build.gradle(Project)

// Top-level build file where you can add configuration options common to all sub-projects/modules.
buildscript {
    repositories {
        google()
        jcenter()
    }
    dependencies {
        classpath "com.android.tools.build:gradle:4.1.3"
        classpath 'com.google.gms:google-services:4.3.10'
        // NOTE: Do not place your application dependencies here; they belong
        // in the individual module build.gradle files
    }
}

allprojects {
    repositories {
        google()
        jcenter()
    }
}

task clean(type: Delete) {
    delete rootProject.buildDir
}

build.gradle (App)

plugins {
    id 'com.android.application'
    id 'com.google.gms.google-services'
}

android {
    compileSdkVersion 31
    buildToolsVersion "30.0.3"

    defaultConfig {
        applicationId "com.learnoset.googlesignin"
        minSdkVersion 19
        targetSdkVersion 31
        versionCode 1
        versionName "1.0"

        testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"
    }

    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'
        }
    }
    compileOptions {
        sourceCompatibility JavaVersion.VERSION_1_8
        targetCompatibility JavaVersion.VERSION_1_8
    }
}

dependencies {

    implementation 'androidx.appcompat:appcompat:1.3.1'
    implementation 'com.google.android.material:material:1.4.0'
    implementation 'androidx.constraintlayout:constraintlayout:2.1.1'
    testImplementation 'junit:junit:'
    implementation platform('com.google.firebase:firebase-bom:28.4.2')
    androidTestImplementation 'androidx.test.ext:junit:1.1.3'
    androidTestImplementation 'androidx.test.espresso:espresso-core:3.4.0'
    implementation 'com.google.android.gms:play-services-auth:19.2.0'
}

google-services.json

// Replace this file with your own google-service.json file. You ca get this file from Firebase
{
  "project_info": {
    "project_number": "306407888156",
    "project_id": "signin-15b66",
    "storage_bucket": "signin-15b66.appspot.com"
  },
  "client": [
    {
      "client_info": {
        "mobilesdk_app_id": "1:306407888156:android:421d29c936e0905b3896d9",
        "android_client_info": {
          "package_name": "com.learnoset.googlesignin"
        }
      },
      "oauth_client": [
        {
          "client_id": "306407888156-9lbrvac246v834h3eepjlnjsonjvqcmv.apps.googleusercontent.com",
          "client_type": 1,
          "android_info": {
            "package_name": "com.learnoset.googlesignin",
            "certificate_hash": "ce5dbf10dd6cb3a8952c3fde991ee53a45caa940"
          }
        },
        {
          "client_id": "306407888156-dq4fa1cefqmdo5d6g7u77ehk4s2bcdtk.apps.googleusercontent.com",
          "client_type": 3
        }
      ],
      "api_key": [
        {
          "current_key": "AIzaSyCtKJCHQcaDOFoN0ViuyiJz2dpCbP6zrJM"
        }
      ],
      "services": {
        "appinvite_service": {
          "other_platform_oauth_client": [
            {
              "client_id": "306407888156-dq4fa1cefqmdo5d6g7u77ehk4s2bcdtk.apps.googleusercontent.com",
              "client_type": 3
            }
          ]
        }
      }
    }
  ],
  "configuration_version": "1"
}

MainActivity.java

package com.learnoset.googlesignin;

import android.content.Intent;
import android.net.Uri;
import android.os.Bundle;
import android.view.View;
import android.widget.ImageView;
import android.widget.Toast;

import androidx.activity.result.ActivityResult;
import androidx.activity.result.ActivityResultCallback;
import androidx.activity.result.ActivityResultLauncher;
import androidx.activity.result.contract.ActivityResultContracts;
import androidx.appcompat.app.AppCompatActivity;

import com.google.android.gms.auth.api.signin.GoogleSignIn;
import com.google.android.gms.auth.api.signin.GoogleSignInAccount;
import com.google.android.gms.auth.api.signin.GoogleSignInClient;
import com.google.android.gms.auth.api.signin.GoogleSignInOptions;
import com.google.android.gms.common.api.ApiException;
import com.google.android.gms.tasks.Task;

public class MainActivity extends AppCompatActivity {

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

        final ImageView signInBtn = findViewById(R.id.signInBtn);

