String Reverse Confusion in java - String - StringBulider

Question

String str="inputstring";
StringBuilder sb=new StringBuilder(str);
String rev=sb.reverse().toString();
//System.out.println(sb+" "+rev);             //this prints the same reverse text
if(rev.equals(sb))
  System.out.println("Equal");
else
  System.out.println("Not Equal");

When I print this code StringBuilder and String prints the same output as "gnirtstupni gnirtstupni", but when I checked whether they are equal using if condition they print "Not Equal".

This is really confusing, please explain.

Answer 1

You are comparing a String and a StringBuilder object. That will never lead to an equal result! The fact that the StringBuilder currently contains the same content as some string doesn't matter!

In other words: assume you got an egg and an egg in a box. Is that box (containing an egg) "equal" to that other egg?!

Theoretically the StringBuilder class could @Override the equals() method to specially check if it is equal to another string. But that would be rather confusing. Because if you would do that, you end up with:

new StringBuilder("a").equals("a") // --> true

giving a different result than:

"a".equals(new StringBuilder("a")) // --> false

Finally: StringBuilder uses the implementation for equals() inherited from java.lang.Object. And this implementation is simply doing a "this == other" check.

Answer 2

StringBuilder and String are two different classes, so objects of those types should never equal one another, regardless of their content.