### Question:
MsgFmt { static Pair<String, Optional<Throwable>> fmt(String msg, Object... args) { if (Util.isEmpty(args)) { return Pair.of(msg, Optional.<Throwable>empty()); } boolean hasThrowable = false; if (args[args.length - 1] instanceof Throwable) { hasThrowable = true; } if (msg == null) { msg = Strings.EMPTY; } StringBuilder sb = new StringBuilder(msg.length()); int head = 0; for (int indexOfBrace, argI = 0; (indexOfBrace = msg.indexOf("{}", head)) != -1; ) { for (int i = head; i < indexOfBrace; i++) { sb.append(msg.charAt(i)); } if (argI < args.length) { sb.append(args[argI++]); } else { sb.append("{}"); } head = indexOfBrace + 2; } if (head < msg.length()) { for (int i = head; i < msg.length(); i++) { sb.append(msg.charAt(i)); } } return Pair.of(sb.toString(), hasThrowable ? Optional.of((Throwable) args[args.length - 1]) : Optional.<Throwable>empty()); }    }### Answer:
@Test public void fmt() { Pair<String, Optional<Throwable>> formatted = MsgFmt.fmt("abc"); assertEquals("abc", formatted.getKey()); assertFalse(formatted.getValue().isPresent()); formatted = MsgFmt.fmt("ab{}d{}", "c", 'e'); assertEquals("abcde", formatted.getKey()); assertFalse(formatted.getValue().isPresent()); formatted = MsgFmt.fmt("ab{}d{} {}", "c", 'e'); assertEquals("abcde {}", formatted.getKey()); assertFalse(formatted.getValue().isPresent()); formatted = MsgFmt.fmt("ab{}d", "c", 'e'); assertEquals("abcd", formatted.getKey()); assertFalse(formatted.getValue().isPresent()); Exception ex = new Exception(); formatted = MsgFmt.fmt("ab", ex); assertEquals("ab", formatted.getKey()); assertEquals(ex, formatted.getValue().get()); ex = new RuntimeException(); formatted = MsgFmt.fmt("ab{}", "c", ex); assertEquals("abc", formatted.getKey()); assertEquals(ex, formatted.getValue().get()); ex = new Exception(); formatted = MsgFmt.fmt(null, 1, 2, ex); assertTrue(Strings.isEmpty(formatted.getKey())); assertEquals(ex, formatted.getValue().get()); }