ListBox change event

Could some one please tell me why this doesn't generate any change events:

<ui:UiBinder xmlns:ui="urn:ui:com.google.gwt.uibinder"

xmlns:g="urn:import:com.google.gwt.user.client.ui">

<ui:style>

</ui:style>

<g:HTMLPanel>

<g:ListBox ui:field="category"></g:ListBox>

</g:HTMLPanel>

</ui:UiBinder> 

public class Invoices extends Composite {

private static InvoicesUiBinder uiBinder = GWT.create(InvoicesUiBinder.class);

interface InvoicesUiBinder extends UiBinder<Widget, Invoices> {

}

@UiField

ListBox category;

public Invoices() {

initWidget(uiBinder.createAndBindUi(this));

category.setVisibleItemCount(1);

category.addItem("--- Select ---");

category.addItem("H");

category.addItem("T");

category.addItem("S");

}

@UiHandler("category")

void categoryChange(ChangeEvent event) {

Window.alert("changed");

}

}

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