alert_ed = (EditText) a.findViewById(R.id.alert_ed_queue_num);

alert_ed.addTextChangedListener(mTextWatcher);

private TextWatcher mTextWatcher = new TextWatcher() {

    public void afterTextChanged(Editable s) {
        // 先去掉监听器，否则会出现栈溢出
        alert_ed.removeTextChangedListener(mTextWatcher);
        alert_ed.setText(s.toString().replace("d",""));
        alert_ed.setSelection(alert_ed.getText().length());
        alert_ed.addTextChangedListener(mTextWatcher);
    }
    public void beforeTextChanged(CharSequence s, int start, int count, int after) {
    }
    public void onTextChanged(CharSequence s, int start, int before, int count) {
    }
};