What is the result of the following code?

13: String a = "";
14: a += 2;
15: a += 'c';
16: a += false;
17: if ( a == "2cfalse") System.out.println("==");
18: if ( a.equals("2cfalse")) System.out.println("equals");

❌ A. Compile error on line 14.
❌ B. Compile error on line 15.
❌ C. Compile error on line 16.
❌ D. Compile error on another line.
❌ E. ==
✅ F. equals
❌ G. An exception is thrown

Explanation:
a += 2 expands to a = a + 2. A String concatenated with any other type gives a String. Lines 14, 15, and 16 all append to a, giving a result of "2cfalse". The if statement on line 18 returns false because the values of the two String objects are the same using object equality. The if statement on line 17 returns false because the two String objects are not the same in memory. One comes directly from the string pool and the other comes from building using String operations.