Java set focus on JButton when pressing enter

Something like this should work:

textField.addActionListener(new ActionListener()
{
    @Override
    public void actionPerformed(ActionEvent e)
    {
        button.requestFocusInWindow();
    }
});

You should use an Action for the JButton:

Action sendAction = new AbstractAction("Send") {
    public void actionPerformed(ActionEvent e) {
         // do something
    }
};

JButton  button = new JButton(sendAction);

Then you can set the same action for a JTextField or even on a MenuItem if you want the same action to be available in the Menu:

JTextField textField = new JTextField();
textField.setAction(sendAction);