How to use null in switch

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

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

This is not possible with a switch statement in Java. Check for null before the switch:

if (i == null) {
    doSomething0();
} else {
    switch (i) {
    case 1:
        // ...
        break;
    }
}

You can't use arbitrary objects in switch statements^*. The reason that the compiler doesn't complain about switch (i) where i is an Integer is because Java auto-unboxes the Integer to an int. As assylias already said, the unboxing will throw a NullPointerException when i is null.

^* Since Java 7 you can use String in switch statements.

More about switch (including example with null variable) in Oracle Docs - Switch

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

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.

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.

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) {
   }
}

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;    
}

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.

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

I just learned today that you dont have to put another layer of indentation/curly brackets just because of the if that is checking for null. You can do either of these:

if (i == null) {
    // ...
} else switch (i) {
    case 1:
        // ...
        break;
    default:
        // ...
}

or

if (i != null) switch (i) {
    case 1:
        // ...
        break;
    default:
        // ...
} else {
    // ...
}

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.