### Question:
Int2IntOpenHashMapWritable extends Int2IntOpenHashMap implements Writable { public long dot(Int2IntOpenHashMapWritable m) { int s = 0; for (Int2IntMap.Entry e : m.int2IntEntrySet()) { int key = e.getIntKey(); if (this.containsKey(key)) { s += this.get(key) * e.getIntValue(); } } return s; } Int2IntOpenHashMapWritable(); @Override void readFields(DataInput in); void decode(); boolean hasBeenDecoded(); @Override void write(DataOutput out); byte[] serialize(); static Int2IntOpenHashMapWritable create(DataInput in); static Int2IntOpenHashMapWritable create(byte[] bytes); void plus(Int2IntOpenHashMapWritable m); long dot(Int2IntOpenHashMapWritable m); void increment(int key); void increment(int key, int n); static void setLazyDecodeFlag(boolean b); static boolean getLazyDecodeFlag(); int[] getKeys(); int[] getValues(); void lazyplus(Int2IntOpenHashMapWritable m); Int2IntMap.Entry[] getEntriesSortedByValue(); Int2IntMap.Entry[] getEntriesSortedByValue(int k);  }### Answer:
@Test public void testDot() throws IOException { Int2IntOpenHashMapWritable m1 = new Int2IntOpenHashMapWritable(); m1.put(1, 2); m1.put(2, 1); m1.put(3, 3); Int2IntOpenHashMapWritable m2 = new Int2IntOpenHashMapWritable(); m2.put(1, 1); m2.put(2, 4); m2.put(4, 5); long s = m1.dot(m2); assertEquals(6, s); }