Fixing the Billion Dollar Mistake JavaLand

22.03.23
Fixing the
Billion Dollar Mistake
(Java Lens)
Richard Gross (he/him)
IT-Tailor Archaeologist Auditor
richargh.de/
speakerdeck.com/richargh
@arghrich

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Slides by richargh.de and
5
€ Dealing
with
“nothing”
Effective
“nothing”
Fixing the
Mistake in
Java

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What is the most
dangerous error
programmers make?
6

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2022 CWE Top 20 Most Dangerous1
Software Weaknesses
7
1„most common and impactful issues experienced over the previous two calendar years.”
https://cwe.mitre.org/top25/archive/2022/2022_cwe_top25.html & Comment by Jeff Atwood
#11 Null
Pointer
Dereference
Input Validation
› ‚Cross-Site Scripting‘
› ‚SQL Injection‘
Auth
› Missing Authentication / Authorization
Manual Memory
› Out-of-bounds read/write
› Use after Free

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Handling null is
important but not the
most reported mistake
8

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How often does null
cause business
problems?
9

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97% of All Logged Errors are Caused by
only 10 Unique Errors
10
https://www.overops.com/blog/we-crunched-1-billion-java-logged-errors-heres-what-causes-97-of-them-2/
https://www.overops.com/blog/the-top-10-exceptions-types-in-production-java-applications-based-on-1b-events/
97%
3%
All Logged Errors
10 Unique Errors Other Errors
97% of Logged Errors by Frequency
1. NullPointerException
2. NumberFormatException
3. IllegalArgumentException
4. …

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NullPointer
The most popular exception
11

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12
I call it my billion-dollar mistake. It was
the invention of the null reference in 1965
[ALGOL]. […] This has led to innumerable
errors, vulnerabilities, and system crashes,
which have probably caused a billion dollars
of pain and damage in the last forty years.
Tony Hoare
Null References:
The Billion Dollar Mistake
2009 InnoQ
Did not design C#, Java, or
Python though. J

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14
€ Dealing
with
“nothing”
Effective
“nothing”
Fixing the
Mistake in
Java

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Designing a Map
1. public class Addresses {
2. private final Map all = new HashMap<>();
3.
4. public Address getById(PersonId personId) {
5. return all.get(personId);
6. }
7. /* … */
15
Applying the Principle of Least Surprise:
What should this return when person is unknown?

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Modelling “Nothing”
16
3.
4. public Address getById(PersonId personId) {
5. return all.get(personId);
6. }
7. /* … */
Implicit
Throw Exception
C#, Python
throw KeyNotFoundException
Implicit
Return null
Java
return null;
Implicit
Return “default”
return Address.NULL;
Explicit
Require a default
return GetOrDefault(
personId,
defaultAddress);
Explicit
Model absence
Scala, Kotlin, F#, …
return ???;

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Explicit abscence
1. const address = addresses.find(personId);
17
2. // cannot be ignored
3. log(`Street is ${address.street}`);
4. // has to be checked before use
5. if(address != null)
6. log(…)
Compile error
Cannot access without explicit check

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Wouldn‘t it be nice if
Java had explicit
absence?
18

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19
More secure
languages
by taking
away
options
„Goto
considered
harmful“ *
If, else
for
While
return
Manual
memory
considered
harmful
Garbage Collection,
Reference Counting
Ownership (Rust J)
Implicit null
considered
harmful
Optional
Maybe T
Nullable?
*Edsger Dijkstra „A Case Against the Goto Statement“ aka Goto Statement Considered Harmful.

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F# 2005
F# empowers everyone
to write succinct, robust
and performant code
Has null but you cannot
assign it to anything.
20
Rust 2010
A language empowering
everyone to build reliable
and efficient software.
Doesn‘t even have null.
Kotlin 2011
Modern, concise and
safe programming
language
/* Get rid of those
pesky
NullPointerExceptions,
you know, The Billion
Dollar Mistake*/
Swift 2014
write software that is
incredibly fast and safe by
design.
Has nil for Objective-C
interop.
There is a trend here
https://en.wikipedia.org/wiki/Void_safety

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Luckily, older
languages evolve as
well
21

