How do I access an instance of a class that is inside an arrayList

Question

I am doing a MOOC and am supposed to return a number that is associated with a name. The number name combo is held in an object called Phonebook that is an ArrayList. The arrayList holds information of Person, a class that I created. I need to perform getNumber() on the Phonebook object but I can't since getNumber() only works on objects of type Person.

package problem94_phonebook;
import java.util.ArrayList;

public class Phonebook {

    private ArrayList<Person> phonebook;


    public Phonebook(){
    this.phonebook = new ArrayList<Person>(); 
    }

    public String searchNumber(String name){
    if (this.phonebook.contains(name)){
    return this.phonebook.Person.getNumber(); // here is the problem
    }
    } 

}

package problem94_phonebook;
import java.util.ArrayList;


public class Person {

    private String Name;
    private String Numb;
    private ArrayList<String> Phonebook;

    public Person(String name, String numb){
    this.Name = name;
    this.Numb = numb;
    this.Phonebook = new ArrayList<String>();
    }

    public String getName() {
        return Name;
    }

    public String getNumber() {
        return Numb;
    }

    public String toString(){
    return this.Name +"  " +"nummber:  " + this.Numb;   
    }

    public void changeNumber(String newNumber){
    this.Numb = newNumber;
    }

    public void add(String name, String number){
    this.Phonebook.add(name);
    this.Phonebook.add(number);

    }

    public void printAll(){
    for(String i : this.Phonebook){
        System.out.println(i);
      }
    }
}

Note: `phonebook.contains(name)` doesn't work. You need to explicitly loop over the objects, then compare the names. While comparing them, you'll have an index to access the list for the number — OneCricketeer, Jul 11 '19 at 04:02
@cricket_007, I'm reopening because I don't think the associated duplicate applies. That's asking how to retrieve a list element by index, but this is asking how to retrieve by content, to which the answer is "use a map, for-each loop, or stream find". — chrylis -cautiouslyoptimistic-, Jul 11 '19 at 04:03
I used Mark's suggestion and it worked. Some of the other suggestions may work I didn't try them all. Thanks — john, Jul 12 '19 at 18:09

score 1 · Answer 1 · answered Jul 11 '19 at 04:09

you can try with this

   for (Person  person : phonebook) 
    { 
        if(person.getName().equals(name)){
               return person.getNumber();
         }
    }

for use this you need to change return type of String searchNumber(String name) String to ArrayList<String>

score 1 · Answer 2 · answered Jul 11 '19 at 04:16

1

You could use streams for this, but this has a subtle bug if there's two of the same name in the list, it only gets the first number

return this.phonebook.stream()
    .filter(p -> p.getName().equals(name))
    .map(Person::getNumber)
    .findFirst().orElse(null);

answered Jul 11 '19 at 04:16

OneCricketeer

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So do the `for `loop solutions. And which is why you want to look up `Person `s, not `String `s. – daniu Jul 11 '19 at 04:29
The object type doesn't matter... Both the other answers also stop at the first match. You need to loop over all elements to find all matching names, regardless – OneCricketeer Jul 11 '19 at 04:32

score 0 · Answer 3 · answered Jul 11 '19 at 04:09

After looking your code, change like below to get the number for a name from person:

public String searchNumber(String name){
        for(int i=0;i<this.phonebook.size();i++){
            if(this.phonebook.get(i).getName().equals(name)){
                return this.phonebook.get(i).getNumber();
            }
        }

        return null;
    }

i hope your problem will get resolve if any need please let me know.

Mark Melgo · Accepted Answer · 2019-07-12T00:23:05.593

EDIT

.contains won't work since .contains

Returns true if and only if this list contains at least one element e such that (o==null ? e==null : o.equals(e)).

(More info on how .contains works here)

which means that unless your class overrides the .equals method to compare the names this part -> o.equals(e) will always return false since .equals is comparing two different objects. (more info on how .equals work here)

Thus if you really want to use .contains you need to override .equals method in your Person class. But it still would not work since your second problem is you are accessing an element of an array list wrong.

But since you are a beginner, I suggest that you just try changing your method to this:

public ArrayList<String> searchNumber(String name){
    ArrayList<String> result = new ArrayList<>(); // list incase of persons with the same name
    for (Person p : phonebook){ // iterate through the array
        if (p.getName().equals(name)){ // check if the current person's name is equal to anme
            result.add(p.getNumber()); // add the person's phone number
        }
    }
    return result ;
}

Also I noticed that you have an attribute Phonebook in your Person class. It is just an array of Strings but I think it is better if you change it to an array of Phonebook so that you can make use of your class Phonebook. This will be your new Person class.

package problem94_phonebook;
import java.util.ArrayList;
import Phonebook;


public class Person {

    private String Name;
    private String Numb;
    private ArrayList<Phonebook> phonebook; // updated

    public Person(String name, String numb){
        this.Name = name;
        this.Numb = numb;
        this.phonebook= new ArrayList<Phonebook>();// updated
    }

    public String getName() {
        return Name;
    }

    public String getNumber() {
        return Numb;
    }

    public String toString(){
        return this.Name +"  " +"nummber:  " + this.Numb;   
    }

    public void changeNumber(String newNumber){
        this.Numb = newNumber;
    }

    public void add(Person personToBeAdded){ // changed
        boolean isPresent = false;
        // check if person to be added is already in the phonebook to avoid duplicates
        for (Person person:this.phonebook){
            if (person.getName().equals(personToBeAdded.getName()) && person.getNumber().equals(personToBeAdded.getNumber())){
                isPresent = true;
                break;
            }    
        }
        if (!isPresent){
            this.phonebook.add(personToBeAdded);
        }
    }

    public void printAll(){
      for(Person person : this.phonebook){
        System.out.println(person.toString());
      }
    }
}

Thanks that worked. At first I tried .contains but that didn't work so then I did .equals like you and it started working. Why doesn't .contains work? Doesn't it search for name and if found return true? — john, Jul 11 '19 at 15:57
@john I have edited my answer and provided an explanation why .contains won't work. Kindly read it. Also if I solved your problem don't forget to upvote my answer and mark it as the correct answer so that SO will know that this question was already answered. — Mark Melgo, Jul 12 '19 at 00:24
Thanks, I upvoted your answer but because I have less than 15 reputation I'm not sure if it showed or not. — john, Jul 12 '19 at 18:08

How do I access an instance of a class that is inside an arrayList

4 Answers4