public Set<K> getKeySet() {
        if (mKeySet == null) {
            mKeySet = new KeySet();
        }
        return mKeySet;
    }
Helper for writing standard Java collection interfaces to a data
structure like {@link ArrayMap}.

@author RePlugin Team
replugin-host-library/replugin-host-lib/src/main/java/com/qihoo360/replugin/utils/basic/MapCollections.java