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Everyone tries to deal with null
22
Python
Java
C#
https://docs.microsoft.com/en-us/dotnet/csharp/whats-new/csharp-version-history
JSR-305 https://jcp.org/en/jsr/detail?id=305
FindBugs is dead: https://github.com/spotbugs/spotbugs.github.io/issues/24
Nullaway Paper https://www.uber.com/blog/research/nullaway-practical-type-based-null-safety-for-java/
‘94
‘96
‘02
‘05
‘06
‘14
‘15
‘19
C# 02
› Nullable Value
Types à int?
› Null-coalescing
operator ??
C# 06
› Null-propagator
?. and ?[]
C# 08 .NET Core 3+
› Nullable reference
types à User?
Java 8
› Optional
instead of return
null;
Python 3.5
› Type hints
› Optional[T]
JSR 305
› @Nullable
› FindBugs initiative
› Dormant since ‘12
Nullaway
› Compile-time
@Nullable check

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23
€ Dealing
with
“nothing”
Effective
“nothing”
Fixing the
Mistake in
Java

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Where’s the error?
24
1. public String findNotebookMaker(EmployeeId id) {
2. var employee = company.getById(id);
3. if (employee == null)
4. return "Employee does not exist";
5. /* ... */
6.
7. return employee.notebook().maker();
8. }

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Why is this an error?
5. /* ... */
6.
8. }
25
But, but, but, an Employee always has a
notebook!!1! I designed it that way!
java.lang.NullPointerException:
Cannot invoke "Notebook.maker()"
because the return value of
"Employee.notebook()" is null

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The error is somewhere else
26
Under certain conditions (f.ex. notebook is being repaired),
an employee does not have a notebook. This feature was
added later, or maybe you just forgot. The type in employee
does not reflect this.
1. public void startNotebookRepair(EmployeeId id) {
4. return;
5. /* ... */
6.
7. company.put(id, employee.withNotebook(null));
8. }
1. public record Employee(
2. EmployeeId id,
3. String name,
4. Notebook notebook) {
5. }

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Stack traces are useless
for NullPointerExceptions
27

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Add compile-time null checks with
Nullaway
1. // add processor to pom.xml (or .gradle):
2.
3.
4. maven-compiler-plugin>
5.
6.
7.
8. -Xplugin:ErrorProne
9. -Xep:NullAway:ERROR
10. -XepOpt:NullAway
11. :AnnotatedPackages=de.richargh>>
12.
13.
14.
15. com.google.errorprone>>
16.
17. com.uber.nullaway>>
28
Nullaway has instructions for Maven, Gradle, … https://github.com/uber/NullAway
1. // add annotation to *.java file:
2. public class {
3. // can now signal null
4. private @Nullable String text1;
5. { /* … */}
6. private @Nullable data;
7. { /* … */}
8. // this is not nullable
9. private String text2;
10.}

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After Enable (1)
29
Compile error
4. return;
5. /* ... */
6.
7. company.put(id, employee.withNotebook(null));
8. }
[NullAway] passing @Nullable parameter
'null' where @NonNull is required

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(2) Mandatory nullability modelling
30
Mark it as nullable
2. EmployeeId id,
3. String name,
4. @Nullable Notebook notebook)
{
5. }

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(3) Cascading compile error(s)
31
5. /* ... */
6.
8. }
Compile error
[NullAway] dereferenced expression
employee.notebook() is @Nullable

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The compiler guides you
through the code base
32

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We fixed the root cause, not a symptom
33
4. return;
5. /* ... */
6.
7. company.put(id, employee.withoutNotebook());
8. }
2. EmployeeId id,
3. String name,
4. @Nullable Notebook notebook)
{
5. /* withoutNotebook() … */
6. }
5. /* ... */
6.
7. return employee.notebook() != null
8. ? employee.notebook().maker()
9. : EMPLOYEE_DOES_NOT_HAVE_A_NOTEBOOK;
10.}

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Nullaway helps us find&fix
the root cause of bugs
34

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Migration is worth it. You‘ll
catch so many bugs
37

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Suppose your system looks like this
38
…
UI
Logic
Data
Adapter
commons.lang
jar
Edge Elements
commons.error
commons….
UI
Logic
Data
Adapter
UI
Logic
Data
Adapter
UI
Logic
Data
Adapter
More, Focused steps:
migrate one at a time
Common jar(s) next
Don’t trust the edge
Migrate a simple jar
first
AnnotatedPackages +
UnannotatedSubPackages

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39
€ Dealing
with
“nothing”
Effective
“nothing”
Fixing the
Mistake in
Java
What
about
Optional?

