JAVA Accept only numbers in TextField - KeyListener

This simple Java Swing exercise`s goal was to create textfield and restrict that only numbers from 0 to 9 will be accepted as input. We can do it by assigning keylistener to it and checking that current character is between the range of '0' and '9'
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[grzechu]

Could you advise how to put string "Invalid character" into some JLabel field?

[DeathlySecurity]

i have a .setcontentpane panel thing and it doesnt seem to find the keytyped thing

[ebelechukwunmoemenam]

what could possibly be wrong? I've tried to run this code, only that i'm using JOptionPane, to output an error message "invalid characters". It tells me it cannot find or load main class.