textAppearanceLarge" />

</FrameLayout>

logcat（只有红色部分）：

java.lang.RuntimeException: Unable to start activity 
ComponentInfo{eu.eurohardware24.tictactoe/eu.eurohardware24.tictactoe.MainActivity}: 
java.lang.NullPointerException
   at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1997)
   at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2018)
   at android.app.ActivityThread.access$500(ActivityThread.java:139)
   at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1124)
   at android.os.Handler.dispatchMessage(Handler.java:99)
   at android.os.Looper.loop(Looper.java:152)
   at android.app.ActivityThread.main(ActivityThread.java:4636)
   at java.lang.reflect.Method.invokeNative(Native Method)
   at java.lang.reflect.Method.invoke(Method.java:491)
   at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:841)
   at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:599)
   at dalvik.system.NativeStart.main(Native Method)

Caused by: java.lang.NullPointerException
   at android.view.ViewGroup.addViewInner(ViewGroup.java:3303)
   at android.view.ViewGroup.addView(ViewGroup.java:3193)
   at android.view.ViewGroup.addView(ViewGroup.java:3163)
   at eu.eurohardware24.tictactoe.MainActivity.onCreate(MainActivity.java:28)
   at android.app.Activity.performCreate(Activity.java:4521)
   at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1072)
   at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1961)
   ... 11 more

Answer 1

您的应用程序崩溃，因为您没有在任何地方初始化 text2。

也许您打算创建一个新的TextView：

text2 = new TextView(this);