Why does the 'contains' method return false when I enter the exact name on my ArrayList?

Question

I am trying to make a program that can add a customer's name, age, contact number and email. And I want to search for the name that the user wants, but it does not search the name even if I entered the same name exactly. How can I fix this?

Here is my code:

package com.company;
import java.util.ArrayList;
import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        Scanner scan = new Scanner(System.in);
        ArrayList<customers> customers = new ArrayList<>();
        customers.add(new customers("Zen",19,"0912121212","zen@gmail.com"));
        customers.add(new customers("Mary",20,"09134343434","mary@gmail.com"));
        System.out.println("Enter name: ");
        String name = scan.nextLine();
        System.out.println(customers.contains(name));
    }
}
class customers{
    private String name;
    private int age;
    private String contactNumber;
    private String email;

    public customers(String name, int age, String contactNumber, String email) {
        this.name = name;
        this.age = age;
        this.contactNumber = contactNumber;
        this.email = email;
    }
}

You have added fruit hamper to array list and asking if there are any mangoes. — VIAGC, May 13 '21 at 05:30

tquadrat · Accepted Answer · 2021-05-14T14:26:39.157

List.contains()uses Object.equals() to determine whether an Object is already in that List.

So one approach could be to overwrite that method:

public class Customer
{
  private String m_Name;
  private int m_Age;
  …

  @Override
  public final boolean equals( final Object o )
  {
    return o instanceof String name && name.equals( m_Name );
  }
}

Although this will work, it is not recommended to implement equals() in this way (see here as a starting point).

Instead you should search for the name in the list:

String name = scan.nextLine();
System.out.println( customers.stream().anyMatch( c -> c.getName().equals( name ) ) );

A completely different approach would be to store the Customer objects not in an instance of List but in an instance of Map, with the name as the key:

public class Main 
{
  public static void main( String... args ) 
  {
    Scanner scan = new Scanner(System.in);
    Map<String,Customer> customers = new HashMap<>();
    var customer = new Customer( "Zen", 19, "0912121212", "zen@gmail.com" );
    customers.put( customer.getName(), customer );
    customer = new Customer( "Mary", 20, "09134343434", "mary@gmail.com" );
    customers.put( customer.getName(), customer );
    System.out.println( "Enter name: " );
    String name = scan.nextLine();
    System.out.println( customers.containsKey( name ) );
  }
}

Finally, it would help in general if you would follow the basic naming conventions for the Java language: class names are starting with a Capital letter.

Can you link to those conventions? (An authoritative reference.) — Peter Mortensen, Oct 18 '21 at 11:06
@PeterMortensen – Try this: https://www.oracle.com/technetwork/java/codeconventions-150003.pdf — tquadrat, Oct 19 '21 at 07:56

Eran · Answer 2 · 2021-05-13T05:43:04.197

name is a String. Your List contains customers instances, not Strings. Therefore your List doesn't contain name.

In order to lookup an instance of one type by a key of another type, you can use a Map:

public static void main(String[] args) {
    Scanner scan = new Scanner(System.in);
    Map<String,customers> customers = new HashMap<>();
    customers.put("Zen",new customers("Zen",19,"0912121212","zen@gmail.com"));
    customers.put("Mary",new customers("Mary",20,"09134343434","mary@gmail.com"));
    System.out.println("Enter name: ");
    String name = scan.nextLine();
    System.out.println(customers.containsKey(name));
}

Or, if you want to search for a customers instance having a certain name, you can iterate over the elements of your List (either with a loop or with a Stream).

For example:

System.out.println(customers.stream().anyMatch(c -> c.getName().equals(name)));

This is assuming your customers class has a getName() getter method.

I don't think he has read the DS map. – VIAGC May 13 '21 at 05:41 — VIAGC, May 13 '21 at 05:41

score 0 · Answer 3 · answered May 13 '21 at 05:39

Add another constructor to your class for just name and iterate over all objects in array list for the same name.

import java.util.ArrayList;
import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        Scanner scan = new Scanner(System.in);
        ArrayList<customers> customers = new ArrayList<>();
        customers.add(new customers("Zen",19,"0912121212","zen@gmail.com"));
        customers.add(new customers("Mary",20,"09134343434","mary@gmail.com"));
        System.out.println("Enter name: ");
        String name = scan.nextLine();
        customers obj = new customers(name);
        customers toBeChecked;
        for (int i=0; i<customers.size(); i++) {
            toBeChecked = customers.get(i);
            if(toBeChecked.getName().equals(obj.getName())) {
                System.out.println("Same name");
            }
        }
    }
}
class customers{
    private String name;
    private int age;
    private String contactNumber;
    private String email;

    public customers(String name, int age, String contactNumber, String email) {
        this.name = name;
        this.age = age;
        this.contactNumber = contactNumber;
        this.email = email;
    }
    public customers (String name) {
    this.name = name;
    }
    public String getName() {
        return name;
    }

}

score 0 · Answer 4 · answered May 13 '21 at 06:37

The problem here is that you're not defining the ArrayList with a String type. If you want to keep the customers in your list, you can try this solution:

for(customers c:customers){
if(c.getName().equals(name)){
System.out.println(true);
}
 }

Make sure to create a getter ( getName() ).

Why does the 'contains' method return false when I enter the exact name on my ArrayList?

4 Answers4