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

user_8339668 0
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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

        }
    }
}

P.S. 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).

compile-time-constant switch-statement constants null java
2 Answers

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.

user_55150600

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)

user_2677190
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