one way to intersect sets

set-intersection.java

import java.util.HashSet;
import java.util.Set;
import java.util.function.Consumer;

class Scratch {
  public static void main(String[] args) {
    final Set<Integer> a = Set.of(1, 2, 3);
    final Set<Integer> b = Set.of(3, 4, 5);
    System.out.println("intersection: " + intersect(a, b));
  }


  static <T> Set<T> intersect(final Set<T> a, final Set<T> b) {
    return returning(new HashSet<T>(a), copy -> copy.retainAll(b));
  }

  /**
   * Return {@param val} after invoking {@param f} for side-effects.
   * @param val is the value to return
   * @param f is invoked for side-effects
   * @param <T> is any class
   * @return {@param val}
   */
  public static <T> T returning(final T val, final Consumer<T> f) {
    f.accept(val);
    return val;
  }

}