Compilation 2007Compilation 2007

Java 1.5 FeaturesJava 1.5 Features

Michael I. Schwartzbach

BRICS, University of Aarhus

2Java 1.5 Features

Java 1.5 Features (That We Will Use)Java 1.5 Features (That We Will Use)

enums autoboxing improved for loop generic collections generic classes @override annotations covariant return types

3Java 1.5 Features

Constant Values as Static Fields (1/2)Constant Values as Static Fields (1/2)

public class Test {

public static final int TrafficSignal_RED = 0;

public static final int TrafficSignal_YELLOW = 1;

public static final int TrafficSignal_GREEN = 2;

public static int duration(int color) {

switch (color) {

case TrafficSignal_RED: return 35;

case TrafficSignal_YELLOW: return 15;

case TrafficSignal_GREEN: return 40;

}

return 0;

}

public static String name(int color) {

switch (color) {

case TrafficSignal_RED: return "RED";

case TrafficSignal_YELLOW: return "YELLOW";

case TrafficSignal_GREEN: return "GREEN";

}

return "";

}

4Java 1.5 Features

Constant Values as Static Fields (2/2)Constant Values as Static Fields (2/2)

public static void main(String[] args) throws Exception {

int[] states = { TrafficSignal_RED, TrafficSignal_YELLOW,

TrafficSignal_GREEN, TrafficSignal_YELLOW };

int i = 0;

while (true) {

System.out.println(name(states[i]));

Thread.sleep(1000*duration(states[i]));

i = (i+1)%states.length;

}

}

}

• Not typesafe

• No proper namespace

• Constants compiled into clients

• Annoying to use

5Java 1.5 Features

Type-Safe EnumsType-Safe Enums

public class Test {

public enum TrafficSignal { RED, YELLOW, GREEN };

public static int duration(TrafficSignal color) {

switch (color) {

case RED: return 35;

case YELLOW: return 15;

case GREEN: return 40;

}

return 0;

}

public static void main(String[] args) throws Exception {

TrafficSignal[] states = { TrafficSignal.RED, TrafficSignal.YELLOW,

TrafficSignal.GREEN, TrafficSignal.YELLOW };

int i = 0;

while (true) {

System.out.println(states[i]);

Thread.sleep(1000*duration(states[i]));

i = (i+1)%states.length;

}

}

}

6Java 1.5 Features

Constructors, Fields, and MethodsConstructors, Fields, and Methods

public class Test {

public enum TrafficSignal {

RED(35), YELLOW(15), GREEN(40);

protected int duration;

TrafficSignal(int duration) { this.duration = duration; }

int getDuration() { return duration; }

};

public static void main(String[] args) throws Exception {

TrafficSignal[] states = { TrafficSignal.RED,

TrafficSignal.YELLOW,

TrafficSignal.GREEN,

TrafficSignal.YELLOW };

int i = 0;

while (true) {

System.out.println(states[i]);

Thread.sleep(1000*states[i].getDuration());

i = (i+1)%states.length;

}

}

}

7Java 1.5 Features

int i1 = 42;

Integer i2;

i2 = new Integer(i1);

i1 = i2.intValue();

Map m = new HashMap();

m.put(new Integer(87), new Integer(i1));

i2 = (Integer)m.get(new Integer(87));

Annoying ConversionsAnnoying Conversions

8Java 1.5 Features

int i1 = 42;

Integer i2;

i2 = new Integer(i1);

i1 = i2.intValue();

Map m = new HashMap();

m.put(new Integer(87), new Integer(i1));

i2 = (Integer)m.get(new Integer(87)); int i1 = 42;

int i1 = 42;

Integer i2;

i2 = i1;

i1 = i2;

Map m = new HashMap();

m.put(87, i1);

i2 = (Integer)m.get(87);

Automatic Boxing and UnboxingAutomatic Boxing and Unboxing

9Java 1.5 Features

Getting Rid of the Last CastGetting Rid of the Last Cast

int i1 = 42;

Integer i2;

i2 = i1;

i1 = i2;

Map m = new HashMap();

m.put(87, i1);

i2 = (Integer)m.get(87);

int i1 = 42;

Integer i2;

i2 = i1;

i1 = i2;

Map<Integer,Integer> m = new HashMap<Integer,Integer>();

m.put(87, i1);

i2 = m.get(87);

10Java 1.5 Features

Generic CollectionsGeneric Collections

Type parameters Type checker inserts the required casts The type checker ensures casts will never fail The type checker warns if it is not sure No runtime improvement, casts still take place

(unless JVM optimizes) Required to be backward compatible

11Java 1.5 Features

Generic Collections in ActionGeneric Collections in Action

Student johnni = new Student(”Johnni");

List<Student> phds = new ArrayList<Student>();

phds.add(johnni);

Student phd = phds.get(0); // no cast needed

phds.add(”Janus"); // cool type error

for (Iterator<Student> i = phds.iterator();

i.hasNext(); ) {

Student p = i.next(); // no cast needed

System.out.println(p.name());

if (p.papers() < 3) i.remove();

}

12Java 1.5 Features

No Type Information at RuntimeNo Type Information at Runtime

List<Student> phds = new ArrayList<Student>();

phds instanceof List // true

phds instanceof List<Student> // syntax error

13Java 1.5 Features

Mixing Raw and Generic CollectionsMixing Raw and Generic Collections

Student johnni = new Student(“Johnni");

List phds = new ArrayList();

phds.add(johnni);

phds.add(“Janus"); // no type error

for (Iterator<Student> i = phds.iterator(); i.hasNext(); ) {

Student p = i.next(); // no cast needed

System.out.println(p.name());

if (p.papers() < 3) i.remove();

}

Compiles, but fails at runtime. But the compiler warns you:

Note: Test.java uses unchecked or unsafe operations.

