如何在Facebook用Firebase登录后用displayName显示一条吐司消息？

import android.os.Bundle;
import android.widget.Toast;

import androidx.annotation.NonNull;
import androidx.appcompat.app.AppCompatActivity;

import com.facebook.AccessToken;
import com.facebook.AccessTokenTracker;
import com.facebook.CallbackManager;
import com.facebook.FacebookCallback;
import com.facebook.FacebookException;
import com.facebook.login.LoginManager;
import com.facebook.login.LoginResult;
import com.facebook.login.widget.LoginButton;
import com.google.firebase.auth.AuthCredential;
import com.google.firebase.auth.AuthResult;
import com.google.firebase.auth.FirebaseAuth;
import com.google.firebase.auth.FirebaseUser;
import com.google.firebase.auth.OAuthProvider;

public class MainActivity extends AppCompatActivity {

    private FirebaseAuth mAuth;
    private CallbackManager mCallbackManager;

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

        // 初始化Firebase身份验证
        mAuth = FirebaseAuth.getInstance();

        // 初始化Facebook登录按钮
        LoginButton loginButton = findViewById(R.id.login_button);
        mCallbackManager = CallbackManager.Factory.create();
        loginButton.setPermissions("email", "public_profile");
        loginButton.registerCallback(mCallbackManager, new FacebookCallback<LoginResult>() {
            @Override
            public void onSuccess(LoginResult loginResult) {
                handleFacebookAccessToken(loginResult.getAccessToken());
            }

            @Override
            public void onCancel() {
                // 用户取消了登录
            }

            @Override
            public void onError(FacebookException error) {
                // 登录出现错误
            }
        });

        // 监听Facebook访问令牌的更改
        AccessTokenTracker accessTokenTracker = new AccessTokenTracker() {
            @Override
            protected void onCurrentAccessTokenChanged(AccessToken oldAccessToken, AccessToken currentAccessToken) {
                if (currentAccessToken == null) {
                    // 用户已注销
                    mAuth.signOut();
                }
            }
        };

        // 自定义Facebook登录按钮的外观
        loginButton.setText("使用Facebook登录");
    }

    // 处理Facebook访问令牌的身份验证
    private void handleFacebookAccessToken(AccessToken token) {
        AuthCredential credential = OAuthProvider.newBuilder("facebook.com").setAccessToken(token.getToken()).build();
        mAuth.signInWithCredential(credential)
                .addOnCompleteListener(this, new OnCompleteListener<AuthResult>() {
                    @Override
                    public void onComplete(@NonNull Task<AuthResult> task) {
                        if (task.isSuccessful()) {
                            // 用户成功登录
                            FirebaseUser user = mAuth.getCurrentUser();
                            String displayName = user.getDisplayName();
                            showToast("欢迎，" + displayName + "!");
                        } else {
                            // 登录失败
                            showToast("登录失败，请重试");
                        }
                    }
                });
    }

    // 显示Toast消息
    private void showToast(String message) {
        Toast.makeText(this, message, Toast.LENGTH_SHORT).show();
    }

    @Override
    protected void onActivityResult(int requestCode, int resultCode, Intent data) {
        super.onActivityResult(requestCode, resultCode, data);
        mCallbackManager.onActivityResult(requestCode, resultCode, data);
    }
}