SJSU CS - Cay Horstmann - JavaProperties

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The Very First Proposal

Similar to other proposals, but deserves mention since it's the earliest proposal on record.

public property String foo;
a->Foo = b->Foo;

http://blogs.sun.com/dannycoward/resource/Java7Overview_Prague_JUG.pdf

Marking Properties

A property Keyword

We need to somehow tag properties. The most commonly proposed choice of keyword is, duh, property.

Steven Colebourne:

public bean Person {
    // properties
    property String forename;
    property String surname;
    // normal fields and methods
    private String ordinaryField;
    public boolean validate() { ... }
  }

http://www.jroller.com/scolebourne/entry/a_bean_keyword

Final properties

Peter von der Ahe suggests that read-only properties can be marked with the final keyword.

final property String name = "Name";
property String address;

becomes

final private String name = "Name";
public String getName() { return name; }
private String address;
public String getAddress() { return address; }
public void setAddress(String address) { this.address = address; }

http://blogs.sun.com/ahe/entry/java_se_7_wish_list

Context-sensitive keywords

Remi Forax points out that property can be a "context-sensitive" keyword. It is only taken as a keyword in a specific location. If you have existing code that has a method or variable named "property", that code will continue to compile.

Similarly, get and set can be context-sensitive keywords for controlling the getter and setter generation.

class MyBean {
    public property String name1; // read-write
    public property String name2 get; // read-only
    public property String name3 set; // write-only
    public property String name4 get set; // illegal
    public property String name5 get { return "hello"; } // read-only
    public property String name6 set(String name) { } // write-only
    private String _name7;
    public property String name7
        get { return _name7; }
        set(String name) { _name7 = name; firePropertyChange(name7); } // read-write
}

http://weblogs.java.net/blog/forax/archive/2007/01/a_property_prop_1.html http://weblogs.java.net/blog/forax/archive/2007/01/property_reload.html http://weblogs.java.net/blog/forax/archive/2007/05/java_property_d.html

Using a Token

Nobody likes the idea of introducing new keywords to the language. Remember the pain when enum was added and you had to rename all your variables that were so named? An alternative is to use some kind of token.

public class LineItem
{
    public int @ quantity;
    ...
}

http://weblogs.java.net/blog/cayhorstmann/archive/2006/06/say_no_to_prope.html http://weblogs.java.net/blog/cayhorstmann/archive/2007/01/arrows_in_the_b.html

Here, the @ token is used to mark a property because the same proposal uses the .@ operator for property access. One would want to use a token that is similar to that of the property access operator. For example, if you use :: for propery access

person::name

then a property might be marked as

public String ::name

Using the Presence of Getters and Setters

class MyBean {
    public String name1 get set; // read-write property
    public String name2 get; // read-only property
    public String name3 set; // write-only property
    public String name4; // public field
    public String name5 get { return "hello"; } // read-only, custom getter
    public String name6 get set { name6 = value; } // read-write, custom setter
}

Using Braces

This proposal marks properties simply with braces.

// Trivial case:
    // type name lowercased is property name, public getter and setter, protected member variable
    public RuntimeOptions {}
    // protected getter and setter, static, nondefault name
    protected static RuntimeOptions options {}
    RuntimeOptions options {
        private value;      // member variable is private instead of protected
        public  getValue(); // streamlined syntax for the obvious trivial case
        public  setValue(); // streamlined syntax for the obvious trivial case
    }
    RuntimeOptions options {
        public getValue();
        public setValue(RuntimeOptions newValue) {
            value = newValue;
            sideEffects();
    			}
    }

http://yost.com/computers/java/property-style/index.html

Always Autogenerate Getters/Setters

This proposal auto-generates getters and setters for all fields.

public class User {
    private String name;
    private int age;
}
User u = new User();
u.setName(”fname lname”);
assertEquals(”fname lname”, u.getName());

http://zeusville.wordpress.com/2007/07/25/java-getterssetters/

No Syntax for Declaring Properties

This proposal suggests that that getters/setters are defined as usual, only access/mutation is enhanced.

public class Test {
    private int val;
    private NotifyObject not;
    public int getVal() { return val; }
    public void setVal(int val) {
        this.val = val;
        not.valChanged(val);
    }
}
Test t = new Test();
t.val = 6;
System.out.println(”Val: ” + t.val);

http://matthewbot.wordpress.com/2007/09/09/java-7/

Variants of the Property Access Operator

A common complaint is that using the dot operator for properties makes it hard to distinguish between member access and property access. Here are some of the various operators that have been proposed as alternatives:

The Arrow Operator

struct FooBean { Foo foo; }
struct Point { int x, y; }
Point p = new Point();
p->x = p->y = 0; // oh joy

http://weblogs.java.net/blog/hansmuller/archive/2007/01/property_syntax.html

Double Colon

The -> operator was, of course, lifted from C/C++. The other C++ operator worth considering would be the :: (scope resolution) which you can consider a "fat dot".