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Optional
Awesome but often misunderstood
40

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It’s actually OptionalReturn
and useful for for fluent apis
41
Optional is not for optional
fields
Optional is not for optional
method parameters
Optional does not eliminate
NullPointerExceptions

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While drafting the fluent Java 8 Streams API they realized a
problem2:
1. String employeeNameById(EmployeeId id){
2. return employees.stream()
3. .search(it -> it.id() == id)
4. .name();
5. }
It’s really OptionalReturn1 as in
method return.
42
1 https://mail.openjdk.org/pipermail/lambda-dev/2012-September/005952.html
2 Optional - The Mother of All Bikesheds https://www.youtube.com/watch?v=Ej0sss6cq14
In the actual Stream Apithey added a limited mechanism to
represent “no result” and keep the Api fluent2:
1. String employeeNameById(EmployeeId id){
2. return employees.stream()
3. .filter(it -> it.id() == id)
4. .findFirst()
5. // now we have an Optional
6. .map(Employee::name)
7. .orElse(“UNKNOWN”);
8. }
Non-obvious
NullPointerException
Will only call .name()
when non-null

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Optional does not eliminate
NullPointerExceptions
43
1. // this is valid without Nullaway
2. Optional employeeName = null;
7. // and improper use still results in Exceptions
8. employees.stream()
9. .filter(it -> it.id() == id)
10. .findFirst()
11. .get(); // potential NoSuchElementException
3. // so is this
4. var person = new Person(null);
5. record Person(
6. Optional employer) {}

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Optional for optional fields: error-prone
and cluttered
44
Optional, rather inconvenient if you don’t want fluidity
1. if(sth.a().isEmpty()
2. && sth.b().isPresent()
3. && sth.b().get().bb().isPresent()){
4.
5. aMethod(sth.b().get(), sth.c().get());
6. }
With @Nullable, less clutter, actual compile-safety
1. if(sth.a() == null
2. && sth.b() != null
3. && sth.b().bb() != null){
4.
5. aMethod(sth.b(), sth.c());
6. }
NoSuchElementException:
Forgot to check .c().isPresent()
[NullAway] dereferenced expression
something.c() is @Nullable

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Optional for optional parameters: quite
noisy and cluttered
45
With Optional
1. myMethod(a, Optional.of(b), c, Optional.of(d))
With @Nullable, less clutter, compile-safety
1. myMethod(a, b, c, d)
NullPointerException: d is null,
so this should’ve been
Optional.ofNullable(d)

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Modelling Absence in Java with Nullaway
and sometimes Optional
48
Explicit
Model absence
Scala, Kotlin, F#, Java
3.
4. public Optional getById(PersonId personId) {
5. return Optional.ofNullable(all.get(personId));
6. }
7. /* … */
1. record Person(
2. PersonId id,
3. @Nullable 123){
4.
5. Address withoutNotebook(){ /* … */ }
6. }
Compile-time
safety
Optional where
Fluent Api useful

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49
€ Dealing
with
“nothing”
Effective
“nothing”
Fixing the
Mistake in
Java

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#4 Make non-null your default
• NullAway assumes this
• JSR-305 has @ParametersAreNonnullByDefault for classes
and packages
• JSpecify has @NullMarked
50
1

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51
#4 Model Optional Fields with @Nullable
2
1. public class {
2. private @Nullable Address address;
3. }
4.
5. public record (
6. @Nullable Address address){
7. }

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52
#6 Never pass null, consider explicit
methods instead
1. var p1 = notebookPriceFor(
2. employeeId, notebookType);
3.
4. // what does passing null mean?
5. // what is this supposed to return?
7. null, notebookType);
Clean Code Chapter 7
3
2. employeeId, notebookType);
3.
4. // create an explicit method that
indicates a query without an employee is
just an estimate
5. var p2 = estimateNotebookPrice(
6. notebookType);

