This document discusses the evolution of the Java type system, including changes in Java 5-8 and future projects. Java 5 introduced generics and wildcards. Java 8 added generalized type inference and type annotations. The Checker Framework provides type checkers for properties like nullness and tainting. Project Valhalla aims to add value types and generic specialization to Java.

1. Back to the future:
Evolution of the Java Type System
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2. Agenda
 History: Java 5 Type System
 Generalized Target-Type Inference
 Annotated Types
 Checker Framework
 Project Valhalla
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3. Java 5 Type System
 Parametric polymorphism
public class Sample<T> {
 Bounded wildcards
 Intersection types
 Type erasure
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private T data;
public T getData() { ... }
}
Collection<? extends Serializable>
public class Box<T extends Comparable & Clonable> { ...

4. Java 8 Type System
 JEP 101: Generalized Target-Type Inference
- Add support for method type-parameter inference in method context
- Add support for method type-parameter inference in chained calls
 JSR 308: Annotations on Java Types
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5. Target Typing
Map<Integer, List<String>> map = new HashMap<>();
...
map.put(key, Collections.emptyList());
public <E> void process(List<E> list, Class<E> type) {...}
...
process(new ArrayList<>(), String.class)
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6. Target Typing
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List<Number> numbers = Arrays.asList(1,2,3);
Iterator<String> iterator = new ArrayList<>().iterator();

7. Annotation Type Target
 ANNOTATION_TYPE
 CONSTRUCTOR
 FIELD
 LOCAL_VARIABLE
 METHOD
 PACKAGE
 PARAMETER
 TYPE
 TYPE_PARAMETER
 TYPE_USE
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Java 8

8. Type annotations usage
 generic type arguments
List<@Localized Document> files;
 type parameter bounds
Collection<? @NotNull Number> files;
 class inheritance
class UnmodifiableList<T> implements @Readonly List<T> {...}
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9. Type annotations location
 object creation
new @Interned MyObject();
 casting
myString = (@NonNull String) myObject;
 throwing exceptions
void update() throws @Critical DataAccessException { ... }
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The Java 8 release does not
provide a type checking framework

11. Checker Framework
 Nullness Checker
 Interning Checker
 Lock Checker
 Fake Enum Checker
 Tainting Checker
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 Regex Checker
 Format String Checker
 Property File Checker
 Signature Checker
 Immutability Checker

12. Example 1. NulnessChecker
import org.checkerframework.checker.nullness.qual.Nullable;
public class NullnessExample {
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public String buildMessage(@Nullable String str) {
return "Message: " + str.toLowerCase();
}
}
javac -processor org.checkerframework.checker.nullness.NullnessChecker ...
Compile-time error

13. Example 1. NulnessChecker
import org.checkerframework.checker.nullness.qual.Nullable;
public class NullnessExample1 {
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public String buildMessage(@Nullable String str) {
if (str != null) {
return "Message: " + str.toLowerCase();
}
return "";
}
}
Compiles successfully

14. Example 2. Tainting Checker
import org.checkerframework.checker.tainting.qual.Tainted;
import org.checkerframework.checker.tainting.qual.Untainted;
public class TainedExample {
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public void execute(@Tainted String id) {
String query = buildQuery(id);
//...
}
public String buildQuery(@Untainted String id) {
return "SELECT p.name FROM product p where p.id = " + id;
}
}
javac -processor org.checkerframework.checker.tainting.TaintingChecker ...
Compile-time error

15. Example 2. Tainting Checker
import org.checkerframework.checker.tainting.qual.Tainted;
import org.checkerframework.checker.tainting.qual.Untainted;
public class TainedExample {
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public void execute(@Tainted String id) {
String query = buildQuery(verify(id));
//...
}
public @Untainted String verify(@Tainted String id) {
if (id.matches(".*D.*")) {
throw new IllegalArgumentException("String contains invalid chars!");
}
return new @Untainted String(id);
}
public String buildQuery(@Untainted String id) {
return "SELECT p.name FROM product p where p.id = " + id;
}
}
Compiles successfully

16. Example 3. Immutability Checker
import org.checkerframework.checker.javari.qual.ReadOnly;
import java.util.*;
public class MutabilityExample {
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public void example() {
ImmutableClass obj = new ImmutableClass();
obj.getValues().add("test");
}
private class ImmutableClass {
private List<String> values = new ArrayList<>();
public @ReadOnly List<String> getValues() {
return values;
}
}
}
javac -processor org.checkerframework.checker.javari.JavariChecker ...

17. Project Valhalla
 Value Types
 Generic Specialization
 Reified generics ?
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18. “Codes like a class, works like an int!” ©
Java type system aggregate data types
• heterogeneous aggregates with identity (classes)
• homogeneous aggregates with identity (arrays)
Object identity => footprint and performance costs
Value types: identityless, immutable, pointer-free aggregates
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19. Value Types use cases
 Numerics
 Native types
 Algebraic data types
 Tuples
 Cursors
 Flatting
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20. Generic Specialization
Goal: support generics over primitives and value types
Specialization approaches
• ahead-of-time (Scala)
• on-demand (C#)
class Box<T> {
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private final T t;
public Box(T t) { this.t = t; }
public T get() { return t; }
}

21. Bytecode (current JVM version)
class Box extends java.lang.Object{
private final java.lang.Object t;
public Box(java.lang.Object);
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Code:
0: aload_0
1: invokespecial #1; //Method java/lang/Object."<init>":()V
4: aload_0
5: aload_1
6: putfield #2; //Field t:Ljava/lang/Object;
9: return
public java.lang.Object get();
Code:
0: aload_0
1: getfield #2; //Field t:Ljava/lang/Object;
4: areturn
}

22. Bytecode marked up to preserve erasure information (proposed)
class Box extends java.lang.Object{
private final java.lang.Object*T t;
public Box(java.lang.Object*T);
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Code:
0: aload_0
1: invokespecial #1; //Method java/lang/Object."<init>":()V
4: aload_0
5: aload_1*T
6: putfield #2; //Field t:Ljava/lang/Object*T;
9: return
public java.lang.Object* get();
Code:
0: aload_0
1: getfield #2; //Field t:Ljava/lang/Object*T;
4: areturn*T
}

23. Bytecode after specialization
class Box${T=int} extends java.lang.Object{
private final int t;
public Box${T=int}(int);
Code:
0: aload_0
1: invokespecial #1; //Method java/lang/Object."<init>":()V
4: aload_0
5: iload_1
6: putfield #2; //Field t:int;
9: return
public int get();
Code:
0: aload_0
1: getfield #2; //Field t:int;
4: ireturn
}
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24. Links
1. https://www.oracle.com/javaone/sessions/index.html Daniel Smith. Type Inference in Java SE 8.
2. http://2013.javapoint.ru/talks/21/ Александр Ильин. Type annotations in Java 8. И почему это хорошо.
3. http://types.cs.washington.edu/checker-framework/ The Checker Framework
4. http://mail.openjdk.java.net/pipermail/valhalla-dev/2014-July/000000.html Welcome to Valhalla
5. http://cr.openjdk.java.net/~jrose/values/values-0.html John Rose, Brian Goetz, and Guy Steele. State of
the Values. April 2014: Infant Edition
6. http://cr.openjdk.java.net/~briangoetz/valhalla/specialization.html Brian Goetz. State of Specialization.
Jule 2014: Infant Edition
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Q/A