        GoogleSignInOptions googleSignInOptions = new GoogleSignInOptions.Builder(GoogleSignInOptions.DEFAULT_SIGN_IN)
                .requestEmail()
                .build();

        GoogleSignInClient googleSignInClient = GoogleSignIn.getClient(this, googleSignInOptions);

        GoogleSignInAccount googleSignInAccount = GoogleSignIn.getLastSignedInAccount(this);

        // checking if user already signed in
        if (googleSignInAccount != null) {

            // open home activity
            startActivity(new Intent(MainActivity.this, HomeActivity.class));
            finish();
        }

        ActivityResultLauncher<Intent> activityResultLauncher = registerForActivityResult(new ActivityResultContracts.StartActivityForResult(), new ActivityResultCallback<ActivityResult>() {
            @Override
            public void onActivityResult(ActivityResult result) {

                // getting signed in account after user selected an account from google accounts dialog
                Task<GoogleSignInAccount> task = GoogleSignIn.getSignedInAccountFromIntent(result.getData());
                handleSignInTask(task);
            }
        });

        signInBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Intent signInIntent = googleSignInClient.getSignInIntent();
                activityResultLauncher.launch(signInIntent);
            }
        });
    }

    private void handleSignInTask(Task<GoogleSignInAccount> task) {

        try {
            GoogleSignInAccount account = task.getResult(ApiException.class);

            // getting account data
            final String getFullname = account.getDisplayName();
            final String getEmail = account.getEmail();
            final Uri getPhotoUrl = account.getPhotoUrl();

            // opening HomeActivity
            startActivity(new Intent(MainActivity.this, HomeActivity.class));
            finish();

        } catch (ApiException e) {
            e.printStackTrace();
            Toast.makeText(this, "Failed or Cancelled", Toast.LENGTH_SHORT).show();
        }

    }
}

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout 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">

    <ImageView
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:src="@drawable/google_icon"
        android:adjustViewBounds="true"
        android:layout_marginStart="60dp"
        android:layout_centerInParent="true"
        android:id="@+id/signInBtn"
        android:layout_marginEnd="60dp"/>

</RelativeLayout>

activity_home.xml

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".HomeActivity">

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_centerInParent="true"
        android:gravity="center"
        android:orientation="vertical">

        <TextView
            android:id="@+id/fullName"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="Full Name" />

        <TextView
            android:id="@+id/emailId"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_marginTop="20dp"
            android:text="Email Id" />

        <androidx.appcompat.widget.AppCompatButton
            android:id="@+id/signOutBtn"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_marginTop="20dp"
            android:text="Sign Out" />

    </LinearLayout>
</RelativeLayout>

HomeActivity.java

package com.learnoset.googlesignin;

import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.widget.TextView;

import androidx.appcompat.app.AppCompatActivity;
import androidx.appcompat.widget.AppCompatButton;

import com.google.android.gms.auth.api.signin.GoogleSignIn;
import com.google.android.gms.auth.api.signin.GoogleSignInAccount;
import com.google.android.gms.auth.api.signin.GoogleSignInClient;
import com.google.android.gms.auth.api.signin.GoogleSignInOptions;

public class HomeActivity extends AppCompatActivity {

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

        final TextView email = findViewById(R.id.emailId);
        final TextView fullname = findViewById(R.id.fullName);
        final AppCompatButton signOutBtn = findViewById(R.id.signOutBtn);


        GoogleSignInOptions googleSignInOptions = new GoogleSignInOptions.Builder(GoogleSignInOptions.DEFAULT_SIGN_IN)
                .requestEmail()
                .build();

        GoogleSignInClient googleSignInClient = GoogleSignIn.getClient(this, googleSignInOptions);

        GoogleSignInAccount googleSignInAccount = GoogleSignIn.getLastSignedInAccount(this);

        // get details for signed in user
        final String getFullName = googleSignInAccount.getDisplayName();
        final String getEmail = googleSignInAccount.getEmail();

        email.setText("Email : " + getEmail);
        fullname.setText("FullName : " + getFullName);

        signOutBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {

                // sign out
                googleSignInClient.signOut();

                // opening MainActivity to sign in again
                startActivity(new Intent(HomeActivity.this, MainActivity.class));
                finish();
            }
        });
    }
}

Output