Update: Pattern matching for switch arrived in Java 21.
tl;dr
👉 switch expressions != pattern matching with switch
You are mixing up the relatively new feature of switch expressions with the still-not-released feature of pattern matching with switch.
The switch feature in Java has been evolving through 3 phases, two completed:
- ☑️
switch statement (original feature in Java 1)
- ☑️
switch expression (Java 14+)
- ❌ Pattern matching for
switch, including case null (previewed in Java 17, 18, 19, & 20)
Do not conflate switch expressions with pattern matching
Classic switch
Understand that historically, the Java switch statement has been hostile to null checks. See this Question, How to use null in switch. As shown there, code such as this:
switch (i) {
case null:
doSomething0();
break;
}
… was not possible.
switch expressions
Fast forward to Java 14, when switch expressions was added to Java. See JEP 361: Switch Expressions. That allows the syntax seen in your first code example:
return switch(testEnum) {
case foo -> newFoo();
case bar -> newBar();
}
But read the JEP. No mention of null — « crickets ».
Pattern matching for switch
Fast forward further, to JEP 406: Pattern Matching for switch (Preview). Note that this is a preview feature in Java 17, not a final, officially released feature. (To understand how preview features work, read JEP 12: Preview Features.)
👉 In that JEP 406, notice its second goal: Allow the historical null-hostility of switch to be relaxed when desired.
Now search that page for “null” — 73 hits! That page explains the former policy of the Java language:
Traditionally, switch statements and expressions throw NullPointerException if the selector expression evaluates to null …
Notice the mention of statements, the original switch syntax, and additionally expressions, the new syntax used in your code. In both cases, null check was forbidden.
That page goes on to explain the changes that motivate the inclusion of support for null checks. Read the JEP for will-written details.
👉 The upshot is that you can use case null in a switch in Java 17 — but only if you go out of your way to enable the preview feature.
Still in preview, in Java 20
"We will sell no wine before its time."
There too, in Java 20, you will have to go out of your way to activate the preview feature.
And of course two reminders:
- Preview features should not be used in production for critical apps.
- Preview features are likely to change. That is their purpose, to gather feedback and suggestions leading to changes.
Example
Let's try this code example. Notice how we use plain old syntax here, without the ->. The arrow operator is not related to our discussion here.
String x = null;
switch ( x )
{
case "starburst":
System.out.println( "Is starburst." );
break;
case null:
System.out.println( "Whoops, null." );
break;
default:
System.out.println( "Some other value found." );
break;
}
In Java 17, by default, without enabling preview features we get this error while editing the code for case null:
Patterns in switch are not supported at language level '17'
Set language level to 17 (Preview) - Pattern matching for switch
After enabling preview features in Java 17, the code compiles without complaint. And it runs:
Whoops, null.
javacsay when you compile that file? – Nimwegenjavac), and it gave the same error! I updated the question with this additional information. – Crowdernullcases was present in Java 17, but I guess that is different than pattern matching. So pattern matching is a full feature in Java 17, andnullcase statements is a full feature in Java 17, but pattern matching withnullis only a preview feature in Java 17? That's sort of confusing. – CrowderNullPointerExceptionat runtime. Seems, that support isn’t complete. – Unbated