I am trying to generate an invoice number that contains name, branch and city with current date. I have gathered the information from JComboboxes and SimpleDateFormat. But in the end after we concat all the values and set them to a Jtextfield I get string index out of bound exception. 

I am new to java and don't have much knowledge about it whatever efforts I have made for this are given as follows. I will be grateful to people interested in supporting me. Boots and Bouquets both are welcome.

```
private void bt_generateActionPerformed(java.awt.event.ActionEvent evt) {
    if (evt.getSource() == bt_generate) {

        if ((cb_bkname.getSelectedItem()
                .equals("<html><font color=\"red\">SELECT NAME</font></html>"))
            || (cb_brname.getSelectedItem()
                .equals("<html><font color=\"red\">SELECT BRANCH</font></html>"))
            || (cb_plname.getSelectedItem()
                .equals("<html><font color=\"red\">SELECT PLACE</font></html>"))
        ){

        } else {
            String datePrefix = new SimpleDateFormat("MMMM dd, YYYY")
                    .format(new Date());
            tf_rm_dt.setText(datePrefix);
            Object name = cb_bkname.getSelectedItem();
            String bn = name.toString().substring(0, 3);
            Object branch = cb_brname.getSelectedItem();
            String br = branch.toString().substring(0, 4);
            Object city = cb_bkname.getSelectedItem();
            String pl = city.toString().substring(0, 4);
            String curdt = new SimpleDateFormat("dd-MM-YY")
                    .format(new Date());
            tf_rm_id.setText("" + bn + "/" + br + "/" + "/" + curdt);
        }
    }

}

```

The StackTrace is as follows:

```
run:
Exception in thread "AWT-EventQueue-0" java.lang.StringIndexOutOfBoundsException: String index out of range: 4
    at java.lang.String.substring(String.java:1955)
    at myproj.DATAENTRY.bt_generateActionPerformed(DATAENTRY.java:2215)
    at myproj.DATAENTRY.access$2300(DATAENTRY.java:20)
    at myproj.DATAENTRY$24.actionPerformed(DATAENTRY.java:597)
    at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2018)
    at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2341)
    at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:402)
    at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:259)

```