Summary
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Add getModel() method to ButtonModel interface.

Problem
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As stated in JDK-8182577, when a custom ButtonModel is used and keyboard focus is moved by pressing TAB key, then in LayoutFocusTraversalPolicy.accept() method, it typecasts buttonmodel object to JToggleButton.ToggleButtonModel which results in ClassCastException.

Solution
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Added getModel() method to ButtonModel interface [src/java.desktop/share/classes/javax/swing/ButtonModel.java] would result in custom ButtonModel to implement this method. Also, LayoutFocusTraversalPolicy.accept() has been modified to store the model object as ButtonModel object instead of wrongly assuming as JToggleButton.ToggleButtonModel.
Review thread : http://mail.openjdk.java.net/pipermail/swing-dev/2017-June/007521.html

Specification
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    /**
     * Returns the group that the button belongs to.
     * Normally used with radio buttons, which are mutually
     * exclusive within their group.
     *
     * @implSpec By default, this method returns {@code null}.
     * Subclasses should return the group set by setGroup().
     * 
     * @return the <code>ButtonGroup</code> that the button belongs to
     * @since 10
     */
    default ButtonGroup getGroup() {
        return null;
    }