# Interface FailureCallback<T>

All Known Subinterfaces:
  `Callback<T>`

All Known Implementing Classes:
  `CallbackAdapter`

public interface FailureCallback<T>
Simple interface that allows asynchronous code to perform a parameterized callback in a similar way to GWT's callback interface. This is the base interface for `com.codename1.util.Callback` where the logic is broken down to single method interfaces so Java 8 lambdas can be used.

- ## Method Summary

  Modifier and Type
  Method
  Description
  `void`
  `onError(java.lang.Object sender, Throwable err, int errorCode, String errorMessage)`
  Invoked when the async method throws an exception

- ## Method Details

  - ### onError

    void onError(java.lang.Object sender, Throwable err, int errorCode, String errorMessage)

    Invoked when the async method throws an exception

    ###### Parameters

    - `sender`: the source of the error e.g. connection request that triggered the error, could be null

    - `err`: in case of an error triggered by an exception

    - `errorCode`: if applicable e.g. with http error codes, -1 for unknown

    - `errorMessage`: if applicable a user displayable message (can be null)