StringBuilder .equals Java

Question

class strb
{

    static public void main(String...string)
    {
         StringBuilder s1 = new StringBuilder(\"Test\");
         StringBuilder s2 = new StringBuild         


        

Answer 1

StringBuilder class doesn't have the implementation of equals() method like one in the String class.

So it executes default Object class functionality, which again checks only for address equivalency, which is not same in this case. so it prints false.

Note 1: You can use == operator on objects also, but it simply checks if the address of both the objects are same or not.

Note 2: == operator plays good role in comparing two String objects created in string constant pool, to find out if it is really creating a new object in string constant pool or not.

Answer 2

StringBuilder and StringBuffer not override the equals function of Object class.but string override the equals method. the function of Object is this

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

you could write your code like this.

System.out.println(s1.toString() == s2.toString());
System.out.println(s1.toString().equals(s2.toString()));

Answer 3

Agreed with both of the above responses, but I'm worth noting to actually compare contents you can do:

System.out.println(s1.toString().equals(s2.toString())); //Line 1

Answer 4

Check the contract of equals method:

it must be consistent (if the objects are not modified, then it must keep returning the same value).

That's why StringBuilder does not override it regardless of its content.

Let's take example above.

     StringBuilder s1 = new StringBuilder("Test");
     StringBuilder s2 = new StringBuilder("Test");

Maybe, to you it is expected that s1.equals(s2) returns true due to current run time values.

But what about if you change add line:

    s1.append("abc");

Then s1 and s2 will have different string contents and s1.equals(s2) is expected to be false. But it is in contradiction with consistency.

Answer 5

The StringBuilder class does not provide an overriden equals() method. As such, when that method is called on an instance of StringBuilder, the Object class implementation of the method is executed, since StringBuilder extends Object.

The source code for that is

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

Which simply compares reference equality.

Answer 6

for your first answer check @abmitchell 's Answer

And for your Edit:

In Java, String is an object and we can't compare objects for value equality by using ==

== is used for comparing primitives values or object references.

To compare Object values we use equals() in Java