使用Firebase创建登录UI，但转到Main Activity

implementation 'com.google.firebase:firebase-auth:21.0.1'

FirebaseAuth firebaseAuth = FirebaseAuth.getInstance();
String email = "user@example.com";
String password = "password123";

firebaseAuth.signInWithEmailAndPassword(email, password)
    .addOnCompleteListener(this, new OnCompleteListener<AuthResult>() {
        @Override
        public void onComplete(@NonNull Task<AuthResult> task) {
            if (task.isSuccessful()) {
                // 登录成功，转到Main Activity
                Intent intent = new Intent(LoginActivity.this, MainActivity.class);
                startActivity(intent);
                finish();
            } else {
                // 登录失败，显示错误信息
                Toast.makeText(LoginActivity.this, "登录失败", Toast.LENGTH_SHORT).show();
            }
        }
    });

FirebaseAuth firebaseAuth = FirebaseAuth.getInstance();
FirebaseUser currentUser = firebaseAuth.getCurrentUser();

if (currentUser == null) {
    // 用户未登录，转到登录UI
    Intent intent = new Intent(MainActivity.this, LoginActivity.class);
    startActivity(intent);
    finish();
} else {
    // 用户已登录，继续显示Main Activity
}