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#4 Mark nullable method returns
53
4a
1. @Nullable myMethod(A a, B b) {
2. /*…*/
3. }

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6. // Choice A: caller says null is not allowed
7. var account = accountsfindAccountById(id).orElseThrow();
#4 Occasionally consider Optional for
method returns to give choices to caller
54
4b
1. // Optional is quite useful for repositories
2. Optional getAddressById(employeeId id) {
3. return Optional.ofNullable(addresses.get(id));
4. }
8. // Choice B: caller says default is possible
9. var account = accounts.findAccountById(id).orElse(Account.none());
10.// Choice C: callers uses fluent chain
11.var budget = accounts.findAccountById(id)
12. .map(Account::budgetId)
13. .flatMap(budgets::getById)

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56
#5 Return empty collections or arrays, not
nulls
1. // not like this
2. @Nullable List recentAddresses(){
3. return employee.isTrackingAllowed()
4. ? recentAddresses
5. : null;
6. }
1. // like this
2. List recentAddresses(){
3. return employee.isTrackingAllowed()
4. ? recentAddresses
5. : Collections.emptyList();
6. }
Effective Java 3rd Edition Item 54
Makes Api more
difficult to use for all
callers.
5

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57
#5 Use JDK 16 records for domain types
6
1. public class Address {
2. private final String city;
3. private final String street;
4.
5. Address(
6. String city, String street){
7. /*…*/
8. }
9.
10. @Override boolean equals /*…*/
11.
12. @Override int hashCode /*…*/
13.
14. @Override String toString /*…*/
15.}
1. public record Address(
2. String city,
3. @Nullable String street
4. ) {}

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Records provide value-equality and
immutability
1. public record Address(
2. String city,
3. @Nullable String street
4. );
58
5. var address1 = new Address(
6. “Brühl”, "Berggeiststraße 31-41");
7. var address2 = new Address(
8. “Brühl”, "Berggeiststraße 31-41");
9.
10. address1.equals(address2); // true

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59
#5 Don’t trust types from the edge of your
system
7
1. public record RenterDto(
2. String id,
3. String name)
4. var json = """{ ”name": ”Alex" }""";
5. var result = objectMapper.readValue(json, RenterDto.class);
6. // result = RenterDto[id=null, name=Alex]
7. result.id() == null; // true…
Mapping-Frameworks like Jackson
see null-checks as validation.
And they don’t do validation.
We can write our own
ConstraintValidator however

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The Future…?
61
Java
Checker Frameworkhttps://checkerframework.org/
Nullaway Paper https://www.uber.com/blog/research/nullaway-practical-type-based-null-safety-for-java/
JSpecify https://jspecify.dev/
‘08
‘19
‘23
‘25
Checker Framework
› Compile-time
checks
› NullChecker is
alternative to
Nullaway
Nullaway
› Compile-time
@Nullable check
Primitive Optional<>?
› Optional becomes
Primitive
› Can then only be
present or empty,
never null
JSpecify Ref. Impl.
› Unifying the JSR305
annotation-jungle
› @Nullable,
@NullMarked
JDK Null-Safety?
› Maybe here
› Might happen if C#
Null-Addition works
out well

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Slides by @arghrich
Thanks
62
IT-Tailor
Richard Gross (he/him)
Archaeologist Auditor
richargh.de/
speakerdeck.com/richargh
@arghrich
Works for maibornwolff.de/
People. Code. Commitment.
DE TN ES
Code at https://github.com/Richargh/fixing-the-billion-dollar-mistake

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Backup

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2022 CWE Top 20 Most Dangerous
Software Weaknesses
1. Out-of-bounds Write
2. ‘Cross-site Scripting’
3. ‘SQL Injection’
4. Improper Input Validation
5. Out-of-bounds Read
6. ‘OS Command Injection’
7. Use After Free
8. ‘Path Traversal’
9. Cross-Site Request Forgery
10. Unrestricted File Upload with ‘Evil’ Type
11. NULL Pointer Dereference
12. Deserialization of Untrusted Data
13. Integer Overflow or Wraparound
14. Improper Authentication
15. Use of Hard-Coded Credentials
16. Missing Authorizations
17. ‘Command Injection’
18. Missing Auth. for Critical Function
19. Improper Restriction … of Memory Bounds
20. Incorrect Default Permissions
64
Rough categories
Manual Memory
Input Validation
Auth

