For Facebook login in android, first of all, register your app.

Registering Your Facebook App 

Step 1:  First you log in using your Facebook account to https://developers.facebook.com/apps/.

Step 2: Click on create a new app. And set the app name and the email id as shown in the below pic.

Step 3:  You shall see the dashboard in the next screen. Choose Facebook Login Setup and select the platform as Android as shown in the below images. 

Step 4:    After that, add the following dependencies in your app‘s build.gradle. 

buildscript {  
repositories {

mavenCentral()

}

}  

Step 5: Use the latest version of the SDK dependency. 

implementation 'com.facebook.android:facebook-android-sdk:4.31.0' 
or  
implementation 'com.facebook.android:facebook-android-sdk:[4,5)'

Step 6:     To check the SDK is successfully added in our project add the following import statements in your MainActivity.java class file as shown below. If there’s no error, the build was successful. 

import com.facebook.FacebookSdk; 
import com.facebook.appevents.AppEventsLogger;

Step 7:    Add your Facebook App ID in the string.xml file.

<string name="facebook_app_id">2048405422006344</string>

<string name="fb_login_protocol_scheme">fb2048405422006344</string>

Step 8:    Enter your package name and default activity name with the fully qualified path as shown below. After saving click on Use this package name to confirm. 

Step 9:    The next window asks for the add key hashes. That can be done by running the following commands on windows cmd. 

keytool -exportcert -alias androiddebugkey -keystore "C:\Users\USERNAME\.android\debug.keystore" | "PATH_TO_OPENSSL_LIBRARY\bin\openssl" sha1 -binary | "PATH_TO_OPENSSL_LIBRARY\bin\openssl" base64

 You will need the following steps for generating a hash key.

  1.   Download OpenSSL library for Windows from the https://code.google.com/archive/p/openssl-for-windows/downloads.

  2.  Open your java bin path location and copy it. And after that open cmd and write some command .

      Cd C:\Program Files\Java\jdk-9.0.4\bin (click enter)

     Write some key tool command-

   keytool -exportcert -alias androiddebugkey -keystore "C:\Users\msclient009\.android\debug.keystore" | "C:\openssl-0.9.8e_X64\bin\openssl" sha1 -binary | "C:\openssl-0.9.8e_X64\bin\openssl" base64 

Above command will generate a 28-character key hash unique to your development environment. And after that add key hashes in your Facebook page.

Step 10:   Add internet permissions in the AndroidManifest.xml file by adding the following line in the manifest file. 

<uses-permission android:name="android.permission.INTERNET"/>

Step 11:    Add Following meta-data tag in your AndroidManifest.xml file.

<meta-data android:name="com.facebook.sdk.ApplicationId"  
        android:value="@string/facebook_app_id"/>

    <activity android:name="com.facebook.FacebookActivity"

        android:configChanges=

                "keyboard|keyboardHidden|screenLayout|screenSize|orientation"

        android:label="@string/app_name" />

    <activity

        android:name="com.facebook.CustomTabActivity"

        android:exported="true">

        <intent-filter>

            <action android:name="android.intent.action.VIEW" />

            <category android:name="android.intent.category.DEFAULT" />

            <category android:name="android.intent.category.BROWSABLE" />

            <data android:scheme="@string/fb_login_protocol_scheme" />

        </intent-filter>

    </activity>

Step 12:   Add some code in your activity_main.xml file

<FrameLayout

    android:id="@+id/FrameLayout1"

    android:layout_width="match_parent"

    android:layout_height="@dimen/button_height"

    android:layout_below="@+id/sign_in_button"

    android:layout_marginTop="@dimen/card"

    android:layout_weight="1"

    android:background="@drawable/login_button"

    android:elevation="3dp">
    <com.facebook.login.widget.LoginButton
        android:id="@+id/fb_login_button"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout_marginTop="@dimen/bottom"
        android:background="#4285F4"
        android:textSize="@dimen/text_size" />
    <Button
        android:id="@+id/fb"
        android:layout_width="match_parent"
        android:layout_height="50dp"
        android:background="#4285F4"
        android:drawableStart="@drawable/fb_icon"
        android:padding="@dimen/bottom"
        android:text="@string/com_facebook"
        android:textColor="@color/back"
        android:textSize="@dimen/text_size" />
</FrameLayout>

MainActivity.java file

loginButton = (LoginButton) findViewById(R.id.fb_login_button);
FacebookSdk.sdkInitialize(getApplicationContext());
AppEventsLogger.activateApp(this);
callbackManager = CallbackManager.Factory.create(); // create instance of callback manager class
if (Utils.isNetworkAvailable(getApplicationContext())) {
   loginButton.setReadPermissions(Arrays.asList(EMAIL, PUBLIC_PROFILE));
    loginButton.registerCallback(callbackManager, new FacebookCallback<LoginResult>() {
        @Override
        public void onSuccess(LoginResult loginResult) {
          accessTokenFacebook = loginResult.getAccessToken();
            final GraphRequest request = GraphRequest.newMeRequest(accessTokenFacebook, new GraphRequest.GraphJSONObjectCallback() {
                @Override
                public void onCompleted(JSONObject object, GraphResponse response) {
                    Log.i("GraphResponse", response.toString());
                    Log.i("JSONObject", object.toString());
                    // Get facebook data from login
                    String email="";
                    try {
                       String userName = object.getString("name");
                       String id = object.getString("id");
                       email=object.getString("email");
                        String firstName = " ", lastName = " ";
                        if (userName != null) {
                            int end = userName.lastIndexOf(' ');
                            if (end >= 0) {
                                firstName = userName.substring(0, end);
                                lastName = userName.substring(end + 1, userName.length());
                            }
                        }
                        Toast.makeText(getApplicationContext(),"Facebook Login Successful",Toast.LENGTH_SHORT).show();
                        if(email.equals("")) {
Toast.makeText(getApplicationContext(),getResources().getString(R.string.email_permission),Toast.LENGTH_SHORT).show();
                        }
                        else {
                            loginApi("facebook", id, firstName, lastName, email);
                        }
                    } catch (Exception e) {
                        e.printStackTrace();
                    }
                }
            });
            Bundle parameters = new Bundle();
            parameters.putString("fields", "id,name,email"); // Add some field as per user requirement
            request.setParameters(parameters);
            request.executeAsync();
        }
        @Override
        public void onCancel() {
            Log.d("facebook Login","On cancel");
        }
        @Override
        public void onError(FacebookException error) {
            Log.v("Error", error.toString());
        }
    });
} else {
    Toast.makeText(this, getResources().getString(R.string.no_internet), Toast.LENGTH_SHORT).show();
}
// Login Api AsyncTask Code
public void loginApi(String provider, String providerId, String firstName, String lastName, String emailId) {
    LoginAsyncTask asyncTask = new LoginAsyncTask(getApplicationContext(), provider, providerId, firstName, lastName, emailId);
    asyncTask.delegate = this;
    asyncTask.execute();
}
@Override
public void onActivityResult(int requestCode, int resultCode, Intent data) {
    super.onActivityResult(requestCode, resultCode, data);
    Log.v("Request Code", String.valueOf(requestCode));
    if (requestCode == REQ_CODE_GOOGLE_SIGN_IN) {
        Task<GoogleSignInAccount> task = GoogleSignIn.getSignedInAccountFromIntent(data);
        handleSignInResult(task);
    }
   if(requestCode == REQ_CODE_FB_SIGN_IN){
       callbackManager.onActivityResult(requestCode, resultCode, data);
    }
}