I'd like to simplify the nested if statement, but I'd like you to teach me how to write it well.

The condition that returns true is
if new is empty and old.epual(new) is false. In other words,

private boolean isChange(old,new){
  if(StringUtils.isEmpty(new)){
    return true;
  } else {
    return!old.equal(new)
  }
}

If I want to make it shorter, I think I can go downstairs.
I don't recommend it because it's less readable.

return StringUtils.isEmpty(new)?(true): (!old.equal(new))

"Also, ""new"" is a reserved word, so it is better to use the appropriate variable name."
There is no argument type declaration, so it should be appropriate.

I would like to know how to correct the following implementation.

Based on the isChange method name, I interpreted it as a way to create a method that returns true if there is a change between old and new, and false if there is no change.

The following is an example of code modification.I don't use StringUtils.
Do not identify null with the " empty string. If you change from null to ", you determine that the reverse has also changed.

private boolean isChange(String oldStr, String newStr){
    if(oldStr==null&newStr==null){
        return false;// No variation
    }
    else if(oldStr==null||newStr==null){
        return true; // with variation
    }
    else if(newStr.equals(oldStr)){
        return false;// No variation
    } else{
        return true; // with variation
    }
}

I looked into StringUtils by referring to metropolis's comments.
The following code identifies the null and the " empty string, and if you change from null to ", the reverse is also determined to be unchanged.

if(StringUtils.isEmpty(oldStr)&StringUtils.isEmpty(newStr)){
    return false;
}
else if(StringUtils.equals(oldStr, newStr)){
    return false;
} else{
    return true;
}

The following code does not identify the null with the " empty string. If you change from null to ", the reverse is also considered variable.

private boolean isChange(String oldStr, String newStr){
    return!StringUtils.equals(oldStr, newStr);
}