Q:

android.view. WindowInsets ClassNotFoundException

android.view. WindowInsets is added in API level 20.  
I import android.view. WindowInsets in my CustomLayout and override onApplyWindowInsets(WindowInsets insets), but ClassNotFoundException occurs in some phones, whose api level are from 14 to 21.  What is the reason? 
Occurred on:
Rooted Nexus 5, Android 4.4.2
Stack trace:
Fatal Exception: java.lang. NoClassDefFoundError: android/view/WindowInsets
   at java.lang. Class.getDeclaredMethods(Class.java)
   at java.lang. Class.getDeclaredMethods(Class.java:656)
   at android.view. ViewDebug.getExportedPropertyMethods(ViewDebug.java:960)
   at android.view. ViewDebug.exportMethods(ViewDebug.java:1047)
   at android.view. ViewDebug.dumpViewProperties(ViewDebug.java:997)
   at android.view. ViewDebug.dumpViewProperties(ViewDebug.java:983)
   at android.view. ViewDebug.dumpView(ViewDebug.java:900)
   at android.view. ViewDebug.dumpViewHierarchy(ViewDebug.java:855)
   at android.view. ViewDebug.dumpViewHierarchy(ViewDebug.java:867)
   at android.view. ViewDebug.dumpViewHierarchy(ViewDebug.java:867)
   at android.view. ViewDebug.dumpViewHierarchy(ViewDebug.java:867)
   at android.view. ViewDebug.dumpViewHierarchy(ViewDebug.java:867)
   at android.view. ViewDebug.dumpViewHierarchy(ViewDebug.java:867)
   at android.view. ViewDebug.dumpViewHierarchy(ViewDebug.java:867)
   at android.view. ViewDebug.dump(ViewDebug.java:793)
   at android.view. ViewDebug.dispatchCommand(ViewDebug.java:416)
   at android.view. ViewRootImpl$W.executeCommand(ViewRootImpl.java:6258)
   at android.view. IWindow$Stub.onTransact(IWindow.java:65)
   at android.os. Binder.execTransact(Binder.java:404)
   at dalvik.system. NativeStart.run(NativeStart.java)
Caused by java.lang. ClassNotFoundException: Didn't find class "android.view. WindowInsets" on path: DexPathList[[zip file "/data/app/***-1.apk"],nativeLibraryDirectories=[/data/app-lib/***-1, /vendor/lib, /system/lib]]
   at dalvik.system. BaseDexClassLoader.findClass(BaseDexClassLoader.java:56)
   at java.lang. ClassLoader.loadClass(ClassLoader.java:497)
   at java.lang. ClassLoader.loadClass(ClassLoader.java:457)
   at java.lang. Class.getDeclaredMethods(Class.java)
   at java.lang. Class.getDeclaredMethods(Class.java:656)
   at android.view. ViewDebug.getExportedPropertyMethods(ViewDebug.java:960)
   at android.view. ViewDebug.exportMethods(ViewDebug.java:1047)
   at android.view. ViewDebug.dumpViewProperties(ViewDebug.java:997)
   at android.view. ViewDebug.dumpViewProperties(ViewDebug.java:983)
   at android.view. ViewDebug.dumpView(ViewDebug

A:

What happens
The system traverses all public methods of a view and encounters the overridden onApplyWindowInsets with WindowInsets parameter.  This type does not exist in the system hence the crash. 
Lollipop introduced the View.onApplyWindowInsets method but it also introduced the OnApplyWindowInsetsListener, which if set, is invoked instead of the aforementioned method. 
When does it happen
I've had reports of this on Samsung devices running Android 4.4. 
It can be triggered by dumping view hierarchy. 
What to do about it
So far this doesn't solve anything.  To the rescue comes support-v4 library:
public class SampleView extends View {
    public SampleView(final Context context) {
        this(context, null);
    }

    public SampleView(final Context context, @Nullable final AttributeSet attrs) {
        this(context, attrs, 0);
    }

    public SampleView(final Context context, @Nullable final AttributeSet attrs, final int defStyleAttr) {
        super(context, attrs, defStyleAttr);

        ViewCompat.setOnApplyWindowInsetsListener(this, new android.support.v4.view. OnApplyWindowInsetsListener() {
            @Override
            public WindowInsetsCompat onApplyWindowInsets(final View v, final WindowInsetsCompat insets) {
                // Do whatever you needed to do in the first place...
                return insets.consumeSystemWindowInsets();
            }
        });
    }
}

Use the above in your common constructor.  WindowInsetsCompat is provided by the support-v4 library so it's always present, it does not expose any non-existent future classes directly on the view, and the code is effective only since Lollipop (where actual WindowInsets were introduced). 
Why is this happening
Beats me.