Java：在最终类中实现接口的类

Question

这可能是一个愚蠢的问题，但我在这里感到困惑。我有以下情况：

Main.java

public class Main {

    public static void main (String args[]){
        GenericTag[] arr = new GenericTag[2];
        arr[0] = new Authentication("", "", "", "");
        arr[1] = new Document("", "", "", "");
        byte[] foo= Base64.decodeBase64(XmlBuilder.generate(arr));
        System.out.println(new String(foo));
    }

XmlBuilder.java

public final class XmlBuilder {
    private static final String OPEN_TAG = "";
    private static final String CLOSE_TAG = "";

    public static byte[] generate(GenericTag[] tags){

        String xml = OPEN_TAG;
        for(int i=0; i<tags.length; i++){
            xml += tags[i].xml;
        }
        xml += CLOSE_TAG;

        return Base64.encodeBase64(xml.getBytes());
    }

    public interface GenericTag{
         public String getXml();
    }

    public class Authentication implements GenericTag{
        private static final String OPEN_TAG = "<authentication>";
        private static final String CLOSE_TAG = "</autentication>";
        //some tags

        public Authentication (/*some parameters*/){
            xml = OPEN_TAG;
            //xml building
            xml += CLOSE_TAG;
        }

        @Override
        public String getXml() {
            return xml;
        }
    }

    public class Document implements GenericTag{
        private static final String OPEN_TAG = "<document>";
        private static final String CLOSE_TAG = "</document>";
        //some tags

        public String xml;

        public Documento (/*some params*/){
            xml = OPEN_TAG;
            //xml building
            xml += CLOSE_TAG;
        }
    @Override
    public String getXml() {
        return xml;
    }
    }
}

我无法让它工作。编译器说 Authentication 和 Document 都不能解析为一个类型。如果我明确声明 new XmlBuilder.Authentication 它会说

No enclosing instance of type XmlBuilder is accessible. Must qualify the allocation with an enclosing instance of type XmlBuilder (e.g. x.new A() where x is an instance of XmlBuilder).

我在这里做错了什么？

Answer 1

创建课程Authentication 和Document public static。由于这些不是static，因此您只能从XmlBuilder 实例中实例化它们。

Java inner class and static nested class - 你可以在这里找到更多信息

Answer 2

您尝试创建抽象类的内部类的实例。 使这些类独立。

Answer 3

首先，您有一些语法错误，其次，您需要将内部类设为静态，以便它们不依赖外部实例而是独立存在。您需要将它们重构为不依赖父类的任何字段。更好的方式是将它们放在自己的源文件中，但在一个文件中，您的代码的编译版本将是：

import java.util.Base64;

public abstract class XmlBuilder {
    private static final String OPEN_TAG = "";
    private static final String CLOSE_TAG = "";
    private String xml;

    public static byte[] generate(GenericTag[] tags){

        String xml = OPEN_TAG;
        for(int i=0; i<tags.length; i++){
            xml += tags[i].getXml();
        }
        xml += CLOSE_TAG;

        return Base64.getEncoder().encode(xml.getBytes());
    }

    public static interface GenericTag{
        public String getXml();
    }

    public static class Authentication implements GenericTag{
        private static final String OPEN_TAG = "<authentication>";
        private static final String CLOSE_TAG = "</autentication>";
        private static String xml;
        //some tags

        public Authentication (/*some parameters*/){
            xml = OPEN_TAG;
            //xml building
            xml += CLOSE_TAG;
        }

        @Override
        public String getXml() {
            return xml;
        }
    }

    public static class Document implements GenericTag{
        private static final String OPEN_TAG = "<document>";
        private static final String CLOSE_TAG = "</document>";
        //some tags

        public String xml;

        public Document (/*some params*/){
            xml = OPEN_TAG;
            //xml building
            xml += CLOSE_TAG;
        }
    @Override
    public String getXml() {
        return xml;
    }
    }
}

还有主类：

import java.util.Base64;

public class Main {

    public static void main(String[] args)
    {
        XmlBuilder.GenericTag[] arr = new XmlBuilder.GenericTag[2];
        arr[0] = new XmlBuilder.Authentication();
        arr[1] = new XmlBuilder.Document();
        byte[] foo= Base64.getDecoder().decode(XmlBuilder.generate(arr));
        System.out.println(new String(foo));
    }

}