在将 LinearLayout 嵌套到 RelativeLayout 时设置 LinearLayout 的 LayoutParams

Question

我想知道为什么在运行程序期间尝试通过代码动态更改嵌套在 RelativeLayout 内的 LinearLayout 宽度和高度时抛出 ClassCastException。

更重要的是，我该如何解决？我是java和android的新手，请尽可能清楚地说明。感谢大家的热心帮助！！！

Java 文件 (MainActivity.java)

import android.app.Activity;
import android.os.Bundle;
import android.view.ViewGroup;
import android.widget.LinearLayout;

public class MainActivity extends Activity {

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

        //To search for the LinearLayout
        LinearLayout ll = (LinearLayout) findViewById(R.id.ll);
        //To set new height and width of Linear Layout which is nested inside the Relative Layout
       //Even changing the class name "View Group" below to "Linear Layout" throw Class Cast Exception as well 
        ll.setLayoutParams(new ViewGroup.LayoutParams(100,200));

}
}

Xml 文件 (activity_main.xml)

    <RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
        xmlns:tools="http://schemas.android.com/tools"
        android:id="@+id/RelativeLayout1"
        android:layout_width="match_parent"
        android:layout_height="match_parent" >

        <LinearLayout
<!-- The width and height here is going to be changed by code while the program run -->
            android:layout_width="100dp"
            android:layout_height="100dp"
            android:background="#FF0000"
            android:orientation="vertical" 
            android:id="@+id/ll">
        </LinearLayout>

    </RelativeLayout>

抛出的异常

03-31 12:55:21.862: D/AndroidRuntime(734): Shutting down VM
03-31 12:55:21.862: W/dalvikvm(734): threadid=1: thread exiting with uncaught exception (group=0x409c01f8)
03-31 12:55:21.872: E/AndroidRuntime(734): FATAL EXCEPTION: main
03-31 12:55:21.872: E/AndroidRuntime(734): java.lang.ClassCastException: android.view.ViewGroup$LayoutParams cannot be cast to android.widget.RelativeLayout$LayoutParams
03-31 12:55:21.872: E/AndroidRuntime(734):  at android.widget.RelativeLayout$DependencyGraph.findRoots(RelativeLayout.java:1317)
03-31 12:55:21.872: E/AndroidRuntime(734):  at android.widget.RelativeLayout$DependencyGraph.getSortedViews(RelativeLayout.java:1264)
03-31 12:55:21.872: E/AndroidRuntime(734):  at android.widget.RelativeLayout.sortChildren(RelativeLayout.java:292)
03-31 12:55:21.872: E/AndroidRuntime(734):  at android.widget.RelativeLayout.onMeasure(RelativeLayout.java:314)
03-31 12:55:21.872: E/AndroidRuntime(734):  at android.view.View.measure(View.java:12723)
03-31 12:55:21.872: E/AndroidRuntime(734):  at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:4698)
03-31 12:55:21.872: E/AndroidRuntime(734):  at android.widget.FrameLayout.onMeasure(FrameLayout.java:293)
03-31 12:55:21.872: E/AndroidRuntime(734):  at android.view.View.measure(View.java:12723)
03-31 12:55:21.872: E/AndroidRuntime(734):  at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:4698)
03-31 12:55:21.872: E/AndroidRuntime(734):  at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1369)
03-31 12:55:21.872: E/AndroidRuntime(734):  at android.widget.LinearLayout.measureVertical(LinearLayout.java:660)
03-31 12:55:21.872: E/AndroidRuntime(734):  at android.widget.LinearLayout.onMeasure(LinearLayout.java:553)
03-31 12:55:21.872: E/AndroidRuntime(734):  at android.view.View.measure(View.java:12723)
03-31 12:55:21.872: E/AndroidRuntime(734):  at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:4698)
03-31 12:55:21.872: E/AndroidRuntime(734):  at android.widget.FrameLayout.onMeasure(FrameLayout.java:293)
03-31 12:55:21.872: E/AndroidRuntime(734):  at com.android.internal.policy.impl.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2092)
03-31 12:55:21.872: E/AndroidRuntime(734):  at android.view.View.measure(View.java:12723)
03-31 12:55:21.872: E/AndroidRuntime(734):  at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1064)
03-31 12:55:21.872: E/AndroidRuntime(734):  at android.view.ViewRootImpl.handleMessage(ViewRootImpl.java:2442)
03-31 12:55:21.872: E/AndroidRuntime(734):  at android.os.Handler.dispatchMessage(Handler.java:99)
03-31 12:55:21.872: E/AndroidRuntime(734):  at android.os.Looper.loop(Looper.java:137)
03-31 12:55:21.872: E/AndroidRuntime(734):  at android.app.ActivityThread.main(ActivityThread.java:4424)
03-31 12:55:21.872: E/AndroidRuntime(734):  at java.lang.reflect.Method.invokeNative(Native Method)
03-31 12:55:21.872: E/AndroidRuntime(734):  at java.lang.reflect.Method.invoke(Method.java:511)
03-31 12:55:21.872: E/AndroidRuntime(734):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784)
03-31 12:55:21.872: E/AndroidRuntime(734):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:551)
03-31 12:55:21.872: E/AndroidRuntime(734):  at dalvik.system.NativeStart.main(Native Method)
03-31 12:55:22.152: I/dalvikvm(734): threadid=3: reacting to signal 3
03-31 12:55:22.162: I/dalvikvm(734): Wrote stack traces to '/data/anr/traces.txt'
03-31 12:55:22.392: I/dalvikvm(734): threadid=3: reacting to signal 3
03-31 12:55:22.402: I/dalvikvm(734): Wrote stack traces to '/data/anr/traces.txt'
03-31 12:55:32.152: I/Process(734): Sending signal. PID: 734 SIG: 9

Answer 1

您必须使用的LayoutParams 的类型由封装您要更改参数的事物的Layout 的类型指定。

例如，如果您想将LayoutParams 更改为您的LinearLayout，您需要检查封装它的Layout 类型是什么。在您的情况下，LinearLayout 被RelativeLayout 封装，因此您需要使用RelativeLayout.LayoutParams 而不是ViewGroup.LayoutParams。

Answer 2

LinearLayout 在您的 xml 文件 (activity_main.xml) 中的 RelativeLayout 内。使用RelativeLayout.LayoutParams 解决此问题。

改变这个：

ll.setLayoutParams(new ViewGroup.LayoutParams(100,200));

到：

ll.setLayoutParams(new RelativeLayout.LayoutParams(100,200));