### Question:
Int2LongOpenHashMapWritable extends Int2LongOpenHashMap implements Writable { public void plus(Int2LongOpenHashMapWritable m) { for (Int2LongMap.Entry e : m.int2LongEntrySet()) { int key = e.getIntKey(); long value = e.getLongValue(); if (this.containsKey(key)) { this.put(key, this.get(key) + value); } else { this.put(key, value); } } } 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 testPlus() throws IOException { Int2LongOpenHashMapWritable m1 = new Int2LongOpenHashMapWritable(); m1.put(1, 5L); m1.put(2, 22L); Int2LongOpenHashMapWritable m2 = new Int2LongOpenHashMapWritable(); m2.put(1, 4L); m2.put(3, 5L); m1.plus(m2); assertEquals(3L, m1.size()); assertEquals(9L, m1.get(1)); assertEquals(22L, m1.get(2)); assertEquals(5L, m1.get(3)); }