Java StringBuilder append() return?

Question

I'm just new in Java programming and it's really confusing me that what's the return type of method append() in StringBuilder??

I've checked about the API documents on https://docs.oracle.com/javase/9/docs/api/index.html?java/lang/String.html and the return is "StringBuilder", so I should write the code like:

StringBuilder a=new StringBuilder("hello");
...(another StringBuilder object)=a.append("world");

and a will still be "hello", and another stringbuilder will become"helloworld" because it has a return value??

But actually a itself also become "helloworld". Why??Did I misunderstand something?

Fluent Interface.

Elliott Frisch
– Elliott Frisch

2018-02-05 14:21:47 +00:00
Commented Feb 5, 2018 at 14:21 — Elliott Frisch
– Elliott Frisch, Commented Feb 5, 2018 at 14:21
Also: en.wikipedia.org/wiki/Method_chaining

Christophe Roussy
– Christophe Roussy

2018-02-05 14:31:34 +00:00
Commented Feb 5, 2018 at 14:31 — Christophe Roussy
– Christophe Roussy, Commented Feb 5, 2018 at 14:31

Tim M. · Accepted Answer · 2018-02-06 17:46:31Z

5

It is reasonable that StringBuilder follows the Builder Pattern, as it is used for building a String.

In a builder pattern each method (mostly) returns the current instance so that the return is a modified instance.

But actually a itself also become "helloworld". Why??

Because you are using the same instance to append again., so it already stored the previous value and was given the updated instance.

so I should write the code like:

StringBuilder a=new StringBuilder("hello"); ...(another StringBuilder object)=a.append("world");

You need not to. Since that returning the instance you can chain your method calls. That is the beauty of builder pattern.

StringBuilder a=new StringBuilder("hello");
a.append("World").append(" Mr.").append("Blah");

edited Feb 6, 2018 at 17:46

Tim M.

6186 silver badges15 bronze badges

answered Feb 5, 2018 at 14:24

Suresh Atta

122k38 gold badges206 silver badges315 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

kevinHuang Over a year ago

Thanks for the answer. But how should I know whether this method has this "builder pattern" or not?

Suresh Atta Over a year ago

As I said earlier, most of the methods from that class return the same type and methods have return this

Suresh Atta Over a year ago

Here is an example implementation en.wikipedia.org/wiki/Builder_pattern#Java

kevinHuang Over a year ago

Ok, I got it. Thanks for helping me.

jrtapsell · Accepted Answer · 2018-02-05 14:26:14Z

3

Why append returns a `StringBuilder`

So that you can append "a", "b" and "c" like this:

StringBuilder sb = new StringBuilder()
        .append("a")
        .append("b")
        .append("c");

What is returned

The StringBuilder returned is the same as the one that was called on, so this:

StringBuilder sb1 = new StringBuilder();
StringBuilder sb2 = sb1.append("a")
        .append("b")
        .append("c");
System.out.println(sb1 == sb2);

Prints out true, this is why the 2 StringBuilders in your example had the same text, because they are the same StringBuilder.

answered Feb 5, 2018 at 14:26

jrtapsell

7,0491 gold badge30 silver badges50 bronze badges

Comments

Mena · Accepted Answer · 2018-02-05 14:24:07Z

The StringBuilder class (as well as its thread-safe sibling, StringBuffer) offers a fluent builder pattern.

Most methods return the modified instance of your StringBuilder after applying the method, so you can chain-invoke methods on it.

You an either invoke append as you would a void returning method, and disregard the returned StringBuilder for one shot operations, or you can chain invocations, in the safety that you're still manipulating the same StringBuilder step by step.

Example

StringBuilder sb = new StringBuilder();
sb.append("a"); // void-like invocation
sb.append("b").append("c"); // chained invocation
// terminal invocation as it doesn't return an instance of 
// StringBuilder, but (immutable) String
String s = sb.toString(); // "abc"

Christophe Roussy · Accepted Answer · 2018-02-05 14:33:18Z

2

This is in order to allow chaining:

sb.append("a").append("b").append("c"). ...

Without this you would have to repeat sb: sb.append("a"); sb.append("b"); ...

This pattern is common in builders and most famous with jQuery. In general this can be useful, but when the chain becomes hard to read it can also introduce bugs.

edited Feb 5, 2018 at 14:33

answered Feb 5, 2018 at 14:23

Christophe Roussy

17.2k5 gold badges94 silver badges87 bronze badges

Comments

Naresh_Nalla · Accepted Answer · 2021-05-19 19:00:34Z

0

StringBuilder actually returns this ( current object) in append method of it.

answered May 19, 2021 at 19:00

Naresh_Nalla

211 silver badge7 bronze badges

Collectives™ on Stack Overflow

Java StringBuilder append() return?

5 Answers 5

4 Comments

Why append returns a `StringBuilder`

What is returned

Comments

Comments

Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

5 Answers 5

4 Comments

Why append returns a StringBuilder

What is returned

Comments

Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related

Why append returns a `StringBuilder`