I have made a very basic Bottom Sheet for my sample project to do a flowing page. 1st I made the layout :

<?xml version="1.0" encoding="utf-8"?>
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"
android:id="@+id/mainContent"
android:fitsSystemWindows="true"
tools:context=".MainActivity">

android:layout_height="match_parent">

android:layout_height="match_parent"
android:orientation="vertical"
android:paddingTop="24dp">

<Button
android:id="@+id/btn_1"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:text="Button 1"
android:padding="16dp"
android:layout_margin="8dp"
android:textColor="@android:color/white"
android:background="@android:color/holo_green_dark"/>




android:id="@+id/bottom_sheet"
android:layout_width="match_parent"
android:layout_height="350dp"
android:clipToPadding="true"
android:background="@android:color/holo_orange_light"
app:layout_behavior="android.support.design.widget.BottomSheetBehavior">

android:layout_height="match_parent"
android:text="Bottom Sheets"
android:paddingTop="16dp"
android:textSize="16sp">





In MainActivity i've written these codes:

public class MainActivity extends AppCompatActivity implements View.OnClickListener {

private BottomSheetBehavior mBottomSheetBehavior;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
View bottomSheet = findViewById(R.id.bottom_sheet);

Button btn1 = findViewById(R.id.btn_1);
btn1.setOnClickListener(this);

mBottomSheetBehavior = BottomSheetBehavior.from(bottomSheet);
mBottomSheetBehavior.setState(BottomSheetBehavior.STATE_HIDDEN);

}

@Override
public void onClick(View view) {
switch (view.getId()) {
case R.id.btn_1:
mBottomSheetBehavior.setState(BottomSheetBehavior.STATE_EXPANDED);
break;
}
}
}

But when i run the app on the emulator the BottomSheetBehavior is EXPANDED! When i changed the code to this:

switch (view.getId()) {
case R.id.btn_1:
mBottomSheetBehavior.setState(BottomSheetBehavior.STATE_COLLAPSED);
break;

And press btn_1 BottomSheetBehavior does not collapse, what is the issue? This is my gradle:

dependencies {
implementation fileTree(include: ['*.jar'], dir: 'libs')
implementation 'com.android.support:appcompat-v7:28.0.0-alpha3'
implementation 'com.android.support.constraint:constraint-layout:1.0.2'
testImplementation 'junit:junit:4.12'
androidTestImplementation 'com.android.support.test:runner:1.0.1'
androidTestImplementation 'com.android.support.test.espresso:espresso-core:3.0.1'
implementation 'com.android.support:design:28.0.0-alpha3'
}