Check if Arraylist has identical items

Question

I have the class Pessoa:

String Nome, Cidade;

public Pessoa(String Nome, String Cidade){
    this.Nome = Nome;
    this.Cidade = Cidade;
}


public String getNome() {
    return Nome;
}

public String getCidade() {
    return Cidade;
}

I have the class that adds the Pessoas in a ArrayList:

ArrayList<Pessoas> pessoas = new ArrayList<>();

public void cadastrarPessoas(){
    Pessoa pessoa = new Pessoa(Nome, Cidade);
    pessoas.add(pessoa);
}

How can I return true if case in the list of persons contains the equal added element (example below): Person = new Person

Leandro|SP
Leandro|SP

As I wished:

Leandro|SP
Leandro|RC

I tried this way but nothing happens:

Pessoa pessoa2 = new Pessoa(Nome, Cidade);

    for (Pessoa pessoa : pessoas){
                if (pessoa.equals(pessoa2)){
                    iguais
                }else{
                    pessoas.add(pessoa2);
                    diferentes
                }
            }

Answer 1

When you create the class Pessoa:

public class Pessoa {
    private String nome;
    private String cidade;

    public Pessoa(String nome, String cidade) {
        this.nome = nome;
        this.cidade = cidade;
    }

    // ... getters e setters
}

How do I know if 2 instances of this class are equal? The first idea is to use the operator ==, since it works for numbers, so why wouldn’t it work with objects?

// primeira tentativa de comparar 2 pessoas
Pessoa p1 = new Pessoa("Fulano", "SP");
Pessoa p2 = new Pessoa("Fulano", "SP");
System.out.println(p1 == p2); // false

Only this code prints false. That’s because the operator == checks whether p1 and p2 are the same object. As each one was created with new, they are 2 different objects (although with the same values).

Ah, but wait, there’s the method equals. Let’s try to use it?

// segunda tentativa de comparar 2 pessoas
Pessoa p1 = new Pessoa("Fulano", "SP");
Pessoa p2 = new Pessoa("Fulano", "SP");
System.out.println(p1.equals(p2)); // false

This code also prints false. This happens because the method equals was inherited from Object, and internally this method uses the operator ==. I traded six for half a dozen.

---

For me to consider 2 instances of Pessoa equal based on their values, I must overwrite the method equals. That is, the class Pessoa must have its own version of equals, with the rule that defines what makes 2 people equal.

In this case, 2 people are equal if their names and cities are the same, then the method would look like this:

public class Pessoa {
    ....

    @Override
    public boolean equals(Object obj) {
        Pessoa other = (Pessoa) obj;
        // verifica se o nome e cidade são iguais
        return this.nome.equals(other.nome) && this.cidade.equals(other.cidade);
    }
}

Of course this implementation can be improved by testing whether the fields are null, if obj is really an instance of Pessoa, etc, but let’s keep it simple for didactic purposes.

It is also important to overwrite the method hashCode. At this link there is a good response detailing the reasons and how to implement it properly. Here I leave the implementation suggested by Eclipse, but I suggest you read the link indicated to better understand this method:

public class Pessoa {
    ....
    // equals, getters, setters, construtor, etc

    @Override
    public int hashCode() {
        final int prime = 31;
        int result = 1;
        result = prime * result + ((cidade == null) ? 0 : cidade.hashCode());
        result = prime * result + ((nome == null) ? 0 : nome.hashCode());
        return result;
    }
}

With that, now the class Pessoa can verify if another Pessoa is just like her:

// terceira tentativa de comparar 2 pessoas
Pessoa p1 = new Pessoa("Fulano", "SP");
Pessoa p2 = new Pessoa("Fulano", "SP");
System.out.println(p1.equals(p2)); // true

Now this code prints true.

---

With that, if you have a list of Pessoa, can easily check if any Pessoa already belongs to the list:

// cria a lista e insere pessoas nela
List<Pessoa> listaPessoas = new ArrayList<>();
listaPessoas.add(new Pessoa("Fulano", "SP"));
listaPessoas.add(new Pessoa("Fulano", "RC"));

// verifica se uma pessoa qualquer pertence à lista
Pessoa pessoa = new Pessoa("Fulano", "SP");
System.out.println(listaPessoas.contains(pessoa));

The method contains uses the method equals of Pessoa to find out if two people are the same, and so checks whether the pessoa already belongs to the list. This code above prints true ("Fulano/SP" belongs to the list).

Of course you could also do the for in the list and compare each person with the method equals, but why do it if contains does exactly the same thing?

Besides, you’re making one for in the list and adding an element to this list within the for, which will generate a java.util.ConcurrentModificationException. You better use contains even:

Pessoa pessoa = new Pessoa("Fulano", "SP");
// se a pessoa não pertence à lista, adiciona
if (! listaPessoas.contains(pessoa)) {
    listaPessoas.add(pessoa);
}

---

But if you want a collection of several people without having repeated, it is better to use a java.util.Set, for a set does not allow equal elements:

// usar Set ao invés de List
Set<Pessoa> setPessoas = new HashSet<>();
// inserir pessoas
setPessoas.add(new Pessoa("Fulano", "SP"));
setPessoas.add(new Pessoa("Fulano", "RC"));
// tentar inserir Fulano/SP de novo
setPessoas.add(new Pessoa("Fulano", "SP"));

// imprime o tamanho do set
System.out.println(setPessoas.size());

This code prints out 2, for the Set will only have 2 elements. The second attempt to insert "Fulano/SP" does not work, because it already belongs to the Set.

If I used List, would have to check if the element already belongs to the list before adding it (which also works, but use Set eliminates this step).

---

If even so you need a List, can create it from the Set:

// cria um List com os elementos do Set
List<Pessoa> listaPessoas = new ArrayList<>(setPessoas);

---

Another way to implement the hashCode is using a java.util.Objects (plural, not to be confused with java.lang.Object (in the singular)):

@Override
public int hashCode() {
    // calcula o hashCode usando nome e cidade
    return Objects.hash(cidade, nome);
}