意外转换为 ViewGroup：布局标签是 RelativeLayout

Question

我正在这个 page 上做“创建第二个活动”教程 我已经小心翼翼地完成了所有事情，但我遇到了一个错误。我不明白为什么会出现错误，尤其是当我只是复制/粘贴他们告诉我复制/粘贴的代码时！

以下代码行：

ViewGroup layout = (VeiwGroup)  findViewById(R.id.activity_display_message);

给我这个错误：

unexpected cast to ViewGroup: layout tag was RelativeLayout.

嗯，我不知道那是在说什么。我知道 xml 文件是一个相对布局。但我不知道为什么这是个问题，也不知道问题到底出在哪里。

Answer 1

将您的 ViewGroup 更改为 RelativeLayout

当您调用 ViewGroup 类时，您会丢失 RelativeLayout 方法的一些方法，因为 ViewGroup 是父类，而相对布局只是它与其他类的高级版本 这是两个类之间的区别

@RemoteView
public class RelativeLayout extends ViewGroup {
public static final int ABOVE = 2;
public static final int ALIGN_BASELINE = 4;
public static final int ALIGN_BOTTOM = 8;
public static final int ALIGN_END = 19;
public static final int ALIGN_LEFT = 5;
public static final int ALIGN_PARENT_BOTTOM = 12;
public static final int ALIGN_PARENT_END = 21;
public static final int ALIGN_PARENT_LEFT = 9;
public static final int ALIGN_PARENT_RIGHT = 11;
public static final int ALIGN_PARENT_START = 20;
public static final int ALIGN_PARENT_TOP = 10;
public static final int ALIGN_RIGHT = 7;
public static final int ALIGN_START = 18;
public static final int ALIGN_TOP = 6;
public static final int BELOW = 3;
public static final int CENTER_HORIZONTAL = 14;
public static final int CENTER_IN_PARENT = 13;
public static final int CENTER_VERTICAL = 15;
public static final int END_OF = 17;
public static final int LEFT_OF = 0;
public static final int RIGHT_OF = 1;
public static final int START_OF = 16;
public static final int TRUE = -1;

public RelativeLayout(Context context) {
    super((Context)null, (AttributeSet)null, 0, 0);
    throw new RuntimeException("Stub!");
}

public RelativeLayout(Context context, AttributeSet attrs) {
    super((Context)null, (AttributeSet)null, 0, 0);
    throw new RuntimeException("Stub!");
}

public RelativeLayout(Context context, AttributeSet attrs, int defStyleAttr) {
    super((Context)null, (AttributeSet)null, 0, 0);
    throw new RuntimeException("Stub!");
}

public RelativeLayout(Context context, AttributeSet attrs, int defStyleAttr, int defStyleRes) {
    super((Context)null, (AttributeSet)null, 0, 0);
    throw new RuntimeException("Stub!");
}

public boolean shouldDelayChildPressedState() {
    throw new RuntimeException("Stub!");
}

public void setIgnoreGravity(int viewId) {
    throw new RuntimeException("Stub!");
}

public int getGravity() {
    throw new RuntimeException("Stub!");
}

public void setGravity(int gravity) {
    throw new RuntimeException("Stub!");
}

public void setHorizontalGravity(int horizontalGravity) {
    throw new RuntimeException("Stub!");
}

public void setVerticalGravity(int verticalGravity) {
    throw new RuntimeException("Stub!");
}

public int getBaseline() {
    throw new RuntimeException("Stub!");
}

public void requestLayout() {
    throw new RuntimeException("Stub!");
}

protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    throw new RuntimeException("Stub!");
}

protected void onLayout(boolean changed, int l, int t, int r, int b) {
    throw new RuntimeException("Stub!");
}

public RelativeLayout.LayoutParams generateLayoutParams(AttributeSet attrs) {
    throw new RuntimeException("Stub!");
}

protected android.view.ViewGroup.LayoutParams generateDefaultLayoutParams() {
    throw new RuntimeException("Stub!");
}

protected boolean checkLayoutParams(android.view.ViewGroup.LayoutParams p) {
    throw new RuntimeException("Stub!");
}

protected android.view.ViewGroup.LayoutParams generateLayoutParams(android.view.ViewGroup.LayoutParams lp) {
    throw new RuntimeException("Stub!");
}

public CharSequence getAccessibilityClassName() {
    throw new RuntimeException("Stub!");
}

public static class LayoutParams extends MarginLayoutParams {
    @ExportedProperty(
        category = "layout"
    )
    public boolean alignWithParent;

    public LayoutParams(Context c, AttributeSet attrs) {
        super((android.view.ViewGroup.LayoutParams)null);
        throw new RuntimeException("Stub!");
    }

    public LayoutParams(int w, int h) {
        super((android.view.ViewGroup.LayoutParams)null);
        throw new RuntimeException("Stub!");
    }

    public LayoutParams(android.view.ViewGroup.LayoutParams source) {
        super((android.view.ViewGroup.LayoutParams)null);
        throw new RuntimeException("Stub!");
    }

    public LayoutParams(MarginLayoutParams source) {
        super((android.view.ViewGroup.LayoutParams)null);
        throw new RuntimeException("Stub!");
    }

    public LayoutParams(RelativeLayout.LayoutParams source) {
        super((android.view.ViewGroup.LayoutParams)null);
        throw new RuntimeException("Stub!");
    }

    public String debug(String output) {
        throw new RuntimeException("Stub!");
    }

    public void addRule(int verb) {
        throw new RuntimeException("Stub!");
    }

    public void addRule(int verb, int subject) {
        throw new RuntimeException("Stub!");
    }

    public void removeRule(int verb) {
        throw new RuntimeException("Stub!");
    }

    public int getRule(int verb) {
        throw new RuntimeException("Stub!");
    }

    public int[] getRules() {
        throw new RuntimeException("Stub!");
    }

    public void resolveLayoutDirection(int layoutDirection) {
        throw new RuntimeException("Stub!");
    }
}
}

ViewGroup class

您可以看到您没有调用 relativelayout 所需的某些方法。这就是您遇到错误的原因。

Answer 2

在文章（您提供的链接）中，您可以找到注释：

注意：以前版本的 Android Studio 生成的 XML 布局 可能不包括 android:id 属性。调用 findViewById() 如果布局没有 android:id 属性，将会失败。如果 在这种情况下，打开 activity_display_message.xml 并添加 属性 android:id="@+id/activity_display_message" 到布局 元素。

添加缺少的属性，错误应该消失了。