Method equals

generics/Holder.java:13–16

(Object o)

  public void set(T val) { value = val; }
  public T get() { return value; }
  @Override public boolean equals(Object o) {
    return o instanceof Holder &&
      Objects.equals(value, ((Holder)o).value);
  }
  @Override public int hashCode() {
    return Objects.hashCode(value);
  }