Point p = new Point(1, 2);
p::x = p::y; // oh bliss

Double Wow

String foo = "bar";
a !! foo = b !! "bar"

http://mult.ifario.us/articles/2006/12/28/no-arrow-notation-in-java-please

Dot and @ Sign

student.@name = "Alex";
student.@major = "CS";
student.@classes = new String[] {"Database Systems", "Compilers"};
System.out.println(student.@name + ", whose major is " + student.@major
   + ", is taking the following class: " + student.@classes[1]);

http://www.geocities.com/adcalves/javaproperty/

No Property Access Operator

Mikael Grev suggests to auto-generate the getters and setters, but not to provide any property access operator.

@Property(bound = true)
public property String donkeyName = "";

would still be accessed with

String s = bean.getDonkeyName();
bean.setDonkeyName(String s);

Come On, Use The Dot

Lots of other languages do just fine using the dot for both member and property access. See jhook's comment in http://weblogs.java.net/blog/cayhorstmann/archive/2007/01/arrows_in_the_b.html#comments

Overriding Getters and Setters

Auto-generated getters and setters will work in many (most?) cases, but sometimes we need to override them.

Add JavaBeans Style Getter/Setter

The easiest approach is to simply add getFoo/setFoo methods when the default ones won't do the job.

Here, the getter is auto-generated, and the setter is explicitly supplied.

public class Student
    public property name;
    public void setName(String value) { 
        if (value != null) name = value; 
    }
    ...    
}

Add get/set Methods After Property

In C#, the getter and setter methods are defined immediately after the property. In C#, one must always supply them, but most proposals in Java use this as a mechanism to override the default generation.

public Color bgColor 
    get {
        return _bgColor;
    } 
    set { // implied 'value' param of type Color
       _bgColor = value;
       repaint(100);
    };
     // public property of type Color (default setter/getter)
     public Color fgColor get set;

http://blogs.sun.com/joe/resource/java-properties-events.pdf

Access Property As default

When accessing a property in a getter or setter, you want to access the underlying field, not make a recursive call. This proposal suggests to use the keyword default to eliminate any confusion.

public String name
    get { return default; }
    set(v/* type expression here is redundant */) {
       validate(v);
       default = v;
    }
    public int trivialRWProperty get set; // read-write property
    public Date justROProperty get { return new Date(); }; // read-only property
    public int x get; // compile-time error (?)
}

(Comment by Valery in http://weblogs.java.net/blog/cayhorstmann/archive/2007/01/arrows_in_the_b.html)

Using Annotations

Note: It is tempting to use annotations, thereby avoiding the introduction of a new keyword. However, it is against the spirit of annotations. Annotations are not supposed to modify the bytecodes of the class that is being compiled. In a managed environment, such as EJB3, code can be edited at load time, but that won't work for client-side Java.

Alexandre Alves

import javax.lang.Property;
public class Student {
/**
  * Property annotation generates getters and setters
  */
    @Property
    private String name;
    @Property
    private String major;
    @Property
    private String[] classes;
    public static void main(String args[]) {
        Student student = new Student();
        student.@name = "Alex";
        student.@major = "CS";
        student.@classes = new String[] {"Database Systems", "Compilers"};
        System.out.println(student.@name + ", whose major is " + student.@major
            + ", is taking the following class: " + student.@classes[1]);
    }
}

Implementation available. (Alexandre modified the Java 5 compiler so that it generates the getters/setters and deals with .@ expressions as lvalues and rvalues.)

http://www.geocities.com/adcalves/javaproperty/

Tom Eugelink

public class Student
{
    @get String name;
    public void setName(name) { if (name != null) this.name = name; }
    . . .
}

Alternate forms:

@get @set String name;
@get @set @PropChange String name;
@get @set @PropChange @VetoChange String name;

http://www.theserverside.com/news/thread.tss?thread_id=29454#142668

Gamigin

class MyClass
{
    // defaults to providing both getter and setter.
    @PublicProperty
    protected int xyz;
}

http://forum.java.sun.com/thread.jspa?threadID=739931

Ramnivas Laddad

public class Person {
    @Property(ReadOnly) private long id;
    @Property private String name;
}

http://ramnivas.com/blog/index.php?p=15

Jesse Wilson

public class Person {
    private @Property(ObservableProperty.class) String surname;
}

http://publicobject.com/2007/01/extensible-properties-for-java-language.html

Danny Lagrouw

public class Person {
    // Will generate getter and setter:
    private String lastname;
    // Will generate getter:
    @FieldReader protected String firstname;
    // Will generate setter:
    @FieldWriter public Integer age;
    ...
}
@FieldAccess(access=NO_ACCESS)
    public class Address {
    // Will not generate anything because of class-level annotation:
    private String street;
}

http://blog.dannynet.net/archives/81

Property Objects

These are not proposals for extending the language with first-class support for properties but rather variations on the theme of simulating properties with fields of a generic Property type.

