How do I declare global variables on Android?

This example demonstrates how to declare global variables in Android.
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"?>
<LinearLayout 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"
   tools:context=".MainActivity"
   android:gravity = "center"
   android:orientation="vertical">
   <TextView
      android:id = "@+id/tvEvent"
      android:textSize = "40sp"
      android:layout_marginTop = "30dp"
      android:layout_width = "wrap_content"
      android:layout_height = "match_parent" />
</LinearLayout>
Step 3  − Add the following code to src/MainActivity.java
package com.example.sample;
import android.os.Build;
import android.os.Bundle;
import android.support.annotation.RequiresApi;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.widget.TextView;
public class MainActivity extends AppCompatActivity {
   TextView tvEvent;
   @RequiresApi(api = Build.VERSION_CODES.LOLLIPOP)
   @Override
   protected void onCreate(Bundle savedInstanceState) {
      super.onCreate(savedInstanceState);
      setContentView(R.layout.activity_main);
      tvEvent = findViewById(R.id.tvEvent);
      tvEvent.setText("Click");
      tvEvent.setOnClickListener(new View.OnClickListener() {
         @Override
         public void onClick(View v) {
            singleToneClass singleToneClass = com.example.sample.singleToneClass.getInstance();
            singleToneClass.setData("this is sample");
            tvEvent.setText(singleToneClass.getData());
         }
      });
   }
}
Step 4  − Create a class called singToneClass and Add the following code to src/SingleToneClass.java
package com.example.sample;
public class singleToneClass {
   String s;
   private static final singleToneClass ourInstance = new singleToneClass();
   public static singleToneClass getInstance() {
      return ourInstance;
   }
   private singleToneClass() {
   }
   public void setData(String s) {
      this.s = s;
   }
   public String getData() {
      return s;
   }
}
Step 5  – Add the following code to app/manifests/AndroidManifest.xml
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
   package="com.example.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 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 –