Cannot compare two objects using instanceof

Question

When trying to compare two objects passed as arguments: first as object, second as array of objects (the same kind). Getting error Incompatible conditional operand types Room and allRooms[]. I am trying to iterate in while cycle until thisRoomType is not equals to some object from rooms. How to fix it?

public abstract class Room { ...}
public class LightRoom extends Room  {...}
public class DarkRoom extends Room {...}

Caller:

release(thisRoomType, rooms)

with parameters

private Room thisRoomType = this; // is defined in DarkRoom and LightRoom 
private Room[] rooms; // is defined in DarkRoom and LightRoom

in method:

public synchronized void release( Room thisRoom,  Room[] allRooms) {

        try {
            int j = 0;
            while(thisRoom instanceof allRooms[j]){     
                jj++;
            }
                        int nextRoom = jj;
...
    }
}

This is an `ArrayIndexOutOfBoundsException` waiting to happen. the `while` condition must include `jj < allRooms.length` — Óscar López, Apr 30 '12 at 20:10

Óscar López · Accepted Answer · 2012-04-30T20:23:56.933

7

Try this instead:

while (j < allRooms.length && thisRoom.getClass().equals(allRooms[j].getClass()))

edited Apr 30 '12 at 20:23

answered Apr 30 '12 at 19:59

Óscar López

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@RCola sorry, you saw a previous version of my answer. Try with the updated one. – Óscar López Apr 30 '12 at 20:03
2

@ewernli I'm not sure if that's a problem. Because as stated, OP's question will _always_ loop until an `ArrayIndexOutOfBoundsException` occurs, a non-null `Room` will always be an instance of any non-null element in a `Room[]`. The code might only make sense if a different kind of room is encountered, and that's what my answer does. – Óscar López Apr 30 '12 at 20:09
The equivalent of instanceof is isAssignableFrom. See http://stackoverflow.com/questions/496928/what-is-the-difference-between-instanceof-and-class-isassignablefrom – ewernli Apr 30 '12 at 21:04

score 2 · Answer 2 · answered Apr 30 '12 at 20:13

Your logic is wrong.
instanceof expects a ReferenceType while you are passing an object instance allRooms[j]. This can never work.
What you should be doing is:

compare with package names or
class names or
compare against the concrete classes (getClass).

Pick according to your needs

Cannot compare two objects using instanceof

2 Answers2