为什么设计（activity_main.xml）和实际应用看起来不同？

Question

i am new in Android development in java

my activity_main.xml file is look like this

But when i run this application in my pixel 3 phone than it's look's like

why this design are different?

Answer 1

check your style file. By default is with Dialog. u can change it with No Dialog

Sorry i mean the line

  <style name="AppTheme" parent="Theme.AppCompat.Light.DarkActionBar">

u can change the theme with no actionbar

<style name="AppTheme" parent="Theme.AppCompat.Light.NoActionBar">

there are many variants

Answer 2

As seen in background, you ignore missing constraints. When working with ConstraintLayout you must provide at least one horizontal and one vertical constraint, otherwise views could jump, and I believe IDE gave u that error.

Answer 3

if you want to have a full screen theme like Design(activity_main.xml):

solution 1 : go into the manifest and use a full screen theme.

 <activity
   . 
   .
   android:theme="@android:style/Theme.NoTitleBar.Fullscreen">

solution 2 :

Just add the following attribute to your current theme:

<item name="android:windowFullscreen">true</item>

for example:

<style name="AppTheme" parent="Theme.AppCompat.Light.NoActionBar">
<!-- Customize your theme here. -->
<item name="colorPrimary">@color/orange</item>
<item name="colorPrimaryDark">@android:color/holo_orange_dark</item>
<item name="android:windowNoTitle">true</item>
<item name="android:windowFullscreen">true</item>

solution 3 :

 public class MainActivity extends AppCompatActivity {
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        requestWindowFeature(Window.FEATURE_NO_TITLE);
        getWindow().setFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN,
        WindowManager.LayoutParams.FLAG_FULLSCREEN);

        setContentView(R.layout.main);
    }

}

and for manage your buttons, read Dorian's answer to Design and actual app look same.