ArrayList.indexOf

[O. Ziggy]

I am looking at the following code snippet in page 580

import java.util.*;  
class Dog {  
    public String name;  
    Dog(String n) { name=n; }  
}  
  
class IteratorTest {  
    public static void main(String[] args) {  
        List<Dog> d=new ArrayList<Dog>();  
        Dog dog=new Dog("Aiko");  
        d.add(dog);  
        d.add(new Dog("clover"));  
        d.add(new Dog("magnolia"));  
        Iterator<Dog> i3=d.iterator(); 
        while(i3.hasNext()) {  
            Dog d2=i3.next();   
            System.out.println(d2.name);  
        }  
        System.out.println("size "+d.size());  
        System.out.println("get1 "+d.get(1).name);  
        System.out.println("aiko "+d.indexOf(dog));  
        d.remove(2);  
        Object[] oa=d.toArray();  
        for (Object o : oa) {  
            Dog d2=(Dog)o;  
            System.out.println("oa "+d2.name);   
        }  
    }     
}  

I am struggling to understand this line

 System.out.println("aiko "+d.indexOf(dog));    

The indexOf method is describe in the Javadocs as

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.

But i still dont understand how it works in the above example because:

- Dog does not override the equals() method
- Dog does not implement Comparable or Comparator interfaces

Infact it would have been correct if == was used but i dont understand how equals works in this case. How exactly does indexOf find that dog is in index 0?

[John Stark]

If you don't override equals() than the equals() method inherited from Object is used. That method just uses ==

John

[O. Ziggy]

John Stark wrote:If you don't override equals() than the equals() method inherited from Object is used. That method just uses ==

John

Oh ofcourse. i forgot about that.

Thanks