使用String作为参数在ArrayList上进行线性搜索

Question

I am trying to search for a String contained in an ArrayList in my method, findContactsByName, using a for-each loop, if the String is found in the Contact then the Contact is returned and if the String is not found then null is returned. However when testing my code null is returned all the time, here is my code:

import java.util.*;

public class Phonebook {
    private String owner;
    private ArrayList<Contact> contacts = new ArrayList<Contact>();

    public void addContact(Contact contact) {
        contacts.add(contact);
    }

    public void show() {
        System.out.println(owner + "'s phonebook");
        for (Contact contact : contacts) {
            System.out.println(contact);
        }
    }

    public Contact findContactByName(String name) {
        for (Contact contact : contacts) {
            if (contacts.contains(name)) {
               return contact;
           }
       }
       return null;
    }
}

Answer 1

Your findContactByName method isn't correct. You are looping through the ArrayList, but each loop you are checking the array list by using ArrayList.contains , which is comparing the objects in the list itself with name .

The correct way to do it would be:

public Contact findContactByName(String name) {
    for (Contact contact : contacts) {
        if (contact.getName().equals(name)) {
            return contact;
        }
   }
   return null;
}

That is assuming that the Contact object has a method called getName . Notice how in each loop i'm using contact from the for loop for (Contact contact : contacts) instead of the ArrayList itself.

Answer 2

public Contact findContactByName(String name) {
    for (Contact Contact : contacts) {
        if (contacts.contains(name)) {
            return Contact;
        }
   }
   return null;
}

This function is the root cause of your problem. Your condition in the if block reads: contacts.contains(name) where contacts is the entire list of contacts and you are looking in there for a string object. That can't work. You want to find out if your contacts contain a contact with a specific name. So something like: contact.getName().equals(name) .

So your function should be something like this:

public Contact findContactByName(String name) {
    for (Contact contact : contacts) {
        if (contact.getName().equals(name)) {
            return contact;
        }
   }
   return null;
}

Answer 3

Example for using contains is

List<String> fruitsList = new ArrayList<>();
fruitsList.add("Apple");
fruitsList.add("Mango");
fruitsList.add("Grapes");

if(fruitsList.contains("Grapes")) {
S.o.p("Found Grapes");
}

but for your case you need to find a specific element from your dto and return it back if specific name is found, so your usecase is:

public Contact findContactByName(String name) {
    for (Contact contact : contacts) {
        if ((null!=contact.getName()) && contact.getName().equals(name)) {
            return contact;
        }
   }
   return null;
}

Assuming your Contact class has name variable. Hope you understood it.

Answer 4

Unless using ArrayList is specifically required, I suggest you better replace it by a HashMap<String, Contact> , so you can avoid coding the search algorithm from the scratch simply by relying on the Map.get(key) method. Maps are intended to search values by some key (indexing):

public class Phonebook {

    private Map<String, Contact> contacts = new HashMap<String, Contact>();

    public void addContact(Contact Contact) {
        contacts.put(contact.getName(), contact);
    }

    public Contact findContactByName(String name) {
        return contacts.get(name);
    }
}

Also, the search becomes then indexed and thus, faster.