Java类的继承

XHTMLMapper继承 XWPFDocumentVisitor：
由于endVisitTableCell是抽象方法，XHTMLMapper中必须要实现；
existErr()子类是否重写都是自由的；

public abstract class XWPFDocumentVisitor<T, O extends Options, E extends IXWPFMasterPage> implements IMasterPageHandler<E> {
	public void start() throws Exception {
        ...
        ...
        existErr()
    }
 	protected abstract void endVisitTableCell(XWPFTableCell cell, T tableContainer, T tableCellContainer)
            throws Exception;
    public boolean existErr(){return false;}
}
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public class XHTMLMapper extends XWPFDocumentVisitor<Object, XHTMLOptions, XHTMLMasterPage> {
	@Override
    protected void endVisitTableCell(XWPFTableCell cell, Object tableContainer, Object tableCellContainer) throws Exception {
        endElement(TD_ELEMENT);
    }
	@Override
    public boolean existErr(){
        return SAXHelper.existErr(contentHandler);
    }
}
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XHTMLMapper mapper = new XHTMLMapper(document, contentHandler, options);
mapper.start(); // 执行父类的start()，start()内部的 existErr()会调用子类的 existErr()
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