### Question:
Holdings { Holdings add(String symbol, int newShares) { int currentShares = 0; if (holdings.containsKey(symbol)) { currentShares = holdings.get(symbol); } return new Holdings(holdings.plus(symbol, currentShares + newShares)); }    }### Answer:
@Test public void add() { Holdings holdings = Holdings.EMPTY .add("ABC", 10); assertEquals(holdings.getShareCount("ABC"), 10); }