When and why does List remove(Object object) return false

Question

When and why does the method boolean java.util.List.remove(Object object) return false?

The documentation states

[The method ret

Answer 1

Since List is an interface, it wouldn't have a concrete implementation for an example. But taking from ArrayList, it will return false if:

• you pass null and there's no null object in the list, or
• you pass an object not found on the list

From ArrayList.java

     public boolean remove(Object o) {
         if (o == null) {
             for (int index = 0; index < size; index++)
                 if (elementData[index] == null) {
                     fastRemove(index);
                     return true;
                 }
         } else {
             for (int index = 0; index < size; index++)
                 if (o.equals(elementData[index])) {
                     fastRemove(index);
                     return true;
                 }
         }
         return false;
     }

Adding the code of fastRemove, for completeness:

 private void fastRemove(int index) {
      modCount++;
      int numMoved = size - index - 1;
      if (numMoved > 0)
          System.arraycopy(elementData, index+1, elementData, index,
                           numMoved);
      elementData[--size] = null; // Let gc do its work
  }

Proving it:

public static void main(String[] args) {
        ArrayList l = new ArrayList();
        System.out.println("Removing null on empty list: " + l.remove(null));
        System.out.println("Removing an object that won't be found: " + l.remove(new Object()));
}

Result:

Removing null on empty list: false
Removing an object that won't be found: false