Comparing boxed Long values 127 and 128

Question

I want to compare two Long objects values using if conditions  When these values are less than 128  the if condition works properly  but when they are greater than or equal to 128  comparison fails   Example   Long num1   127  Long num2   127   if  num1    num2           Works ok     Comparison on the code above works properly  but fails in the code below   Long num1   128  Long num2   128   if  num1    num2           Does NOT work     Why is there a problem in comparing Long variables with values greater than 127  If the variables data types are changed to long primitives  then the comparisons work for all cases

User · Answer

Comparing non-primitives  aka Objects  in Java with    compares their reference instead of their values  Long is a class and thus Long values are Objects   The problem is that the Java Developers wanted people to use Long like they used long to provide compatibility  which led to the concept of autoboxing  which is essentially the feature  that long-values will be changed to Long-Objects and vice versa as needed  The behaviour of autoboxing is not exactly predictable all the time though  as it is not completely specified   So to be safe and to have predictable results always use  equals   to compare objects and do not rely on autoboxing in this case   Long num1   127  num2   127  if num1 equals num2     iWillBeExecutedAlways

User · Answer

TL DR  Java caches boxed Integer instances from -128 to 127  Since you are using    to compare objects references instead of values  only cached objects will match  Either work with long unboxed primitive values or use  equals   to compare your Long objects   Long  pun intended  version     Why there is problem in comparing Long variable with value greater than 127  If the data type of above variable is primitive  long  then code work for all values    Java caches Integer objects instances from the range -128 to 127  That said    If you set to N Long variables the value 127  cached   the same object instance will be pointed by all references   N variables  1 instance  If you set to N Long variables the value 128  not cached   you will have an object instance pointed by every reference   N variables  N instances    That s why this   Long val1   127L  Long val2   127L   System out println val1    val2    Long val3   128L  Long val4   128L   System out println val3    val4     Outputs this      true   false   For the 127L value  since both references  val1 and val2  point to the same object instance in memory  cached   it returns true   On the other hand  for the 128 value  since there is no instance for it cached in memory  a new one is created for any new assignments for boxed values  resulting in two different instances  pointed by val3 and val4  and returning false on the comparison between them   That happens solely because you are comparing two Long object references  not long primitive values  with the    operator  If it wasn t for this Cache mechanism  these comparisons would always fail  so the real problem here is comparing boxed values with    operator   Changing these variables to primitive long types will prevent this from happening  but in case you need to keep your code using Long objects  you can safely make these comparisons with the following approaches   System out println val3 equals val4                           true System out println val3 longValue      val4 longValue         true System out println  long val3     long val4                   true    Proper null checking is necessary  even for castings   IMO  it s always a good idea to stick with  equals   methods when dealing with Object comparisons   Reference links    https   today java net pub a today 2005 03 24 autoboxing html https   blogs oracle com darcy entry boxing and caches integer valueof http   java dzone com articles surprising-results-autoboxing

User · Answer

Java caches the primitive values from -128 to 127  When we compare two Long objects java internally type cast it to primitive value and compare it  But above 127 the Long object will not get type caste  Java caches the output by  valueOf   method   This caching works for Byte  Short  Long from -128 to 127  For Integer caching works From -128 to java lang Integer IntegerCache high or 127  whichever is bigger  We can set top level value upto which Integer values should get cached by using java lang Integer IntegerCache high     For example      If we set java lang Integer IntegerCache high 500      then values from -128 to 500 will get cached and       Integer a 498      Integer b 499      System out println a  b       Output will be  true     Float and Double objects never gets cached    Character will get cache from 0 to 127  You are comparing two objects  so    operator will check equality of object references  There are following ways to do it   1  type cast both objects into primitive values and compare       long val3     long val4   2  read value of object and compare      val3 longValue      val4 longValue     3  Use equals   method on object comparison       val3 equals val4

User · Answer

num1 and num2 are Long objects   You should be using equals   to compare them      comparison might work sometimes because of the way JVM boxes primitives  but don t depend on it   if  num1 equals num1        code

[java] Comparing boxed Long values 127 and 128

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