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The first reason:

Check whether a keyboard listener is added to the control, i.e. addKeyListener() method

The second reason: (mostly this reason)

Focus issue, that is, the controls that need to be monitored do not gain screen focus.

Here are the two methods of right to add screen focus to the control

1. Call the control's setFocusable(true) method, which should be placed in front of setVisible(true)
2. Call the control's requestFocus() method, which should be placed after setVisible(true)