### Question:
HMapID implements MapID, Cloneable, Serializable { @Override public void plus(MapID m) { for (MapID.Entry e : m.entrySet()) { int key = e.getKey(); if (this.containsKey(key)) { this.put(key, this.get(key) + e.getValue()); } else { this.put(key, e.getValue()); } } } HMapID(int initialCapacity, float loadFactor);  HMapID(int initialCapacity);  HMapID();  HMapID(MapID m); int size(); boolean isEmpty(); double get(int key); boolean containsKey(int key); double put(int key, double value); void putAll(MapID m); double remove(int key); void clear(); boolean containsValue(double value); Object clone(); Set<Integer> keySet(); Collection<Double> values(); Set<MapID.Entry> entrySet(); String toString(); @Override void plus(MapID m); @Override double dot(MapID m); @Override void increment(int key); @Override void increment(int key, double value); double length(); void normalize(); MapID.Entry[] getEntriesSortedByValue(); MapID.Entry[] getEntriesSortedByValue(int n);  }### Answer:
@Test public void testPlus() throws IOException { HMapID m1 = new HMapID(); m1.put(1, 5.0); m1.put(2, 22.0); HMapID m2 = new HMapID(); m2.put(1, 4.0); m2.put(3, 5.0); m1.plus(m2); assertEquals(m1.size(), 3); assertTrue(m1.get(1) == 9); assertTrue(m1.get(2) == 22); assertTrue(m1.get(3) == 5); }