Why `switch(null)` is a compile error but `switch(str)` is fine with str being `static final String str = null;`?

Question

While switch(null) is a compile error but switch(str) is fine ( str being static final String str = null; ).

Isn't static final String str = null; a compile-time constant which shall be substituted into switch(str) at compile time and thus be equivalent to switch(null) ?

switch (null) {  // compile Error immediately!

}

BUT:

public class Test {

    // compile-time constant?
    static final String s4 = null; 

    public static void main(String[] args) {

        switch (s4) {  // compiles fine! NPE at runtime

        }
    }
}

PS I suppose static final String str = null; IS NOT a compile-time constant, because only static final String str = 'string literal' is a compile-time constant , which explains example above (s4).

Answer 1

From the error message:

Incompatible types. Found 'null', required: 'char, byte, short, int, Character, Byte, Integer, String, or an enum'

You can see that null is inferred to nothing, it's not even an Object , it is simply the "null type", and not a String (or any other valid switchable type).

So for it to compile you'd need to cast it to String (or one of the other valid types).

switch((String) null) {

Which then throws a RuntimeException when you try to execute it.

This behaviour is not only applicable to switch it is in fact the same as when you do this:

null.isEmpty()

How should java know that you want to invoke String#isEmpty() ? You could aswell mean Collection#isEmpty() . Or any other isEmpty() method. The same goes for the switch -example, java simply doesn't know which type you mean.

Answer 2

From Oracle's doc :

Unlike if-then and if-then-else statements, the switch statement can have a number of possible execution paths. A switch works with the byte, short, char, and int primitive data types. It also works with enumerated types (discussed in Enum Types), the String class, and a few special classes that wrap certain primitive types: Character, Byte, Short, and Integer (discussed in Numbers and Strings).

Hence switch(null) is invalid because you can't switch on any value ( null could be a null Banana object which is not supported)