Some of the shortcomings of properties as objects (identified and discussed by multiple people) are:

Rely on discipline, not enforced by compiler
Not compatible with standard & other java apis JavaBeans / JMX etc without byte-code instrumentation
Not efficient – too many objects on heap – imagine a class with 100s of properties
Not efficient – typecasting with generics or memorization of many new class names (IntProperty, BooleanProperty …)

John McClean

http://www.jroller.com/ie/entry/properties_in_java_without_modifying

2 proposals, first one is like the rest but the second one is interesting: host-aware property objects. Quote:

public class Property<E,T>{
    private T value
    private E host;
    public Property(E host){ this.host =host; }
    public Property(E host,T value){
        set(value);
        this.host = host;
    }
    public T get(){
        return value;
    }
    public E set(T value){
        this.value = value;
        return host;
    }
}
public class Person {
    final public Property<Person,String> name = new Property<Person,String>(this);
    final public Property<Person,Integer> age = new Property<Person,Integer>(this);
}

We can even chain setters

Person p = new Person();
p.name.set("bill").age.set(25);

Joseph Ottinger

http://www.theserverside.com/news/thread.tss?thread_id=44804

public class NewBean {
    public final Property<Integer> x = new PropertyImpl<Integer>();
}
NewBean b = new NewBean();
b.x.set(5);
b.x.get();

Christopher

http://www.javalobby.org/java/forums/t74270.html?start=30#92026711

class Employee extends Person {
    public final Address workAddress = new Address();
    public final USPhone workPhone = new USPhone();
}
/* usage */
public class PersonTest {
    public static void main(String args[]) {
        Employee emp = new Employee();
        emp.firstName.setValue("christopher");
        emp.phone.setValue("5554443333");
        emp.address.street.setValue("  1234 mocking bird lane");
        // output -- note the proper capitalization from NameField
        System.out.println(emp.firstName); // Christopher
        System.out.println(emp.workPhoneName); // (555) 444-3333
        System.out.println(emp.address.street); // 1234 Mocking Bird Lane
    }
}

JP Moresmau

http://jpmoresmau.blogspot.com/2007/03/java-properties-with-no-language.html

public class PersonBean {
    public final IFullProperty name=new NonNullProperty();
    public final IReadProperty id;
    public final IFullProperty email=new FullProperty(){
        @Override
        public void set(String value) {
            if (value.indexOf('@')==-1){
                throw new IllegalArgumentException("email does not contain @");
            }
            super.set(value);
        }
    };
    public PersonBean(int id,String name){
        this.name.set(name);
        this.id=new ReadOnlyProperty(id);
    }
}
bean.getName() -> bean.name.get()
bean.setName(name) -> bean.name.set(name)

Joda

http://joda.sourceforge.net/beans.html

String surname = person.getSurname();      // JavaBeans style, or...
String surname = person.surname().get();   // Joda style

Ricky

http://rickyclarkson.blogspot.com/2007/02/ricky-clarkson-properties-for-java.html

class Person
{
    public final Property<String> name=new DirectProperty<String>("unknown");
}

Matthias Ernst

http://www.mernst.org/blog/rss.xml#Arent-properties-objects

public final RWProperty<String> name = new Slot<String>();
mernst.name.get();
mernst.name.set("matthias");
mernst.name.attachListener(...);
label.bind(mernst.name);

nicba

http://weblogs.java.net/blog/cayhorstmann/archive/2007/01/properties_are.html comment Posted by: nicba on January 20, 2007 at 10:31 AM

public class Example3 {
    final Property name = new Property();
    public static void main(String[] args) {
        new Example3().runExample();
    }
    private void runExample() {
        System.out.println(name.get());
        name.set("John Smith");
    }
}

bean-properties project (Shai Almog)

https://bean-properties.dev.java.net/

public class NewBean {
    public final Property<Integer> x = new PropertyImpl<Integer>();
}

Which you use like this:

NewBean b = new NewBean();
b.x.set(5);
b.x.get();

bean-properties Project Analysis - Nikolay Botev's analysis of the features of the bean-properties project and how they compare to native property syntax in general.

Bean-Independent Properties (Jesse Wilson)

http://publicobject.com/2007/09/bean-independent-properties.html

public property Property<Customer,String> city
      = new BasicPropertyBuilder(property(""))
          .nonNull()
          .observable(##propertyChangeSupport)
          .build();

Which you use like this:

Customer customer = ...
String city = customer#city;
customer#city = "San Francisco";

Version 1.127 last modified by Administrator on 24/04/2008 at 19:06

Comments: 11

XWikiGuest | 29.12.2007 at 05:58 PM

Why not just copy what C# 2, 3 and 3.5 is doing?

Declaring properties:

// Default implementation with simple getter and (protected setter). Creates an anonymous field to go with it. public property String name { get; protected set; } // Supply your implementation if you need it private int _age; public property int age { get { return _age; } set { _age = value; } }

2. Access. Just use a dot. Deal with conflicts between identically named properties and fields the same way that c# does. (the .net runtime does properties with get_name and set_name methods internally, the c# compiler just doesn't let you use them directly).

"Good artists copy, great artists steal"

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