Java SE 8 Programmer I (OCAJP 8): Working with Selected Classes from the Java API

Working with Selected Classes from the Java API

This domain covers String, StringBuilder, ArrayList, key java.time types, and simple lambda expressions with Predicate. The exam focuses on immutability, index rules, overloaded methods, and target typing.

String and StringBuilder

String is immutable. Methods such as substring, replace, trim, toUpperCase, and concat return a new string; the original is unchanged. substring(begin, end) includes begin and excludes end.
StringBuilder is mutable and not thread-safe: append, insert, delete, deleteCharAt, reverse, and toString change or expose its contents. StringBuilder does not override equals, so == and equals both compare references.
ArrayList is a resizable generic list: add, get, set, size, contains, and remove. remove(int index) removes by position; remove(Object o) removes the first equal element. Autoboxing can make the choice subtle.

java.time and lambdas

LocalDate, LocalTime, and LocalDateTime are immutable; plusDays, minusHours, and similar methods return new objects. Period represents a date-based amount, Duration a time-based amount. DateTimeFormatter parses and formats; pattern letters are case-sensitive (MM month, mm minute). A lambda assigned to Predicate<T> implements boolean test(T t).

Worked example

String s = "java";
s.substring(1, 3);
StringBuilder sb = new StringBuilder(s);
sb.append("8").reverse();
System.out.println(s + " " + sb);

substring returns av but the result is discarded, so s remains java. The builder becomes java8, then 8avaj. Output: java 8avaj.

Traps

String.charAt and substring throw StringIndexOutOfBoundsException for invalid indexes.
ArrayList.get(size()) and remove(size()) throw IndexOutOfBoundsException.
List<Integer>.remove(1) selects the index overload, not the value overload.
LocalDate methods do not mutate the receiver.
A lambda needs a functional interface target; Predicate<String> p = s -> s.isEmpty(); is valid.