So, I used a JScrollPane and then I added a JTextArea. I used textArea.setCaretPosition(0) to reset the scroll and it went at the top. All good, until I wanted to set a disabled Button on enable when the scrollbar reaches at the bottom.

How can I do that?

Answer

You can listen for changes to the JScrollPane’s viewport, and compare the bottom of the viewport’s visible rectangle with the height of the viewport’s view (that is, the JTextArea):

JViewport viewport = scrollPane.getViewport();
viewport.addChangeListener(e -> {
    Rectangle rect = viewport.getViewRect();
    int bottom = rect.y + rect.height;
    endButton.setEnabled(bottom >= viewport.getViewSize().height);
});