Is it bad practice to make a setter return “this”?

Question

Is it a good or bad idea to make setters in java return "this"?

public Employee setName(String name){
   this.name = name;
   return this;
}

This pattern can be useful because then you can chain setters like this:

list.add(new Employee().setName("Jack Sparrow").setId(1).setFoo("bacon!"));

instead of this:

Employee e = new Employee();
e.setName("Jack Sparrow");
...and so on...
list.add(e);

...but it sort of goes against standard convention. I suppose it might be worthwhile just because it can make that setter do something else useful. I've seen this pattern used some places (e.g. JMock, JPA), but it seems uncommon, and only generally used for very well defined APIs where this pattern is used everywhere.

Update:

What I've described is obviously valid, but what I am really looking for is some thoughts on whether this is generally acceptable, and if there are any pitfalls or related best practices. I know about the Builder pattern but it is a little more involved then what I am describing - as Josh Bloch describes it there is an associated static Builder class for object creation.

Answer 1

If I'm writing an API, I use "return this" to set values that will only be set once. If I have any other values that the user should be able to change, I use a standard void setter instead.

However, it's really a matter of preference and chaining setters does look quite cool, in my opinion.

Answer 2

I'm in favor of setters having "this" returns. I don't care if it's not beans compliant. To me, if it's okay to have the "=" expression/statement, then setters that return values is fine.

Answer 3

Long ago answer, but my two cents ... Its fine. I wish this fluent interface were used more often.

Repeating the 'factory' variable does not add more info below:

ProxyFactory factory = new ProxyFactory();
factory.setSuperclass(Foo.class);
factory.setFilter(new MethodFilter() { ...

This is cleaner, imho:

ProxyFactory factory = new ProxyFactory()
.setSuperclass(Properties.class);
.setFilter(new MethodFilter() { ...

Of course, as one of the answers already mentioned, the Java API would have to be tweaked to do this correctly for some situations, like inheritance and tools.

Answer 4

I don't think there's anything specifically wrong with it, it's just a matter of style. It's useful when:

• You need to set many fields at once (including at construction)
• you know which fields you need to set at the time you're writing the code, and
• there are many different combinations for which fields you want to set.

Alternatives to this method might be:

1. One mega constructor (downside: you might pass lots of nulls or default values, and it gets hard to know which value corresponds to what)
2. Several overloaded constructors (downside: gets unwieldy once you have more than a few)
3. Factory/static methods (downside: same as overloaded constructors - gets unwieldy once there is more than a few)

If you're only going to set a few properties at a time I'd say it's not worth returning 'this'. It certainly falls down if you later decide to return something else, like a status/success indicator/message.

Answer 5

To summarize:

• it's called a "fluent interface", or "method chaining".
• this is not "standard" Java, although you do see it more an more these days (works great in jQuery)
• it violates the JavaBean spec, so it will break with various tools and libraries, especially JSP builders and Spring.
• it may prevent some optimizations that the JVM would normally do
• some people think it cleans code up, others think it's "ghastly"

A couple other points not mentioned:

• This violates the principal that each function should do one (and only one) thing. You may or may not believe in this, but in Java I believe it works well.

• IDEs aren't going to generate these for you (by default).

• I finally, here's a real-world data point. I have had problems using a library built like this. Hibernate's query builder is an example of this in an existing library. Since Query's set* methods are returning queries, it's impossible to tell just by looking at the signature how to use it. For example:

  Query setWhatever(String what);
  

• It introduces an ambiguity: does the method modify the current object (your pattern) or, perhaps Query is really immutable (a very popular and valuable pattern), and the method is returning a new one. It just makes the library harder to use, and many programmers don't exploit this feature. If setters were setters, it would be clearer how to use it.

Answer 6

It's not bad practice. It's an increasingly common practice. Most languages don't require you to deal with the returned object if you don't want to so it doesn't change "normal" setter usage syntax but allows you to chain setters together.

This is commonly called a builder pattern or a fluent interface.

It's also common in the Java API:

String s = new StringBuilder().append("testing ").append(1)
  .append(" 2 ").append(3).toString();