tree map中的tailmap是如何实现的？

[volcanodia]

treemap中的tailmap是返回大于某个key的子map
查看源代码，不断的trace发现tailmap这个方法在调用treemap的内部类
最后是return new navigablesubmap(xxxx)
在这个this.m=m后，debug看到的this变量是
>Exception occurred in target VM:
java.lang.NullPointerException:
at TreeMap.compare(TreeMap.java:1192)
at TreeMap.getHigherEntry(TreeMap.java:460)
at TreeMap$NavigableSubMap.absLowest(TreeMap.java:1323)
at TreeMap$AscendingSubMap$AscendingEntrySetView.iterator(TreeMap.java:1758)
at TreeMap$NavigableSubMap$EntrySetView.size(TreeMap.java:1531)
at TreeMap$NavigableSubMap.size(TreeMap.java:1404)
at TreeMap$NavigableSubMap.<init>(TreeMap.java:1274)
at TreeMap$AscendingSubMap.<init>(TreeMap.java:1705)
at TreeMap.tailMap(TreeMap.java:909)
at TreeMap.tailMap(TreeMap.java:944)
at NewClass1.main(NewClass1.java:22)
<
在this.toEnd=toEnd后，再看this变量，这个exception已经没了，并且this中是一个想要的submap了（既结果）
求教这个是怎么实现的？另外exception里面为啥会出现at TreeMap$NavigableSubMap.size(TreeMap.java:1404)

NavigableSubMap(TreeMap<K,V> m,
                        boolean fromStart, K lo, boolean loInclusive,
                        boolean toEnd,     K hi, boolean hiInclusive) {
            if (!fromStart && !toEnd) {
                if (m.compare(lo, hi) > 0)
                    throw new IllegalArgumentException("fromKey > toKey");
            } else {
                if (!fromStart) // type check
                    m.compare(lo, lo);
                if (!toEnd)
                    m.compare(hi, hi);
            }

            this.m = m;
            this.fromStart = fromStart;
            this.lo = lo;
            this.loInclusive = loInclusive;
            this.toEnd = toEnd;
            this.hi = hi;
            this.hiInclusive = hiInclusive;
        }