我有A和B类。 “ A”成为“ B”的实例，并调用修改String属性的方法。但是，调用后，“ A”中签入的属性为空。 谁能帮我吗？

Class B

private String Retro;
....
public boolean Login(final String user,final String pass){

        stringRequest = new StringRequest(Request.Method.POST,Url + strLogin,new Response.Listener<String>() {
                    @Override
                    public void onResponse(String response) {
                        // Display the first 500 characters of the response string.
                        Retro = "Done";
                    }
                },new Response.ErrorListener() {
            @Override
            public void onErrorResponse(VolleyError error) {
                **Retro = error.getMessage();**
                Toast.makeText(context,"Error de acceso: "+Retro,Toast.LENGTH_LONG).show();  //error.getMessage()
            }

        }) {
            @Override
            protected Map<String,String> getParams(){
                Map<String,String> parameters = new HashMap<>();
                parameters.put("identifier",user);
                parameters.put("password",pass);
                return parameters;
            }
        };

        // Add the request to the RequestQueue.
        requestQueue.add(stringRequest);
        return Retro == "Done";
    }

Class A

protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Toolbar toolbar = findViewById(R.id.toolbar);
        setSupportactionBar(toolbar);

        FloatingactionButton fab = findViewById(R.id.fab);
        fab.setOnClicklistener(new View.OnClicklistener() {
            @Override
            public void onClick(View view) {
                snackbar.make(view,"Replace with your own action",snackbar.LENGTH_LONG)
                        .setaction("action",null).show();
            }
        });

        accessDB = new StrapiDBaccess(this);

        signIn();
    }

    private void handleSignInResult(Task<GoogleSignInaccount> completedTask) {
        try {
            ...
            if (acct != null) {
                ....
                boolean result = accessDB.Login(parcelaccess.personEmail,"*******");

                tvAuth.setText(tvAuth.getText()+" "+**accessDB.getRetro()**); //
            }

        } catch (ApiException e) {
            // The ApiException status code indicates the detailed failure reason.
            // Please refer to the GoogleSignInStatusCodes class reference for more information.
            parcelaccess.signedIn = false;
            parcelaccess.personName = "Anónimo";
            Toast.makeText(this,"Error de autenticación con Google. Chequee que tiene internet e inténtelo de nuevo.",Toast.LENGTH_SHORT).show();

        }
    }