CODEWITHSUNDEEP
javacollectionshashset
LeMinh411
LeMinh411

Reputation: 3

Two Set contain same elements but they are not equal. Why that result?

I try to check two set and I need both result are TRUE but they are not. I don't know why one of result is return false. Please help me, many thanks.

class CollectionExample:

    Set<String> set1 = new HashSet<>();
    set1.add(new String("A"));
    set1.add(new String("B"));
    Set<String> set2 = new HashSet<>();
    set2.add("A");
    set2.add("B");

    System.out.println("set1.equal(set2): "+ set1.equals(set2));

    Set<Person1> set3 = new HashSet<>();
    set3.add(new Person1("A", "1"));
    set3.add(new Person1("B", "1"));
    Set<Person1> set4 = new HashSet<>();
    set4.add(new Person1("A", "1"));
    set4.add(new Person1("B", "1"));

    System.out.println("set3.equal(set4): "+ set3.equals(set4));

entity Person1:

public class Person1 {

String firstName;
String lastName;

public Person1(String firstName, String lastName) {
    // TODO Auto-generated constructor stub
    this.firstName = firstName;
    this.lastName = lastName;
}

public String firstName() { return firstName;}
public String lastName() { return lastName;}
}

Result:

set1.equal(set2): true
set3.equal(set4): false

Upvotes: 0

Views: 1499

Answers (2)

Arpit Sharma
Arpit Sharma

Reputation: 910

The issue is not because of the comparison of Set but that of the Person class.

If two objects are equal according to the equals(Object) method, then calling the hashCode method on each of the two objects must produce the same integer result.

But it is not necessary that if two objects have same hashcode() then the two objects are equals() as well.

Even if the computed hashcode() is the same, the objects are not equals().
This is because the equals() method inherited from the Object class compares the reference as well. Hence the output.

Upvotes: 0

jacobm
jacobm

Reputation: 14035

You need to override equals() and hashCode() in your Person class. Them implementation of HashSet uses those to determine if two Person instances are equal; if you don't override them then you'll get the default Object implementation in which two Person instances are equal only if they are the exact same reference.

Upvotes: 1

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