Method References

// Lambda
Function<String, Integer> parser = s -> Integer.parseInt(s);

// Method reference
Function<String, Integer> parser = Integer::parseInt;

// Usage
List<Integer> numbers = List.of("1", "2", "3").stream()
    .map(Integer::parseInt)
    .toList();

String prefix = "Hello, ";
Function<String, String> greeter = s -> prefix.concat(s);

// Method reference to a specific instance
Function<String, String> greeter = prefix::concat;

// Usage with a specific logger
Consumer<String> logger = System.out::println;
List.of("a", "b", "c").forEach(System.out::println);

// Lambda: str is the instance, trim() is called on it
Function<String, String> trimmer = str -> str.trim();

// Method reference
Function<String, String> trimmer = String::trim;

// More examples
Predicate<String> isEmpty = String::isEmpty;
Comparator<String> comp   = String::compareTo;

// In streams
List<String> trimmed = names.stream()
    .map(String::trim)
    .filter(Predicate.not(String::isEmpty))
    .toList();

// Lambda
Supplier<ArrayList<String>> factory = () -> new ArrayList<>();

// Constructor reference
Supplier<ArrayList<String>> factory = ArrayList::new;

// With arguments
Function<String, StringBuilder> sbFactory = StringBuilder::new;

// Copying a list
List<String> copy = original.stream()
    .collect(Collectors.toCollection(ArrayList::new));

List<Person> people = ...;

// Extracting fields
List<String> names = people.stream()
    .map(Person::getName)      // instance method reference
    .sorted(String::compareTo) // comparator
    .toList();

// Sorting
people.sort(Comparator.comparing(Person::getName)
    .thenComparing(Person::getAge));

// Predicate composition
Predicate<String> isEmail = String::contains; // needs a parameter — this won't work
// Use lambdas when arguments are needed:
Predicate<String> isEmail = s -> s.contains("@");

// Event handlers
button.addActionListener(this::handleClick);

// Collectors
Map<String, List<Person>> byCity = people.stream()
    .collect(Collectors.groupingBy(Person::getCity));