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As3.0与java 5语法对比

王朝java/jsp·作者佚名 2006-11-16

Below is a short comparison table of major elements/concepts of these two languages for a quick reference.

You can read this table either left-to-right or right-to-left, depending on what’s your primary programming language is today.

This list is not complete, and your input is appreciated.

Concept/Language Construct

Java 5.0

ActionScript 3.0

Class library packaging

.jar

.swc

Inheritance

class Employee extends Person{…}

Variable declaration and initialization

String firstName=”John”;

Date shipDate=new Date();

int i;

int a, b=10;

double salary;

var firstName:String=”John”;

var shipDate:Date=new Date();

var i:int;

var a:int, b:int=10;

var salary:Number;

Undeclared variables

n/a

It’s an equivalent to the wild card type notation *. If you declare a variable but do not specify its type, the * type will apply.

A default value: undefined

var myVar:*;

Variable scopes

block: declared within curly braces,

local: declared within a method or a block

member: declared on the class level

no global variables

No block scope: the minimal scope is a function

local: declared within a function

member: declared on the class level

If a variable is declared outside of any function or class definition, it has global scope.

Strings

Immutable, store sequences of two-byte Unicode characters

Terminating statements with semicolons

A must

If you write one statement per line you can omit it.

Strict equality operator

n/a

===

for strict non-equality use

!==

Constant qualifier

The keyword final

final int STATE=”NY”;

The keyword const

const STATE:int =”NY”;

Type checking

Static (checked at compile time)

Dynamic (checked at run-time) and static (it’s so called ‘strict mode’, which is default in Flex Builder)

Type check operator

instanceof

is – checks data type, i.e. if (myVar is String){…}

The is operator is a replacement of older instanceof

The as operator

n/a

Similar to is operator, but returns not Boolean, but the result of expression:

var orderId:String=”123”;

var orderIdN:Number=orderId as Number;

trace(orderIdN);//prints 123

Primitives

byte, int, long, float, double,short, boolean, char

all primitives in ActionScript are objects.

Boolean, int, uint, Number, String

The following lines are equivalent;

var age:int = 25;

var age:int = new int(25);

Complex types

n/a

Array, Date, Error, Function, RegExp, XML, and XMLList

Array declaration and instantiation

int quarterResults[];

quarterResults =

new int[4];

int quarterResults[]={25,33,56,84};

var quarterResults:Array

=new Array();

var quarterResults:Array=[];

var quarterResults:Array=

[25, 33, 56, 84];

AS3 also has associative arrays that uses named elements instead of numeric indexes (similar to Hashtable).

The top class in the inheritance tree

Object

Casting syntax: cast the class Object to Person:

Person p=(Person) myObject;

var p:Person= Person(myObject);

var p:Person= myObject as Person;

upcasting

class Xyz extends Abc{}

Abc myObj = new Xyz();

class Xyz extends Abc{}

var myObj:Abc=new Xyz();

Un-typed variable

n/a

var myObject:*

var myObject:

packages

package com.xyz;

class myClass {…}

package com.xyz{

class myClass{…}

}

ActionScript packages can include not only classes, but separate functions as well

Class access levels

public, private, protected

if none is specified, classes have package access level

public, private, protected

if none is specified, classes have internal access level (similar to package access level in Java)

Custom access levels: namespaces

n/a

Similar to XML namespaces.

namespace abc;

abc function myCalc(){}

abc::myCalc(){}

use namespace abc ;

Console output

System.out.println();

// in debug mode only

trace();

imports

import com.abc.*;

import com.abc.MyClass;

import com.abc.*;

import com.abc.MyClass;

packages must be imported even if the class names are fully qualified in the code.

Unordered key-value pairs

Hashtable, Map

Hashtable friends = new Hashtable();

friends.put(”good”,

“Mary”);

friends.put(”best”,

“Bill”);

friends.put(”bad”,

“Masha”);

String bestFriend= friends.get(“best”);

// bestFriend is Bill

Associative Arrays

Allows referencing its elements by names instead of indexes.

var friends:Array=new Array();

friends[”good”]=”Mary”;

friends[”best”]=”Bill”;

friends[”bad”]=”Masha”;

var bestFriend:String= friends[“best”]

friends.best=”Alex”;

Another syntax:

var car:Object = {make:”Toyota”, model:”Camry”};

trace (car[”make”], car.model);

// Output: Toyota Camry

Hoisting

n/a

Compiler moves all variable declaration to the top of the function, so you can use a variable name even before it’s been explicitly declared in the code.

Instantiation objects from classes

Customer cmr = new Customer();

Class cls = Class.forName(“Customer”);

Object myObj= cls.newInstance();

var cmr:Customer = new Customer();

var cls:Class = flash.util.getClassByName(”Customer”);

var myObj:Object = new cls();

Private classes

private class myClass{…}

There is no private classes in AS3.

Private constructors

Supported. Typical use: singleton classes.

Not available. Implementation of private constructors is postponed as they are not the part of the ECMAScript standard yet.

To create a Singleton, use public static getInstance(), which sets a private flag instanceExists after the first instantiation. Check this flag in the public constructor, and if instanceExists==true, throw an error.

Class and file names

A file can have multiple class declarations, but only one of them can be public, and the file must have the same name as this class.

A file can have multiple class declarations, but only one of them can be placed inside the package declaration, and the file must have the same name as this class.

What can be placed in a package

Classes and interfaces

Classes, interfaces, variables, functions, namespaces, and executable statements.

Dynamic classes (define an object that can be altered at runtime by adding or changing properties and methods).

n/a

dynamic class Person {

var name:String;

}

//Dynamically add a variable // and a function

Person p= new Person();

p.name=”Joe”;

p.age=25;

p.printMe = function () {

trace (p.name, p.age);

}

p.printMe(); // Joe 25

function closures

n/a. Closure is a proposed addition to Java 7.

myButton.addEventListener(“click”, myMethod);

A closure is an object that represents a snapshot of a function with its lexical context (variable’s values, objects in the scope). A function closure can be passed as an argument and executed without being a part of any object

Abstract classes

supported

n/a

Function overriding

supported

Supported. You must use the override qualifier

Function overloading

supported

Not supported.

Interfaces

class A implements B{…}

interfaces can contain method declarations and final variables.