[java] How to use null in switch

Integer i = ...

switch (i){
    case null:
        doSomething0();
        break;    
    }

In the code above I cant use null in switch case statement. How can I do this differently? I can't use default because then I want to do something else.

switch(i) will throw a NullPointerException if i is null, because it will try to unbox the Integer into an int. So case null, which happens to be illegal, would never have been reached anyway.

You need to check that i is not null before the switch statement.

You can't. You can use primitives (int, char, short, byte) and String (Strings in java 7 only) in switch. primitives can't be null.
Check i in separate condition before switch.

Java docs clearly stated that:

The prohibition against using null as a switch label prevents one from writing code that can never be executed. If the switch expression is of a reference type, such as a boxed primitive type or an enum, a run-time error will occur if the expression evaluates to null at run-time.

You must have to verify for null before Swithch statement execution.

if (i == null)

See The Switch Statement

case null: // will never be executed, therefore disallowed.

switch ((i != null) ? i : DEFAULT_VALUE) {
        //...
}

Just consider how the SWITCH might work,

• in case of primitives we know it can fail with NPE for auto-boxing
• but for String or enum, it might be invoking equals method, which obviously needs a LHS value on which equals is being invoked. So, given no method can be invoked on a null, switch cant handle null.

Some libraries attempt to offer alternatives to the builtin java switch statement. Vavr is one of them, they generalize it to pattern matching.

Here is an example from their documentation:

String s = Match(i).of(
    Case($(1), "one"),
    Case($(2), "two"),
    Case($(), "?")
);

You can use any predicate, but they offer many of them out of the box, and $(null) is perfectly legal. I find this a more elegant solution than the alternatives, but this requires java8 and a dependency on the vavr library...

You can also use String.valueOf((Object) nullableString) like

switch (String.valueOf((Object) nullableString)) {
case "someCase"
    //...
    break;
...
case "null": // or default:
    //...
        break;
}

See interesting SO Q/A: Why does String.valueOf(null) throw a NullPointerException

Based on @tetsuo answer, with java 8 :

Integer i = ...

switch (Optional.ofNullable(i).orElse(DEFAULT_VALUE)) {
    case DEFAULT_VALUE:
        doDefault();
        break;    
}

switch (String.valueOf(value)){
    case "null":
    default: 
}

Given:

public enum PersonType {
    COOL_GUY(1),
    JERK(2);

    private final int typeId;
    private PersonType(int typeId) {
        this.typeId = typeId;
    }

    public final int getTypeId() {
        return typeId;
    }

    public static PersonType findByTypeId(int typeId) {
        for (PersonType type : values()) {
            if (type.typeId == typeId) {
                return type;
            }
        }
        return null;
    }
}

For me, this typically aligns with a look-up table in a database (for rarely-updated tables only).

However, when I try to use findByTypeId in a switch statement (from, most likely, user input)...

int userInput = 3;
PersonType personType = PersonType.findByTypeId(userInput);
switch(personType) {
case COOL_GUY:
    // Do things only a cool guy would do.
    break;
case JERK:
    // Push back. Don't enable him.
    break;
default:
    // I don't know or care what to do with this mess.
}

...as others have stated, this results in an NPE @ switch(personType) {. One work-around (i.e., "solution") I started implementing was to add an UNKNOWN(-1) type.

public enum PersonType {
    UNKNOWN(-1),
    COOL_GUY(1),
    JERK(2);
    ...
    public static PersonType findByTypeId(int id) {
        ...
        return UNKNOWN;
    }
}

Now, you don't have to do null-checking where it counts and you can choose to, or not to, handle UNKNOWN types. (NOTE: -1 is an unlikely identifier in a business scenario, but obviously choose something that makes sense for your use-case).

You have to make a

if (i == null) {
   doSomething0();
} else {
   switch (i) {
   }
}