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2021 CWE Top 20 Most Dangerous
Software Weaknesses
1. Out-of-bounds Write
2. ‘Cross-site Scripting’
3. Out-of-bounds Read
4. Improper Input Validation
5. ‘OS Command Injection’
6. ‘SQL Injection’
7. Use After Free
8. ‘Path Traversal’
9. Cross-Site Request Forgery
10. Unrestricted File Upload with ‘Evil’ Type
11. Missing Auth. for Critical Function
12. Integer Overflow or Wraparound
13. Deserialization of Untrusted Data
14. Improper Authentication
15. NULL Pointer Dereference
16. Use of Hard-Coded Credentials
17. Improper Restriction … of Memory Bounds
18. Missing Authorizations
19. Incorrect Default Permissions
20. Exposure of Sensitive Information …
65
Rough categories
Manual Memory
Input Validation
Auth

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66
€ Dealing
with
“nothing”
Effective
“nothing”
Fixing the
Mistake in
C#
One more thing …

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67
I‘ve enabled nullable reference types but
now I‘ve got null-checks everywhere!!11!eleven

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Great, you‘ve fixed the root
cause of many bugs at
compile time
68

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1. public IResponse CreateEmployee(Request request)
2. {
3. var employee = EmployeeFromBody(request.Path);
4. CheckEmailIsUnique(employee);
5. StoreEmployee(employee);
6. EmailEmployee(employee);
7. return new OkResponse(200);
8. }
69

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2. {
4. if(employee is null)
5. {
6. return new BadResponse(400, “Employee invalid”);
7. }
8. if(EmailIsNotUnique(employee))
9. {
10. return new BadResponse(400, “Email must be unique”);
11. }
13. try {
15. }
16. catch(ServerUnreachableException)
17. {
18. return new BadResponse(500, “could not send email”);
19. }
21.}
70
Where did my
happy path go?
Only one null-
check to keep
example short

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Can we clarify the business
rules again?
71

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A shallow dive into
Railway-Oriented
Programming*
72
*By Scott Wlaschin https://fsharpforfunandprofit.com/rop/

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Happy Path without error handling
73
2. {
4. CheckEmailIsUnique(employee);
8. }

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ROP Path with error handling
74
2. {
3. EmployeeFromBody(request.Path).AsResult(ifnull: “invalid employee passed”)
4. .ThenTry(CheckEmailIsUnique)
5. .Then(StoreEmployee)
6. .ThenTry(EmailEmployee)
7. .Then(_ => new OkResponse(200));
8. }
I can see my
business logic
again

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Focus on Happy Path, Result does the rest
76
J L
thenTry
then
thenTryFix
thenFix
1. Result Then(
2. Func onOk){ /*…*/ }
3. Result ThenTry(
4. Func> onOk){ /*…*/ }

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How to get on the rail
77
J L
Ok(sth) Fail(“why”)
OfResult(sth?)

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Against Railway-Oriented
Programming*
78
*Also Scott Wlaschin https://fsharpforfunandprofit.com/posts/against-railway-oriented-programming/
https://eiriktsarpalis.wordpress.com/2017/02/19/youre-better-off-using-exceptions/

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Don‘t wrap all exceptions
with Results
79

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Use ROP only for Domain
Errors
80

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How to get started?
81
Railway Oriented Programming [Original, Scott Wlaschin 2014]
Against Railway-Oriented Programming [Scott Wlaschin 2019]
Railway Oriented Programming: C# Edition [Tama Waddell 2019]
Switch to F# J Or Copy/Code your
Java Result class
Watch a deep-
dive talk J

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Now we‘re done J
82

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I‘m not the first to talk about ROP :)
• Railway Oriented Programming [Original, Scott Wlaschin 2014]
• Against Railway-Oriented Programming [Scott Wlaschin 2019]
• Railway Oriented Programming: C# Edition [Tama Waddell 2019]
• Railway-Oriented Programming in C# [Marcus Denny 2017]
• Railway-oriented programming in Java [Tom Johnson 2021]
83

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UseCase: Change Employee Address
84
EmployeeId From
Request Path
Find Employee
Address from Path
Change Address
Store changed
Employee
Greet Employee with
new Email

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1. public IResponse ChangeAddress(Request request)
2. {
3. var employeeId = EmployeeIdFromPath(request.Path);
4. var employee = _employees.FindById(employeeId);
5. var address = AddressFromBody(request.Body);
6. employee = employee.ChangeAddress(address);
7. _employees.Store(employee);
8. GreetEmployee(employee.Email);
10.}
85
What about DbExceptions?
Does not compile. All nullable.

