Segregate even and odd numbers in a given array
I strongly doubt that your program does exactly this:

Example:
  Input = {12, 34, 45, 9, 8, 90, 3}
  Output = {12, 34, 8, 90, 45, 9, 3}


AFAIK the output will be 

  Output = {12, 34, 90, 8, ...}


i.e., the elements get reordered because of the swapping. But that's alright according to what's required.



 private static void evenOddFunction(int []data) {


It's not a function, it's a method. And calling something evenOddMethod wouldn't be any better as there's no information therein. something like evenOddSegregator sounds better, but method names should be verbs. So maybe segregateEvenOdd?

    if(data[leftSide] % 2 != 0 &amp;&amp; data[rightSide] % 2 == 0) {
        ...
    } else {
        if(data[leftSide] % 2 == 0) {
            ...
        }
        if(data[rightSide] % 2 == 1) {
            ...
        }


I hate repeated conditions. What about

    if(data[leftSide] % 2 == 0) {
        ...
    } else if(data[rightSide] % 2 == 1) {
        ...
    } else {
        ...
    }


It's not exactly the same, but this doesn't matter.

private static void swappEvenOdd(int []data , int left, int right) {


This is misnomer (and a type, too). There's nothing even nor odd in this method.


  Any chances of improvement/optimizations in this code?


I guess your code is optimal, apart from this low-level optimization: Instead of (x % 2) use x &amp; 1; it's much faster, works better for negative numbers, etc. But the JIT knows this trick too and there's not so much to gain.



An untested, stupid, but funny solution:

Comparator&lt;Integer&gt; comparator = new Comparator&lt;&gt;() {
    public int compare(Integer a, Integer b) {
        return (a &amp; 1) - (b &amp; 1);
    }
}
Collections.sort(Ints.asList(data), comparator);


The comparator claims that an even number is smaller than an odd number and sort does all the work. It's even stable, i.e., it doesn't reorder what it doesn't have to and produces the output from your example. Guava's Ints adapts the array as a List&lt;Integer&gt;.