Java : Equal numbers not matching

Question

I am seeing a strange issue. I have an object, which has a few instance variables, one of which is numeric. This object came out of a socket using ObjectInputStream's readObject(), so it should have been deserialized. However, when i compare it with another object which has the exact same number for that field, using == , it doesn't match. Note: I am not comparing objects, just the integer instance variable, using a getter. Print statements just before the comparison, show that they're exactly the same.

    System.out.println("New book id:"+newBook.getId());
for (Book p: listOfBooks) {
    System.out.println("CurrentBook's id:"+p.getId());
    if (newBook.getId() == p.getId()) {
        System.out.println("Matched CurrentBook's id:"+p.getId()) 
            }
    }

Thanks Ted Hopp, yes, i guess the behaviour is inconsistent for Integer comparisons with ==. I realised that the fields were Integer, not int, and it worked before, so was very confusing why it no longer worked.

Please provide code so that we can diagnose. There's nothing to go off of here — torquestomp, Apr 23 '13 at 00:43
`==` is just comparing the object's reference with the other. You need implement a `obj1.equals(obj2)` method to compare the internal data. — Supericy, Apr 23 '13 at 00:43
Could this be your problem? http://stackoverflow.com/questions/10149959/using-operator-in-java-to-compare-wrapper-objects — sarahTheButterFly, Apr 23 '13 at 00:54

score 3 · Accepted Answer · answered Apr 23 '13 at 00:49

Comparing numeric objects (Integer, etc.) using == can give inconsistent results. Integer, for instance, will autobox all values between -128 and +127 to cached values, but values outside that range may or may not be cached, depending on the implementation. Thus,

Integer.valueOf(-45) == Integer.valueOf(-45)

will be true, but

Integer.valueOf(-145) == Integer.valueOf(-145)

will usually be false.

For numeric quantities, you should either use .equals() or else extract the primitive values (int, long, etc.) before using ==

num1.intValue() == num2.intValue()

score 0 · Answer 2 · answered Apr 23 '13 at 00:45

0

readObject() returns an object, so be sure to cast it to an int

http://docs.oracle.com/javase/6/docs/api/java/io/ObjectInputStream.html#readObject()

answered Apr 23 '13 at 00:45

alex_milhouse

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I cast it to the object that it is, Book, in this case. – stumped Apr 23 '13 at 00:49

score 0 · Answer 3 · edited May 23 '17 at 11:57

0

Print statements just before the comparison, show that they're exactly the same.

I think Alex Rellim is right. And the Print statements reads the object's toString method. That's why you see the numbers are the same. If you use a debugger, check the object's type.

Edit (after OP's code's being pasted): Could this be your problem? Using == operator in Java to compare wrapper objects

edited May 23 '17 at 11:57

Community

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answered Apr 23 '13 at 00:49

sarahTheButterFly

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Java : Equal numbers not matching

3 Answers3