使用Helper时Android中的indexOf（）

Question

I know that the below code gives the index of that particular element in java.

List<String> list = new ArrayList<>();

list .add("100");

Log.d("TAG",String.valueOf(list.indexOf("300")));

But how to get the index of an element while using a helper Class?

List<HelperClass> Arraylist= new ArrayList<>();

Arraylist.add(new HelperClass(name, email, phoneno));

Log.d("TAG", String.valueOf(new HelperClass(Arraylist.indexOf(name,email,phoneno))));

I searched everywhere for this but couldn't find. Can someone tell me how to find index of a particular item in arraylist while using modal to add data?

Obviously what I have tried is wrong and it shows red line under the whole line but I just typed that code for your understanding of what I want to achieve. Can someone give me a way please?

Helper

 @Override
public int hashCode() {
    int result = getName() != null ? getName().hashCode() : 0;
    result = 31 * result + (Email != null ? Emaail.hashCode() : 0);
    result = 31 * result + (PhoneNo!= null ? PhoneNo.hashCode() : 0);

    return result;
}

@Override
public boolean equals(Object o) {
    if (this == o) return true;
    if (!(o instanceof Helper)) return false;

    Helperthat = (Helper) o;

    if (getName() != null ? !getName().equals(that.getName()) : that.getName() != null)
        return false;
    if (Email != null ? !Email.equals(that.Email) : that.Email != null)
        return false;
    if (PhoneNo != null ? !PhoneNo.equals(that.PhoneNo) : that.PhoneNo != null)
        return false;

}

Answer 1

ArrayList#indexOf uses the Object#equals comparison method.

If you want to be able to lookup a HelperClass instance inside a Collection , you need to provide your own, overridden, equals method, and possibly also the hashCode one, for use with other, specific, Collection implementations ( Map , Set , etc.).

class HelperClass {
   ...

   @Override
   public boolean equals(final Object object) {
      if (object == this) {
        return true;
      }

      if (!(object instance of HelperClass)) {
         return false;
      }

      final HelperClass other = (HelperClass) object;
      return name.equals(other.name) &&
             email.equals(other.email) &&
             phone.equals(other.phone);
   }
}

You obviously need to have an appropriate HelperClass instance to find a match.

final String name = "Name";
final String email = "Email";
final String phone = "Phone";

final HelperClass first = new HelperClass(name, email, phone);
final HelperClass second = new HelperClass(name, email, phone);

final List<HelperClass> helpers = new ArrayList<>(8);
helpers.add(first);

final int index = helpers.indexOf(second); // index = 0

Answer 2

indexOf requires the object as input. If it does not find the object you are passing in, it will return -1. You need to pass the object whose location in the arraylist you are looking for as the input into the indexOf function.

Solution : create a HelperClass to pass into the indexOf method:

.indexOf(new HelperClass(name, email, phoneno));

However that change by itself will still return -1. See the api doc for indexOf:

public int indexOf(Object o)

Returns the index of the first occurrence of the specified element in this list, or -1 if this list does not contain the element. More formally, returns the lowest index i such that (o==null ? get(i)==null : o.equals(get(i))), or -1 if there is no such index.

It's using equals to decide whether it's found a match. You should have overridden the equals method on your HelperClass class, so it's using the default implementation in java.lang.Object, which compares the references, and only returns true if the two references HelperClass to the same object.

Override equals and hashcode on your HelperClass class, like:

@Override public boolean equals(Object other) {
    if (!(other instanceof HelperClass)) {
        return false;
    }
    HelperClass otherHelperClass = (HelperClass)other;
    return otherHelperClass.x == this.x && otherHelperClass.y == this.y;
}

@Override public int hashCode() {
    return x + y; // same values should hash to the same number
}