Java: Integer equals vs. ==

As of Java 1.5, you can pretty much interchange Integer with int in many situations.

However, I found a potential defect in my code that surprised me a bit.

The following code:

Integer cdiCt = ...;
Integer cdsCt = ...;
...
if (cdiCt != null && cdsCt != null && cdiCt != cdsCt)
    mismatch = true;

appeared to be incorrectly setting mismatch when the values were equal, although I can't determine under what circumstances. I set a 开发者_开发问答breakpoint in Eclipse and saw that the Integer values were both 137, and I inspected the boolean expression and it said it was false, but when I stepped over it, it was setting mismatch to true.

Changing the conditional to:

if (cdiCt != null && cdsCt != null && !cdiCt.equals(cdsCt))

fixed the problem.

Can anyone shed some light on why this happened? So far, I have only seen the behavior on my localhost on my own PC. In this particular case, the code successfully made it past about 20 comparisons, but failed on 2. The problem was consistently reproducible.

If it is a prevalent problem, it should be causing errors on our other environments (dev and test), but so far, no one has reported the problem after hundreds of tests executing this code snippet.

Is it still not legitimate to use == to compare two Integer values?

In addition to all the fine answers below, the following stackoverflow link has quite a bit of additional information. It actually would have answered my original question, but because I didn't mention autoboxing in my question, it didn't show up in the selected suggestions:

Why can't the compiler/JVM just make autoboxing “just work”?

---

The JVM is caching Integer values. Hence the comparison with == only works for numbers between -128 and 127.

Refer: #Immutable_Objects_.2F_Wrapper_Class_Caching

---

You can't compare two Integer with a simple == they're objects so most of the time references won't be the same.

There is a trick, with Integer between -128 and 127, references will be the same as autoboxing uses Integer.valueOf() which caches small integers.

If the value p being boxed is true, false, a byte, a char in the range \u0000 to \u007f, or an int or short number between -128 and 127, then let r1 and r2 be the results of any two boxing conversions of p. It is always the case that r1 == r2.

---

Resources :

• JLS - Boxing

On the same topic :

• autoboxing vs manual boxing java

---

"==" always compare the memory location or object references of the values. equals method always compare the values. But equals also indirectly uses the "==" operator to compare the values.

Integer uses Integer cache to store the values from -128 to +127. If == operator is used to check for any values between -128 to 127 then it returns true. for other than these values it returns false .

Refer the link for some additional info

---

Integer refers to the reference, that is, when comparing references you're comparing if they point to the same object, not value. Hence, the issue you're seeing. The reason it works so well with plain int types is that it unboxes the value contained by the Integer.

May I add that if you're doing what you're doing, why have the if statement to begin with?

mismatch = ( cdiCt != null && cdsCt != null && !cdiCt.equals( cdsCt ) );

---

The issue is that your two Integer objects are just that, objects. They do not match because you are comparing your two object references, not the values within. Obviously .equals is overridden to provide a value comparison as opposed to an object reference comparison.

---

Besides these given great answers, What I have learned is that:

NEVER compare objects with == unless you intend to be comparing them by their references.

---

As well for correctness of using == you can just unbox one of compared Integer values before doing == comparison, like:

if ( firstInteger.intValue() == secondInteger ) {..

The second will be auto unboxed (of course you have to check for nulls first).