从回调中返回字符串 - Java

does anyone know how I can solve the following problem. I want to return a String from a callback, but I get only "The final local variable s cannot be assigned, since it is defined in an enclosing type", because of final.

 public String getConstraint(int indexFdg) {
    final String s;
    AsyncCallback<String> callback = new AsyncCallback<String>() {
        public void onFailure(Throwable caught) {
            caught.printStackTrace();
        }

        public void onSuccess(String result) {
            s = result;
        }
    };
    SpeicherService.Util.getInstance().getConstraint(indexFdg, callback);
    return s;
    }

推荐答案

异步回调的重点是通知你异步发生的事情，在未来的某个时间.如果要在方法完成运行后设置，则不能从 getConstraint 返回 s.

 String s = getConstraint();
 someGuiLabel.setText(s);

 myCallback = new AsyncCallback<String>() {
     public void onSuccess(String result) {
         someGuiLabel.setText(result);
     }
 }
 fetchConstraintAsynchronously(myCallback);