Java Constructor and static method

Question

When should I use a constructor and when should I use static method?

Can you explain above with small snippet? I skimmed through a few threads but I'm still not clear with this.

Answer 1

Joshua Bloch advises to favor static factory methods instead of constructors (which I think is a good practice). Couple of advantages and disadvantages :

Advantages of static factory methods :

• unlike constructors, they have names
• unlike constructors, they are not required to create a new object each time they're invoked (you can cache instances : e.g. Boolean.valueOf(..)
• unlike constructors, they can return an object of any subtype of their return type (great flexibility)

Disadvantages of static factory methods :

• They are not really distiguishable from other static methods (it's hard to find out how to initialize an object if you are not familiar with the API)
• The main disadvantage (if you use only static factory methods, and make constructors private) is that you cannot subclass that class.

Answer 2

Use a public constructor when you only ever want to return a new object that type and you want simplicity.

A good example is StringBuilder as it's mutable and you are likely to want a new object each time.

public String toString() {
    StringBuilder sb = new StringBuilder();
    // append fields to the sb
    return sb.toString();
}

Use a static factor method when you might want to re-use objects (esp if immutable), you might want to return a sub-class or you want descriptice construction. A good example is EnumSet which has a number of static factories which do different things even though some have the same arguments.

EnumSet.noneOf(RetentionPolicy.class);
// has the same arguments, but is not the same as
EnumSet.allOf(RetentionPolicy.class);

In this case, using a static factory makes it clear what the difference between these two ways of construction the set.

Also EnumSet can return two different implementations, one optimised for enums with a small number of values (<= 64) RegularEnumSet and another for many values called JumboEnumSet