I am just wondering why the Java 7 switch
statement does not support a null
case and instead throws NullPointerException
? See the commented line below (example taken from the Java Tutorials article on switch
):
{
String month = null;
switch (month) {
case "january":
monthNumber = 1;
break;
case "february":
monthNumber = 2;
break;
case "march":
monthNumber = 3;
break;
//case null:
default:
monthNumber = 0;
break;
}
return monthNumber;
}
This would have avoided an if
condition for null check before every switch
use.
As Mark points out in the comments, as of Java 21 a switch
statement does now support case null
thanks to JEP 441. A NullPointerException
will still be thrown by a switch lacking an explicit case null
if null
is passed into it.
Original answer follows:
As damryfbfnetsi points out in the comments, JLS §14.11 has the following note:
The prohibition against using
null
as a switch label prevents one from writing code that can never be executed. If theswitch
expression is of a reference type, that is,String
or a boxed primitive type or an enum type, then a run-time error will occur if the expression evaluates tonull
at run time. In the judgment of the designers of the Java programming language, this is a better outcome than silently skipping the entireswitch
statement or choosing to execute the statements (if any) after thedefault
label (if any).
(emphasis mine)
While the last sentence skips over the possibility of using case null:
, it seems reasonable and offers a view into the language designers' intentions.
If we rather look at implementation details, this blog post by Christian Hujer has some insightful speculation about why null
isn't allowed in switches (although it centers on the enum
switch rather than the String
switch):
Under the hood, the
switch
statement will typically compile to a tablesswitch byte code. And the "physical" argument toswitch
as well as its cases areint
s. The int value to switch on is determined by invoking the methodEnum.ordinal()
. The [...] ordinals start at zero.That means, mapping
null
to0
wouldn't be a good idea. A switch on the first enum value would be indistinguishible from null. Maybe it would've been a good idea to start counting the ordinals for enums at 1. However it hasn't been defined like that, and this definition can not be changed.
While String
switches are implemented differently, the enum
switch came first and set the precedent for how switching on a reference type should behave when the reference is null
.