Q:

Difference between using unary operator !  and (== false) in if statement

I need to know what is difference between using !  operator and ==false in if statement. 
I know output will be same for both.  Am expecting to know internal jvm performance or any memory usage for using false value like that. 
By using !  operator
if(!MyUtil.isFieldEmpty(String value)){
   System. Out.println("Not Empty");
}

By using ==false 
if(MyUtil.isFieldEmpty(String value) == false){
   System. Out.println("Not Empty");
}

MyUtil.java
public static boolean isFieldEmpty(String value){
  if ((value == null) || (value.trim().length() == 0) || value.equals("null")) {
    return true;
  } else {
    return false;
  }
}

Thanks!!!  

A:

In terms of performance, there may (or may not, likely not) be a slight edge in favor of using the simpler (that is, not including ==) form.   Otherwise, they're virtually identical. 
But that's really not the point here. 
Writing your boolean expressions in the form of if(booleanMethod() == true|false) is entirely redundant.   That decreases readability in some aspects, introduces unnecessary code and evaluation statements, and most modern IDEs/Java style checkers will complain about expressions like that. 
As an example, PMD will whine about this being a case of "SimplifyBooleanExpressions". 
If your left-hand expression is a boolean, and all you need is a boolean, then just use the left-hand expression.    ! MyUtil.isFieldEmpty(String value) is perfectly acceptable. 
Also, as an aside, your boolean method could also do with some refactoring, too.   It boils down to this:
public static boolean isFieldEmpty(String value) {
     return (value == null) || (value.trim().length() == 0) || value.equals("null");
}

This is for the same reason:  redundancy.   You're basically saying, "if this is true, or this is true, or this is true, then return true."   Turns out that's just the result of the natural evaluation of the expression. 
Why not just return the result of the expression?   PMD would consider this a case of "SimplifyBooleanReturns".