Java new 关键字调用默认构造器，在反编译后就看不懂了

这个例子是 java 编程思想——17.2.3 小节.使用 abstract 类。这个例子作者就是想，体现一下“享元”设计模式，大概意思就是容器的数据不一定非得存在容器内部。

package net.mindview.util;

import java.util.*;//删掉所有 import，改成*

public class Countries {
    public static final String[][] DATA = new String[][]{{"ALGERIA", "Algiers"}, {"ANGOLA", "Luanda"}, {"BENIN", "Porto-Novo"}, {"BOTSWANA", "Gaberone"}, {"BURKINA FASO", "Ouagadougou"}, {"BURUNDI", "Bujumbura"}};//删掉了一些
    static Map<String, String> map = new Countries.FlyweightMap((Countries.FlyweightMap)null);//这句不懂？
    static List<String> names;

    static {
        names = new ArrayList(map.keySet());
    }

    public Countries() {
    }

    static Map<String, String> select(final int size) {
        return new Countries.FlyweightMap() {
            public Set<java.util.Map.Entry<String, String>> entrySet() {
                return new Countries.FlyweightMap.EntrySet(size);
            }
        };
    }

    public static Map<String, String> capitals() {
        return map;
    }

    public static Map<String, String> capitals(int size) {
        return select(size);
    }

    public static List<String> names() {
        return names;
    }

    public static List<String> names(int size) {
        return new ArrayList(select(size).keySet());
    }

    public static void main(String[] args) {
        Print.print(capitals(10));
        Print.print(names(10));
        Print.print(new HashMap(capitals(3)));
        Print.print(new LinkedHashMap(capitals(3)));
        Print.print(new TreeMap(capitals(3)));
        Print.print(new Hashtable(capitals(3)));
        Print.print(new HashSet(names(6)));
        Print.print(new LinkedHashSet(names(6)));
        Print.print(new TreeSet(names(6)));
        Print.print(new ArrayList(names(6)));
        Print.print(new LinkedList(names(6)));
        Print.print(capitals().get("BRAZIL"));
    }
    //就是个静态内部类
    private static class FlyweightMap extends AbstractMap<String, String> {
        private static Set<java.util.Map.Entry<String, String>> entries;

        static {
            entries = new Countries.FlyweightMap.EntrySet(Countries.DATA.length);
        }

        private FlyweightMap() {
        }

        public Set<java.util.Map.Entry<String, String>> entrySet() {
            return entries;
        }
//太长了，省略

但这是通过 IDEA 查看 jar 包里面的 class 文件的反编译视角看的源码（当初只找到 net.mindview.util 的 jar 包，没找到它的源码），然后发现有个地方不大懂：

就是static Map<String, String> map = new Countries.FlyweightMap((Countries.FlyweightMap)null);这句，这句到原书里面是static Map<String, String> map = new FlyweightMap();

原来只是调用默认构造器，现在是给默认构造器传一个强转后的null，这是什么操作啊？（恕我愚昧，这个问题我确实没百度到）