➜  src /usr/lib/jvm/java-14-openjdk/bin/javac --enable-preview --source 14 TestClass.java
➜  src cat config.xml                                                                                                   
<?xml version="1.0"?>
<!DOCTYPE module PUBLIC
        "-//Checkstyle//DTD Checkstyle Configuration 1.3//EN"
        "https://checkstyle.org/dtds/configuration_1_3.dtd">
<module name="Checker">
    <module name="TreeWalker">
        <module name="NeedBraces">
            <property name="tokens" value="LAMBDA"/>
            <property name="allowSingleLineStatement" value="true"/>
        </module>
    </module>
</module>
➜  src cat TestClass.java                                                                                               
interface MathOperation2 {
    int operation(int a, int b);
}

public class TestClass {
    MathOperation2 tooManyParens(int k) {
        return switch (k) {
            case 1 -> {
                MathOperation2 case5 = (a, b) ->
                        (a + b); // should be violation
                yield case5;
            }
            case (2) -> { // should be violation
                MathOperation2 case6 = (int a, int b) ->
                        (a + b); // should be violation
                yield case6;
            }
            case 3 -> {
                MathOperation2 case7 = (int a, int b) -> { // ok
                    return (a + b);
                };
                yield (case7);
            }
            case 4 -> {
                Runnable r3 = () -> // should be violation
                        String.CASE_INSENSITIVE_ORDER.equals("Hello world two!");
                MathOperation2 case10 = (int x, int y) -> { // ok
                    return (x + y);
                };
                yield case10;            }
            default -> {
                MathOperation2 case8 = (int x, int y) -> { // ok
                    return (x + y);
                };
                yield case8;
            }
        };


    }
}


➜  src java -jar /home/nick/IdeaProjects/checkstyle/target/checkstyle-8.36-SNAPSHOT-all.jar -c config.xml TestClass.java
Starting audit...
com.puppycrawl.tools.checkstyle.api.CheckstyleException: Exception was thrown while processing TestClass.java
        at com.puppycrawl.tools.checkstyle.Checker.processFiles(Checker.java:311)
        at com.puppycrawl.tools.checkstyle.Checker.process(Checker.java:221)
        at com.puppycrawl.tools.checkstyle.Main.runCheckstyle(Main.java:408)
        at com.puppycrawl.tools.checkstyle.Main.runCli(Main.java:331)
        at com.puppycrawl.tools.checkstyle.Main.execute(Main.java:190)
        at com.puppycrawl.tools.checkstyle.Main.main(Main.java:125)
Caused by: java.lang.NullPointerException
        at com.puppycrawl.tools.checkstyle.checks.blocks.NeedBracesCheck.getLastLambdaToken(NeedBracesCheck.java:474)
        at com.puppycrawl.tools.checkstyle.checks.blocks.NeedBracesCheck.isSingleLineLambda(NeedBracesCheck.java:460)
        at com.puppycrawl.tools.checkstyle.checks.blocks.NeedBracesCheck.isSingleLineStatement(NeedBracesCheck.java:344)
        at com.puppycrawl.tools.checkstyle.checks.blocks.NeedBracesCheck.isSkipStatement(NeedBracesCheck.java:308)
        at com.puppycrawl.tools.checkstyle.checks.blocks.NeedBracesCheck.visitToken(NeedBracesCheck.java:236)
        at com.puppycrawl.tools.checkstyle.TreeWalker.notifyVisit(TreeWalker.java:340)
        at com.puppycrawl.tools.checkstyle.TreeWalker.processIter(TreeWalker.java:451)
        at com.puppycrawl.tools.checkstyle.TreeWalker.walk(TreeWalker.java:278)
        at com.puppycrawl.tools.checkstyle.TreeWalker.processFiltered(TreeWalker.java:151)
        at com.puppycrawl.tools.checkstyle.api.AbstractFileSetCheck.process(AbstractFileSetCheck.java:87)
        at com.puppycrawl.tools.checkstyle.Checker.processFile(Checker.java:337)
        at com.puppycrawl.tools.checkstyle.Checker.processFiles(Checker.java:298)
        ... 5 more
Checkstyle ends with 1 errors.