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.

`rev` is a `String`. `sb` is a `StringBuffer`. They are two different things. — Joe C, Aug 13 '17 at 06:55

GhostCat · Accepted Answer · 2017-08-13T06:58:08.740

3

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.

edited Aug 13 '17 at 06:58

answered Aug 13 '17 at 06:55

GhostCat

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Unless you do `egg.equals(eggInBox.toEgg())`, they are not. – Maroun Aug 13 '17 at 06:57
More than just confusing; it's forbidden by the spec. – shmosel Aug 13 '17 at 07:04
@shmosel Well, you could then @ Override String.equals() too . But I hope the message *bad idea* comes over already. – GhostCat Aug 13 '17 at 07:05

score 1 · Answer 2 · answered Aug 13 '17 at 06:55

1

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

answered Aug 13 '17 at 06:55

Mureinik

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You seem to be implying that objects of different types can't be equal to each other as a rule, which isn't true. – shmosel Aug 13 '17 at 07:05

score 1 · Answer 3 · answered Aug 13 '17 at 06:57

1

StringBuilder and String are different objects,

But if you want check equal by string. just use toString()

    if(rev.equals(sb.toString()))
      System.out.println("Equal");
    else
      System.out.println("Not Equal");

answered Aug 13 '17 at 06:57

Ori Marko

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score 1 · Answer 4 · answered Aug 13 '17 at 06:58

1

rev is a String.

sb is a StringBuilder.

They are two different things, and thus will never be equal.

answered Aug 13 '17 at 06:58

Joe C

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String Reverse Confusion in java - String - StringBulider

4 Answers4