Note: Recompile with -Xlint:unchecked for details.

14Java 1.5 Features

Nesting Parameterized TypesNesting Parameterized Types

Map<Advisor, List<Student>> phdSchool;

Map<String,Map<Advisor, List<Student>>> phdSchools;

Advisor michael;

...

System.out.println(

phdSchools.get("BRICS").get(michael).get(0).name()

);

15Java 1.5 Features

Invariant Subtyping of ParametersInvariant Subtyping of Parameters

class Student extends Person { ... }

...

List<Person> people = new ArrayList<Person>();

// perfectly fine since ArrayList is a subtype of List

List<Student> phds = new ArrayList<Student>();

// perfectly fine since ArrayList is a subtype of List

people = phds; // type error

16Java 1.5 Features

And for a Good ReasonAnd for a Good Reason

class Student extends Person { ... }

class Advisor extends Person { ... }

...

List<Person> people = new ArrayList<Person>();

List<Student> phds new ArrayList<Student>();

people = phds; // assume this is allowed

people.add(new Advisor("Michael"));

phds.get(0).studentid();

// runtime error, an Advisor doesn't have a studentid

17Java 1.5 Features

Same Problem with ArraysSame Problem with Arrays

class Student extends Person { ... }

class Advisor extends Person { ... }

...

Student[] phds = new Student[100];

Person[] people = new Person[100];

people = phds; // allowed, but unsound

people[0] = new Advisor();

phds[0].studentid(); // runtime error

Pragmatic (but later regretted) design choice

18Java 1.5 Features

Print All Elements of Any ListPrint All Elements of Any List

public void printList(List x) throws IOException {

for (Iterator i = x.iterator(); i.hasNext(); ) {

System.out.println(i.next().toString());

}

}

List<Student> phds;

List<Person> people;

printList(phds); // unchecked warning

printList(people); // unchecked warning

19Java 1.5 Features

Another Attempt with GenericsAnother Attempt with Generics

public void printList(List<Object> x)

throws IOException {

for (Iterator<Object> i = x.iterator();

i.hasNext(); ) {

System.out.println(i.next().toString());

}

}

List<Student> phds;

List<Person> people;

printList(phds); // type error

printList(people); // type error

20Java 1.5 Features

Wildcards to The RescueWildcards to The Rescue

public void printList(List<?> x) throws IOException {

for (Iterator<?> i = x.iterator(); i.hasNext(); ) {

System.out.println(i.next().toString());

}

}

List<Student> phds;

List<Person> people;

printList(phds); // allowed

printList(people); // allowed

21Java 1.5 Features

Write Your Own Generic ClassWrite Your Own Generic Class

public class Queue<T> {

protected List<T> s;

public Queue() {

s = new ArrayList<T>();

}

public void enter(T x) {

s.add(x);

}

public T leave() {

if (s.size()==0) return null;

else return s.remove(0);

}

}

22Java 1.5 Features

Bounded Type ParametersBounded Type Parameters

class PersonQueue<T extends Person> {

protected List<T> s;

public PersonQueue() {

s = new ArrayList<T>();

}

public void enter(T x) {

s.add(x);

}

public T leave() {

if (s.size()==0) return null;

else return s.remove(0);

}

public String first() {

if (s.size()==0) return null;

return s.get(0).name();

}

}

23Java 1.5 Features

Using InheritanceUsing Inheritance

class PersonQueue<T extends Person> extends Queue<T> {

public String first() {

return s.get(0).name();

}

}

24Java 1.5 Features

For Loop for Collection ClassesFor Loop for Collection Classes

for (Iterator<Person> i = phds.iterator(); i.hasNext(); ) {

Person p = i.next();

System.out.println(p.name());

}

for (Person p: phds) {

System.out.println(p.name());

}

But we cannot remove, need old iterators for that

25Java 1.5 Features

Person phds[] = { new Person(”Johnni"),

new Person(”Janus") };

for (Person p: phds) {

System.out.println(p.name());

}

Works for Arrays as WellWorks for Arrays as Well

26Java 1.5 Features

Override ProblemOverride Problem

public class Graduate extends Person {

public String naem() {

return super.name()+", PhD";

}

}

Nothing happens, the PhD disappeared

27Java 1.5 Features

The The @Override@Override Annotation Annotation

public class Graduate extends Person {

public @Override String naem() {

return super.name()+", PhD";

}

}

Compiler error:

method does not override a method from its superclass

28Java 1.5 Features

Return Type ProblemReturn Type Problem

public class Person {

protected String name;

public Person(String name) {

this.name = name;

}

public Person changeName(String newName) {

this.name = newName;

return this;

}

}

...

public Student extends Person { ... }

...

Student x;

...

x.changeName(”Johnny").papers() // type error

29Java 1.5 Features

Covariant Return TypesCovariant Return Types

public Student extends Person {

public Student changeName(String newName) {

this.name = newName;

return this;

}

...

}

x.changeName(“Johnny").papers() // allowed