How to make an alert dialog fill 50% of screen size on Android device?

This example demonstrates how to make an alert dialog to fill 50% of screen size on an Android device.

Step 1 − Create a new project in Android Studio, go to File ⇒ New Project and fill all required details to create a new project.

Step 2 − Add the following code to res/layout/activity_main.xml.

<?xml version="1.0" encoding="utf-8"?>
<androidx.coordinatorlayout.widget.CoordinatorLayout
   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:fitsSystemWindows="true"
   android:id="@+id/coordinator_layout"
   tools:context=".MainActivity">
   <Button
      android:id="@+id/btnAlert"
      android:layout_width="wrap_content"
      android:layout_height="wrap_content"
      android:text="Show Alert Dialog"
      android:layout_gravity="top|center_horizontal"
      tools:ignore="MissingConstraints" /&glt;
</androidx.coordinatorlayout.widget.CoordinatorLayout>

Step 3 − Add the following code to src/MainActivity.java

package com.app.sample;
import androidx.appcompat.app.AppCompatActivity;
import androidx.coordinatorlayout.widget.CoordinatorLayout;
import android.app.AlertDialog;
import android.os.Bundle;
import android.app.Activity;
import android.content.Context;
import android.util.DisplayMetrics;
import android.view.View;
import android.view.WindowManager;
import android.widget.Button;
public class MainActivity extends AppCompatActivity {
   private Context mContext;
   private Activity mActivity;
   private CoordinatorLayout mCLayout;
   private Button mButton;
   @Override
   protected void onCreate(Bundle savedInstanceState) {
      super.onCreate(savedInstanceState);
      setContentView(R.layout.activity_main);
      mContext = getApplicationContext();
      mActivity = MainActivity.this;
      mCLayout = (CoordinatorLayout) findViewById(R.id.coordinator_layout);
      mButton = (Button) findViewById(R.id.btn_alert);
      mButton.setOnClickListener(new View.OnClickListener() {
         @Override
         public void onClick(View view) {
            AlertDialog.Builder builder = new AlertDialog.Builder(mActivity);
            builder.setTitle("Say Hello!");
            builder.setMessage("Are you want to say it?");
            builder.setPositiveButton("Say",null);
            builder.setNegativeButton("No",null);
            AlertDialog dialog = builder.create();
            dialog.show();
            DisplayMetrics displayMetrics = new DisplayMetrics();
            getWindowManager().getDefaultDisplay().getMetrics(displayMetrics);
            int displayWidth = displayMetrics.widthPixels;
            int displayHeight = displayMetrics.heightPixels;
            WindowManager.LayoutParams layoutParams = new WindowManager.LayoutParams();
            layoutParams.copyFrom(dialog.getWindow().getAttributes());
            int dialogWindowWidth = (int) (displayWidth * 0.5f);
            int dialogWindowHeight = (int) (displayHeight * 0.5f);
            layoutParams.width = dialogWindowWidth;
            layoutParams.height = dialogWindowHeight;
            dialog.getWindow().setAttributes(layoutParams);
         }
      });
   }
}

Step 4 − Add the following code to Manifests/AndroidManifest.xml

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android" package="com.app.sample">
   <application
      android:allowBackup="true"
      android:icon="@mipmap/ic_launcher"
      android:label="@string/app_name"
      android:roundIcon="@mipmap/ic_launcher_round"
      android:supportsRtl="true"
      android:theme="@style/AppTheme">
      <activity android:name=".MainActivity">
         <intent-filter>
            <action android:name="android.intent.action.MAIN" />
            <category android:name="android.intent.category.LAUNCHER" />
         </intent-filter>
      </activity>
   </application>
</manifest>

Let's try to run your application. I assume you have connected your actual Android Mobile device with your computer. To run the app from the android studio, open one of your project's activity files and click Run  icon from the toolbar. Select your mobile device as an option and then check your mobile device which will display your default screen −

Click here to download the project code.