Java Comparison for Beginners

So, we all know about boxing and unboxing in Java, and the fact that we have "int" which is a primitive, and we have "Integer" which is an Object.

So, when comparing "int", we can use ==. When comparing "Integer" it's better to use .equals().

It may seem like it works, but this is only for small values that are cached in the JVM. Do not rely on that!

See the following example:

So, what happens when using different comparisons? let's explain the test below:

It works fine! Exactly as we would expect.

In short, according to the JLS¹, first unboxing takes place, in order to be able to compare two primitives.

Then a widening primitive conversion² takes place, in this case the int will be converted up to type long.

Now the comparison can take place.

References

[1] JLS Java 16 - 15.20. Relational Operators

https://docs.oracle.com/javase/specs/jls/se16/html/jls-15.html#jls-15.20

[2] JLS Java 16 - 5.6. Numeric Contexts

https://docs.oracle.com/javase/specs/jls/se16/html/jls-5.html#jls-5.6

java.lang.IllegalArgumentException: Comparison method violates its general contract!

I got the error message in the title whilst using Flyway to run database scripts at work¹.

I find it very satisfying that sorting methods in the JDK can detect if your comparer is not obeying the contract.

There are a lot of errors that cause the contract to be violated according to [2].

Here are the rules in short as described in [3]:

• sign(compare(x,y)) == -sign(compare(y,x))
• (compare(x, y)>0) && (compare(y, z)>0)) implies compare(x, z)>0 (transitivity)
• compare(x, y)==0 implies that sgn(compare(x, z))==sgn(compare(y, z))

Sounds like an excellent starting point for some good JUnit Tests.

References

[1] GitHub - Flyway - Issue 1249

https://github.com/flyway/flyway/issues/1249

[2] StackOverflow - “Comparison method violates its general contract!”

http://stackoverflow.com/questions/8327514/comparison-method-violates-its-general-contract

[3] Oracle Javadoc - Interface Comparator>T>

https://docs.oracle.com/javase/7/docs/api/java/util/Comparator.html

Comparing Files using Find and CMP

I needed to compare two directories containing binary files. Rsync was confusing, Diff only compares text-files.

I tried the following:

#!/bin/bash
# Example: ~/compare.sh FC30-3DA9
# where $1 is the directory name
# requirement: you are in one of the two parent dirs.
# the directories to compare must have the same name

find $1 -type f -exec sh -c '
  for f; do
    cmp "$f" /directory/of/mr/bear/"$f"
  done
' sh {} +

Seemed to work fine.

References

How to compare files in two folders using cmp?

http://www.unix.com/shell-programming-and-scripting/144821-how-compare-files-two-folders-using-cmp.html