Is this a method or a constructor?

Question

I am reading someone else's code and got confused by this snippet:

public static Builder Builder() {
        return new Builder();
    }

Is this a constructor? Constructor usually has no 'return' statement. Regular method doesn't use the upper case 'Builder()'. I got confused.

Seems like someone dont know about java code conventions but try to use factory design pattern :) — Link182, Sep 06 '18 at 10:36
let not the unconventional method name deceive us. It is a method returning a new Builder object. — padmanabhanm, Sep 06 '18 at 10:37
Not using the uppercase `Builder()` is a convention (and this should be named `builder()`). That constructors have no return type or value is a language *rule* and is always true. — chrylis -cautiouslyoptimistic-, Sep 06 '18 at 10:41
Possible duplicate of [What are static factory methods?](https://stackoverflow.com/questions/929021/what-are-static-factory-methods) — Greggz, Sep 06 '18 at 11:08

Stephen C · Accepted Answer · 2018-09-06T10:53:16.687

The key feature that distinguishes a constructor from a method is the return type. So

    /* optional modifiers */ Builder()

is a constructor¹ for Builder, but

    /* optional modifiers */ Builder Builder()

is a method named Builder that returns a Builder object. It is also an egregious style violation, since Java methods should start with a lower-case letter. Among other things, this makes it easier for human beings to distinguish methods and constructors! (The compiler doesn't care though ...)

There are other telltales too. Some modifiers are allowed for methods, but not for constructors. The static modifier for example.

In short, your example is a method².

^{1 - Note that the constructor name must match the enclosing class name. But if you get that wrong the compiler will still call this a constructor ... in the compilation error.}

^{2 - We can further classify it as a static factory method. However, that is a design classification, not anything to do with the Java language itself.}

score 5 · Answer 2 · answered Sep 06 '18 at 10:36

5

A constructor cannot be static, cannot return anything. So, it's a method.

answered Sep 06 '18 at 10:36

Nishit

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xingbin · Answer 3 · 2018-09-06T10:44:14.680

1

No, see jls 8.8

In all other respects, the constructor declaration looks just like a method declaration that has no result (§8.4.5).

....

Unlike methods, a constructor cannot be abstract, static, final, native, strictfp, or synchronized

The method name should be renamed to builder

edited Sep 06 '18 at 10:44

answered Sep 06 '18 at 10:36

xingbin

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score 0 · Answer 4 · answered Sep 06 '18 at 10:38

0

It is a method that returns a new instance of Builder using its default constructor Builder().

answered Sep 06 '18 at 10:38

deHaar

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score 0 · Answer 5 · answered Sep 06 '18 at 10:39

0

its a Static Factory Method, not a constructor.

answered Sep 06 '18 at 10:39

Raj

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score 0 · Answer 6 · answered Sep 06 '18 at 10:48

It is a method that returns an instance of Builder using no-argument constructor(Default constructor).

Additional to this, below are the Rules for writing Constructor

Constructor(s) of a class must has same name as the class name in which it resides.
A constructor in Java can not be abstract, final, static and Synchronized.
Access modifiers can be used in constructor declaration to control its access i.e which other class can call the constructor.

Is this a method or a constructor?

6 Answers6