How to get JSON error response and toast in android : Here's my code for when i trying to register user and need a toast which is response from server regarding user already exist. i can post successfully to server using json but if there's response i have to idea how to catch it the image shows example when using postman.
    
    public class RegisterActivity extends AppCompatActivity implements View.OnClickListener{

    private EditText signupInputName, signupInputEmail, signupInputPassword, retypeInputPassword;
    private Button btnSignUp;
    private Button btnLinkLogin;
    private String message = "";
    private int code = 0;

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

        signupInputName = (EditText) findViewById(R.id.signup_input_name);
        signupInputEmail = (EditText) findViewById(R.id.signup_input_email);
        signupInputPassword = (EditText) findViewById(R.id.signup_input_password);
        retypeInputPassword = (EditText) findViewById(R.id.signup_retype_password);

        btnSignUp = (Button) findViewById(R.id.btn_signup);
        btnLinkLogin = (Button) findViewById(R.id.btn_link_login);

        btnSignUp.setOnClickListener(this);

        btnLinkLogin.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {

                Intent i = new Intent(getApplicationContext(),LoginActivity.class);
                startActivity(i);
            }
        });
    }

    public String POST(String url, Person person)
    {
        InputStream inputStream = null;
        String result = "";

        try {

            // 1. create HttpClient
            HttpClient httpclient = new DefaultHttpClient();

            // 2. make POST request to the given URL
            HttpPost httppost = new HttpPost(url);

            String json = "";

            // 3. build jsonObject
            JSONObject jsonObject = new JSONObject();
            jsonObject.accumulate("user_name", person.getUsername());
            jsonObject.accumulate("email", person.getEmail());
            jsonObject.accumulate("password", person.getPassword());

            // 4. convert JSONObject to JSON to String
            json = jsonObject.toString();

            // ** Alternative way to convert Person object to JSON string usin Jackson Lib
            // ObjectMapper mapper = new ObjectMapper();
            // json = mapper.writeValueAsString(person);

            // 5. set json to StringEntity
            StringEntity se = new StringEntity(json);

            // 6. set httpPost Entity
            httppost.setEntity(se);

            // 7. Set some headers to inform server about the type of the content
            httppost.setHeader("Accept", "application/json");
            httppost.setHeader("Content-type", "application/json");

            // 8. Execute POST request to the given URL
            HttpResponse httpResponse = httpclient.execute(httppost);

            // 9. receive response as inputStream
            inputStream = httpResponse.getEntity().getContent();

            // 10. convert inputstream to string
            if(inputStream != null)
                result = convertInputStreamToString(inputStream);
            else
                result = "Error! email exist";

        } catch (Exception e) {
            Log.d("InputStream", e.getLocalizedMessage());
        }

        // 11. return result
        return result;
    }

    @Override
    public void onClick(View view) {
        if(validate() == 1)
        {
            Toast.makeText(getBaseContext(), message.toString(), Toast.LENGTH_SHORT).show();
        }
        else if (validate() == 2)
        {
            Toast.makeText(getBaseContext(), message.toString(), Toast.LENGTH_SHORT).show();
        }
        else if (validate() == 3)
        {
            Toast.makeText(getBaseContext(), message.toString(), Toast.LENGTH_SHORT).show();
        }
        else if (validate() == 4)
        {
            //Toast.makeText(getBaseContext(), "Success", Toast.LENGTH_SHORT).show();
            new HttpAsyncTask().execute("http://ip-addressses/api/register");
        }

    }

    private class HttpAsyncTask extends AsyncTask<String, Void, String>
    {
        @Override
        protected String doInBackground(String... urls) {

            person = new Person();
            person.setUsername(signupInputName.getText().toString());
            person.setEmail(signupInputEmail.getText().toString());
            person.setPassword(signupInputPassword.getText().toString());

            return POST(urls[0],person);
        }
        // onPostExecute displays the results of the AsyncTask.
        @Override
        protected void onPostExecute(String result) {

            JSONObject jObject;

            try {
                jObject = new JSONObject(result);
                if (jObject.has("error")) {
                    String aJsonString = jObject.getString("error");
                    Toast.makeText(getBaseContext(), aJsonString, Toast.LENGTH_SHORT).show();
                } else {
                    Toast.makeText(getBaseContext(), "Login Successful", Toast.LENGTH_SHORT).show();
                }
            } catch (JSONException e1) {
                // TODO Auto-generated catch block
                e1.printStackTrace();
            }


        }
    }

    private int validate() {
        if(signupInputName.getText().toString().trim().equals("") || signupInputEmail.getText().toString().trim().equals("") || signupInputPassword.getText().toString().trim().equals("") || retypeInputPassword.getText().toString().trim().equals(""))
        {
            code = 1;
            message = "Complete the form!";
        }
        else if (!(signupInputPassword.getText().toString().equals(retypeInputPassword.getText().toString())))
        {
            code = 2;
            message = "Re-check password";
        }
        else if (!isValidEmail(signupInputEmail.getText().toString()) ) {
            code = 3;
            message = "Invalid email";
        }
        else
            code = 4;

        return code;
    }

    public final static boolean isValidEmail(String target)
    {
        if (target == null) {
            return false;
        } else {
            Matcher match = Patterns.EMAIL_ADDRESS.matcher(target);
            return match.matches();
        }
    }

    private static String convertInputStreamToString(InputStream inputStream) throws IOException{
        BufferedReader bufferedReader = new BufferedReader( new InputStreamReader(inputStream));
        String line = "";
        String result = "";
        while((line = bufferedReader.readLine()) != null)
            result += line;

        inputStream.close();
        return result;

    }
}

[Postman response when email exist][1]


  [1]: https://i.stack.imgur.com/wqesS.png