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2. {
3. var employeeId = EmployeeIdFromPath(request.Path);
4. if(employeeId is null)
5. {
6. return new BadResponse(400, “Employee Id invalid”);
7. }
8. var employee = _employees.FindById(employeeId);
9. if(employee is null)
10. {
11. return new BadResponse(400, “Employee not found”);
12. }
13. var address = AddressFromBody(request.Body);
14. if(address is null)
15. {
16. return new BadResponse(400, “address invalid”);
17. }
18. employee = employee.ChangeAddress(address);
19. try {
20. _employees.Store(employee);
21. }
22. catch(MyDbException ex)
23. {
24. return new BadResponse(500, “could not change email”);
86
Where did my
happy path go?

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ROP Path with error handling
87
2. {
3. EmployeeIdFromPath(request.Path)
4. .ThenTry(_employees.FindById)
5. .ThenTryToPair(_ => AddressFromBody(request.Body))
6. .ThenTry((employee, address) => employee.ChangeAddress(address))
7. .ThenTry(_employees.Store)
8. .ThenTry(employee => GreetEmployee(employee.Email))
9. .Then(_ => new OkResponse(200));
10.}
I can see my
business logic
again

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UseCase: Rent NotebookType
88
Employee doesn’t have
a notebook
Employee has enough
budget?
Notebook with desired
type is available?
Rent notebook for
employee

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1. public IResponse Rent(NotebookType type, EmployeeId
eId)
2. {
3. var employee = _employees.FindById(eId);
4. if (AlreadyHasANotebook(employee))
5. {
6. return Bad(”Already has a notebook);
7. }
8. var notebook = _inventory
9. .FindNotebooksByType(type)
10. .FirstOrDefault(IsAvailable);
11.
12. var budget = _budget.FindById(eId);
13. if (HasNotEnoughBudget(budget, notebook))
14. {
15. return Bad(”Not enough budget”);
16. }
17. var remainingBudget = RentNotebook(
employee, budget, notebook);
18. NotifyOfRent(
employee, notebook, remainingBudget);
19. return Ok(notebook);
20.}
89
Employee doesn’t have
a notebook
Employee has enough
budget?
Notebook with desired
type is available?
Rent notebook for
employee
Notify Employee
Does not compile
What about DbExceptions?
Negative framing

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1. public IResponse Rent(NotebookType type,
EmployeeId eId)
2. {
3. var employee = _employees.FindById(eId);
4. if (employee is null)
5. {
6. return Bad("Employee does not exist);
7. }
8. if (AlreadyHasANotebook(employee))
9. {
10. return Bad(”Already has a notebook);
11. }
12. var notebook = _inventory
13. .FindNotebooksByType(type)
14. .FirstOrDefault(IsAvailable);
15. if (notebook is null)
16. {
17. return Bad("Notebook does not exist);
18. }
19. var budget = _budget.FindById(eId);
20. if (budget is null)
21. {
22. return Bad("Employee has no budget);
23. }
24.
25. if (HasNotEnoughBudget(budget, notebook))
26. {
27. return Bad(”Not enough budget”);
28. }
29.
30. try
31. {
32. var remainingBudget = RentNotebook(
employee, budget, notebook);
33. NotifyOfRent(
employee, notebook, remainingBudget);
34. return Ok(notebook);
35. }
36. catch (MyDbException e)
37. {
38. Console.WriteLine(e);
39. return Bad("Could not notebook");
40. }
41.}
90
Where did all my business
rules go?

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Fixing the Billion Dollar Mistake JavaLand

Fixing the Billion Dollar Mistake JavaLand

Richard

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