StringBuilder Methods called from left to right?

[shweta patiljadhav]

Hi,

in this code,

class EJavaGuruStringBuilder {
 public static void main(String args[]) {
 StringBuilder ejg = new StringBuilder(10 + 2 + "SUN" + 4 + 5);
 ejg.append(ejg.delete(3, 6));
 System.out.println(ejg);
 }
}

what would be the output? if on StringBuilder methods are called from left to right here delete() is called first. and why it's output is not 12SUN4512S5?
Why it is appending out put of delete method twice?

output is 12S512S5 ?

TIA
Shweta

[nick woodward]

shweta patiljadhav wrote:Hi,

in this code,

class EJavaGuruStringBuilder {
 public static void main(String args[]) {
 StringBuilder ejg = new StringBuilder(10 + 2 + "SUN" + 4 + 5);
 ejg.append(ejg.delete(3, 6));
 System.out.println(ejg);
 }
}

what would be the output? if on StringBuilder methods are called from left to right here delete() is called first. and why it's output is not 12SUN4512S5?
Why it is appending out put of delete method twice?

output is 12S512S5 ?

TIA
Shweta

I think you're confusing chaining methods (ie calling methods on an object from left to right), and passing methods as arguments

ejg.append("abc").delete(3,6); vs ejg.append(//argument that happends to be a method);

[Liutauras Vilda]

shweta patiljadhav wrote:what would be the output? if on StringBuilder methods are called from left to right here delete() is called first. and why it's output is not 12SUN4512S5?
Why it is appending out put of delete method twice?

output is 12S512S5 ?

Well, StringBuilder is mutable object at first. Second, when you call a method, you need to pass arguments to it (or not). In order to pass arguments, you need to know what these are.
1. ejg.append(ejg.delete(3, 6)); // evaluates ejg.delete(3, 6) first. And you get 12S5
2. ejg.append(evaluated argument from 1st step) // StringBuilder is mutable, think about it as 12S5.append(12S5).
3. And you get what you get, two times by 12S512S5

[shweta patiljadhav]

Thanks!

[Roel De Nijs]

shweta patiljadhav wrote:what would be the output? if on StringBuilder methods are called from left to right here delete() is called first. and why it's output is not 12SUN4512S5?
Why it is appending out put of delete method twice?

Chained methods will definitely be called from left to right. And that's not only limited to StringBuilder's methods, but applies to any object. Here's an example of a StringBuilder

StringBuilder sb = new StringBuilder("ava ro");
sb.append("cks").insert(0, 'J').append("!").reverse();
System.out.println(sb);

Output: !skcor avaJ

And you can do the same with strings as well

String str = "Java rocks!";
String res = str.toUpperCase().substring(3, 10).toLowerCase().replace('o', 'O').concat("? " + str).replace("a", "abc");
System.out.println(res);

Output: abc rOcks? Jabcvabc rocks!

Remember: there is one very important difference between the first and second code snippet. In the first code snippet you are using a StringBuilder which is a mutable object, so that means that every method call will manipulate the same object. In the second code snippet you are using a String and as you know a String object is immutable. So every method call results in another String object being created. Only the result of the last method call replace("a", "abc") is assigned to the reference variable res and thus is still reachable; all other objects are unreachable and thus eligible for garbage collection. It would be a nice exercise to list all unreachable String objects from the second code snippet

Now in your code snippet

class EJavaGuruStringBuilder {
    public static void main(String args[]) {
        StringBuilder ejg = new StringBuilder(10 + 2 + "SUN" + 4 + 5);
        ejg.append(ejg.delete(3, 6));
        System.out.println(ejg);
    }
}

you didn't use chained methods at all. And that's why these methods are not executed from left to right. In order to call the append method, you need of course to know the String you want to append. That's why the delete method is executed before the the append method: the result of the delete method is required to be able to execute the append method.

Why this code snippet prints 12S512S5 is already well explained by Liutauras, so I won't explain it again. But I'll do a little pop quiz with you to see if you fully understood everything explained in this topic

What will be the output of the above code snippet if we replace the first line of the main method to

// exercise1
StringBuilder ejg = new StringBuilder(10 + (2 + "SUN") + 4 + 5);

// exercise2
StringBuilder ejg = new StringBuilder(10 + 2.0 + "SUN" + 4 + 5);

// exercise3
StringBuilder ejg = new StringBuilder(10 + 2 + "SUN" + (4 + 5));

If you think the code snippet doesn't compile, just say so (and explain why).

Hope it helps!
Kind regards,
Roel