HashSet retainAll using interface

Question

I have some code where I try to use the HashSet.retainAll() function.

In the example code below, the HashSet contains the interface IPerson , but the equals functions in object Person is never reached. I have even tried to expose the equals function in the interface and several other things. I feel I have tried everything. How can I make retainAll() use my implemented equal function?

class Person implements IPerson {
  private String name;
  public Person(String name){
    this.name = name;
  }

  @Override
  public boolean equals(Object obj){
    System.out.println("calling equals");
    return super.equals(Object obj);
  }

}

HashSet<IPerson> persons1 = new HashSet<IPerson>();
persons1.add(new Person("Jane"));
persons1.add(new Person("Joel"));
persons1.add(new Person("Joe"));

HashSet<IPerson> persons2 = new HashSet<IPerson>();
persons2.add(new Person("Jane"));
persons2.add(new Person("Joel"));

persons1.retainAll(persons2);
// expect sysout from Person.equals()

System.out.println(persons1.size());
// prints 0

Answer 1

您需要考虑name以检查是否equal并将其包括在内以计算hashCode并确保您遵循hashcode并equals合同

Answer 2

You need to override hashCode because the hashset uses the hashcode to find the right 'bucket' first and only calls equals after it finds something in it. That's why your equals method is never called. (if you don't override the hashcode method, it gives every new object a different hashcode, so even if you call new Person("name") twice with the same name they won't have the same hashcode)