浅谈对Java双冒号::的理解

list.forEach(item -> System.out.println(item));

list.forEach(item -> {
 int numA = item.getNumA();
 int numB = item.getNumB();
 System.out.println(numA + numB);
});

//不使用双冒号
list.forEach(item -> System.out.println(item));
//使用双冒号
list.forEach(System.out::println);

//化简前
list.forEach(item -> System.out.println(item));
//化简后
list.forEach(System.out::println);

StringBuilder stringBuilder = new StringBuilder();
//化简前
IntStream.range(1, 101).forEach(item -> stringBuilder.append(item));
//化简后
IntStream.range(1, 101).forEach(stringBuilder::append);

public <T, SOURCE extends Collection<T>, DEST extends Collection<T>>
DEST transferElements(SOURCE sourceCollection, Supplier<DEST> collectionFactory) {
  DEST result = collectionFactory.get();
  result.addAll(sourceCollection);
  return result;
}

//化简前
Set<Person> rosterSetLambda = transferElements(roster, () -> new HashSet<>());
//化简后
Set<Person> rosterSet = transferElements(roster, HashSet::new);

@FunctionalInterface
public interface Supplier<T> {

  /**
   * Gets a result.
   *
   * @return a result
   */
  T get();
}

@Data
public class ModelA {
  private String id;

  public ModelA(String id) {
    this.id = id;
  }

  public ModelA() {
  }
}

class ClassB {
  private final List<ModelA> list = new ArrayList<>();

  public void add(String string, Function<String, ModelA> function) {
    list.add(function.apply(string));
  }
}

ClassB classB = new ClassB();d
//化简前
classB.add("ddd", item -> new ModelA(item));
//化简后
classB.add("ddd", ModelA::new);

//化简前
List<String> stringList = list.stream().map(item -> item.getId()).collect(Collectors.toList());
//化简后
List<String> stringList = list.stream().map(ModelA::getId).collect(Collectors.toList());

String[] stringArray = {"Barbara", "James", "Mary", "John"};
//化简前
Arrays.sort(stringArray, (a,b) -> a.compareToIgnoreCase(b));
//化简后
Arrays.sort(stringArray, String::compareToIgnoreCase);