Android activity as a dialog

Set Dialog theme into your manifest file. Like android:theme="@style/DialogTheme"

AndroidManifest.xml

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.androidsimplifycodes.example">

    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        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>
        <activity
            android:name=".DialogActivity"
            android:theme="@style/DialogTheme" />
    </application>

</manifest>

Here, define DialogTheme theme added into your style file and set into manifest file. If you not want to use windows title then make true windowNoTitle attribute.

style.xml

<resources>

    <style name="AppTheme" parent="Theme.AppCompat.Light.DarkActionBar">
        <item name="colorPrimary">@color/colorPrimary</item>
        <item name="colorPrimaryDark">@color/colorPrimaryDark</item>
        <item name="colorAccent">@color/colorAccent</item>
    </style>

    <style name="DialogTheme" parent="Theme.AppCompat.Light.Dialog">
        <item name="android:textStyle">bold</item>
        <item name="android:gravity">center_vertical</item>
        <item name="colorPrimary">@color/colorPrimary</item>
        <item name="colorPrimaryDark">@color/colorPrimaryDark</item>
        <item name="colorAccent">@color/colorAccent</item>
        <item name="windowNoTitle">true</item>
    </style>

</resources>

activity_dialog.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="300dp"
    android:layout_height="match_parent"
    android:orientation="vertical"
    tools:context="com.androidsimplifycodes.example.MainActivity">

    <TextView
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:background="@android:color/black"
        android:gravity="center"
        android:padding="10dp"
        android:text="Set Title Of the Dialog"
        android:textAppearance="?android:textAppearanceMedium"
        android:textColor="@android:color/white"
        android:textStyle="bold" />

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="250dp">

        <TextView
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:gravity="center"
            android:text="Hello World!"
            android:textAppearance="?android:textAppearanceLarge"
            android:textColor="@android:color/holo_red_light" />

    </LinearLayout>

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:background="@android:color/darker_gray"
        android:gravity="center"
        android:padding="5dp">

        <Button
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="Ok" />

        <Button
            android:id="@+id/btnCancelDialog"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="Cancel" />

    </LinearLayout>

</LinearLayout>

DialogActivity.java

package com.androidsimplifycodes.example;

import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.widget.Button;

public class DialogActivity extends AppCompatActivity {

    protected Button btnCancelDialog;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        super.setContentView(R.layout.activity_dialog);
        initView();

        btnCancelDialog.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                finish();
            }
        });
    }

    private void initView() {
        btnCancelDialog = (Button) findViewById(R.id.btnCancelDialog);
    }
}

Output :