Check if a value exists in ArrayList

Question

How can I check if a value exists in an ArrayList?

List<CurrentAccount> lista = new ArrayList<CurrentAccount>();

CurrentAccount conta1 = new CurrentAccount("Alberto Carlos", 1052);
CurrentAccount conta2 = new CurrentAccount("Pedro Fonseca", 30);
CurrentAccount conta3 = new CurrentAccount("Ricardo Vitor", 1534);
CurrentAccount conta4 = new CurrentAccount("João Lopes", 3135);

lista.add(conta1);
lista.add(conta2);
lista.add(conta3);
lista.add(conta4);

Collections.sort(lista);

System.out.printf("Bank Accounts:" + "%n");
Iterator<CurrentAccount> itr = lista.iterator();
while (itr.hasNext()) {
    CurrentAccount element = itr.next();
    System.out.printf(element + " " + "%n");
}
System.out.println();

Answer 1

Just use ArrayList.contains(desiredElement). For example, if you're looking for the conta1 account from your example, you could use something like:

if (lista.contains(conta1)) {
    System.out.println("Account found");
} else {
    System.out.println("Account not found");
}

Edit: Note that in order for this to work, you will need to properly override the equals() and hashCode() methods. If you are using Eclipse IDE, then you can have these methods generated by first opening the source file for your CurrentAccount object and the selecting Source > Generate hashCode() and equals()...

Answer 2

Better to use a HashSet than an ArrayList when you are checking for existence of a value.

Java docs for HashSet says

This class offers constant time performance for the basic operations (add, remove, contains and size)

ArrayList.contains() might have to iterate the whole list to find the instance you are looking for.

Answer 3

We can use contains method to check if an item exists if we have provided the implementation of equals and hashCode else object reference will be used for equality comparison. Also in case of a list contains is O(n) operation where as it is O(1) for HashSet so better to use later. In Java 8 we can use streams also to check item based on its equality or based on a specific property.

Java 8

CurrentAccount conta5 = new CurrentAccount("João Lopes", 3135);
boolean itemExists = lista.stream().anyMatch(c -> c.equals(conta5)); //provided equals and hashcode overridden
System.out.println(itemExists); // true

String nameToMatch = "Ricardo Vitor";
boolean itemExistsBasedOnProp = lista.stream().map(CurrentAccount::getName).anyMatch(nameToMatch::equals);
System.out.println(itemExistsBasedOnProp); //true

Answer 4

Please refer to my answer on this post.

There is no need to iterate over the List; just override the equals method and use .contains.

@Override
public boolean equals (Object object) {
    boolean result = false;
    if (object == null || object.getClass() != getClass()) {
        result = false;
    } else {
        EmployeeModel employee = (EmployeeModel) object;
        if (this.name.equals(employee.getName()) && this.designation.equals(employee.getDesignation())   && this.age == employee.getAge()) {
            result = true;
        }
    }
    return result;
}

Call it like this:

public static void main(String args[]) {

    EmployeeModel first = new EmployeeModel("Sameer", "Developer", 25);
    EmployeeModel second = new EmployeeModel("Jon", "Manager", 30);
    EmployeeModel third = new EmployeeModel("Priyanka", "Tester", 24);

    List<EmployeeModel> employeeList = new ArrayList<EmployeeModel>();
    employeeList.add(first);
    employeeList.add(second);
    employeeList.add(third);

    EmployeeModel checkUserOne = new EmployeeModel("Sameer", "Developer", 25);
    System.out.println("Check checkUserOne is in list or not");
    System.out.println("Is checkUserOne Preasent = ? " + employeeList.contains(checkUserOne));

    EmployeeModel checkUserTwo = new EmployeeModel("Tim", "Tester", 24);
    System.out.println("Check checkUserTwo is in list or not");
    System.out.println("Is checkUserTwo Preasent = ? " + employeeList.contains(checkUserTwo));

}