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A solution is not to put any code into the actionPerfomed method other than a call to another method in the enclosing class. This method should have access to everything you need. Something like...
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CheckedTime callStart = new CheckedTime.fromString(startTime);

Using the new keyword means that you are creating a new object on that line. But your fromString method is creating and returning the...

Two choices.

Do as the error message says and make the variable final (the textfield not the string)
Make the textfield an instance variable and not local to whatever constructor/method you are...