### Question:
Int2LongOpenHashMapWritable extends Int2LongOpenHashMap implements Writable { public long dot(Int2LongOpenHashMapWritable m) { int s = 0; for (Int2LongMap.Entry e : m.int2LongEntrySet()) { int key = e.getIntKey(); if (this.containsKey(key)) { s += this.get(key) * e.getLongValue(); } } return s; } Int2LongOpenHashMapWritable(); void readFields(DataInput in); void decode(); boolean hasBeenDecoded(); void write(DataOutput out); byte[] serialize(); static Int2LongOpenHashMapWritable create(DataInput in); static Int2LongOpenHashMapWritable create(byte[] bytes); void plus(Int2LongOpenHashMapWritable m); long dot(Int2LongOpenHashMapWritable m); void increment(int key); void increment(int key, long n); static void setLazyDecodeFlag(boolean b); static boolean getLazyDecodeFlag(); int[] getKeys(); long[] getValues(); void lazyplus(Int2LongOpenHashMapWritable m); Int2LongMap.Entry[] getEntriesSortedByValue(); Int2LongMap.Entry[] getEntriesSortedByValue(int k);  }### Answer:
@Test public void testDot() throws IOException { Int2LongOpenHashMapWritable m1 = new Int2LongOpenHashMapWritable(); m1.put(1, 2L); m1.put(2, 1L); m1.put(3, 3L); Int2LongOpenHashMapWritable m2 = new Int2LongOpenHashMapWritable(); m2.put(1, 1L); m2.put(2, 4L); m2.put(4, 5L); long s = m1.dot(m2); assertEquals(6, s); }