Confusion with equal method

Question

Whenever I use equals() method with two different StringBuffer objects, I get the result as false, but when I use equals() method with two different String objects, I get the result as true. Why?

    String s=new String("434");
    String s1=new String("434");

    System.out.println(s.equals(s1));//true

   StringBuffer s=new StringBuffer("434");
   StringBuffer s1=new StringBuffer("434");

   System.out.println(s.equals(s1));//false

Answer 1

StringBuffer does not override equals(). As such, Object.equals() is called, which compare the object identity (the memory address). String does override equals and compare the content.

Answer 2

String s.equals will use the string table to compare the actual strings where as the StringBuffer sb.equals will just use the default implementation of the equals method and just compare the object pointers.

Answer 3

At least in my version of the JDK (Sun 1.6), StringBuffer does not implement an equals() method. This means it inherits Object's equals() method, which is the same as ==

If you really want to test two StringBuffers for equality, you could call x.toString().equals(y.toString())

Answer 4

StringBuffer does not override Object#equals(), so you're experiencing reference identity-based checks rather than value-based checks. As these StringBuilder instances are distinct, each with different memory locations, the base Object#equals() implementation will always return false.

Here's the definition as of Java 6:

public boolean equals(Object obj) {
  return (this == obj);
}

See the problem?

Answer 5

If what you are looking to do is compare the String representations of the two StringBuffer objects, then what you want to do is:

StringBuffer sb1 = new StringBuffer("434");
StringBuffer sb2 = new StringBuffer("434");
if (sb1.toString().equals(sb2.toString())) {
  // whatever you want to do if they're equal
} else {
  // whatever you want to do if they're not
}

Otherwise, you're comparing for equality of the two StringBuffer objects, not their contents -- in other words, executing Object#equals(), not (the non-existent) StringBuffer#equals().

Answer 6

equals only returns true on StringBuffer objects when the two objects are the same. To compare StringBuffers the way you want, use this:

System.out.println(s.toString().equals(s1.toString());