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copied!<p>I believe I understand what you're shooting for better now. You're looking to perform user input validation.</p> <p>When you're doing your user validation, there's really two ways to approach it:</p> <ol> <li>Validate immediately after any change takes place and provide feedback</li> <li>Validate when the focus leaves the input area</li> </ol> <p>With both, you'll be using property listeners - it's just a matter of what property listener you're dealing with.</p> <p>In the first case you'll listen directly to the property you're validating:</p> <pre><code> TextField field = new TextField(); field.textProperty().addListener(new ChangeListener<String>(){ @Override public void changed(ObservableValue<? extends String> value, String oldValue, String newValue) { //Do your validation or revert the value }}); </code></pre> <p>In the second case, you'll listen to the <code>focused</code> property, and validate when focus is lost (you can maintain the last validated value in this listener to help revert the value if necessary):</p> <pre><code> TextField field = new TextField(); field.focusedProperty().addListener(new ChangeListener<Boolean>(){ String lastValidatedValue = ""; @Override public void changed(ObservableValue<? extends Boolean> value, Boolean oldValue, Boolean newValue) { if(newValue == false && oldValue == true){ //Do your validation and set `lastValidatedValue` if valid } }}); </code></pre> <p><strong>Note:</strong> I was assuming you just wanted to put in a fail safe for system code updating the UI. I'll leave my previous answer as I believe it provides useful information as well.</p>

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