This page intentionally left blank in Java Development QR Code in Java This page intentionally left blank

How to generate, print barcode using .NET, Java sdk library control with example project source code free download:

This page intentionally left blank generate, create qr code iso/iec18004 none on java projects barcode C HAPTE R F I V E Lookup The Lookup c qr bidimensional barcode for Java lass, which we introduced in 4, is used for much more than just dependency injection. In NetBeans-based applications, it is used for other purposes as well. All of these involve decoupling.

This chapter will introduce some of the other ways Lookup is used in the NetBeans APIs. The basic usage pattern remains the same for any Lookup instance, as shown in Example 5.1.

. Example 5.1: Using Lookup Foo foo = someLookup.lookup ( Foo.

class );. The code in QR-Code for Java Example 5.1 asks a Lookup for an instance of Foo. If there is at least one Foo in the Lookup, it will be returned; otherwise null will be returned.

. Lookup is us ed pervasively in NetBeans. One way to think of a Lookup is as a Map where the keys are Class objects and the value for each key is an instance of that class. Lookup is a typesafe way of asking for an instance of some known interface.

It is used for a number of things in NetBeans:. Expressing a QR-Code for Java n object s capabilities One way of doing decoupling is to get an object that another module s code created and try to cast it to a. 5 . Lookup speci c type . A common pattern in NetBeans, instead, is to have the object the other module created implement Lookup.Provider (an interface with a single method, public Lookup getLookup()) and ask that Lookup for objects your code wants to work with.

Your code asks the object if it has an instance of some known interface. This approach turns is-a relationships into has-a relationships. If you want to save an object representing a le, you do not test if that object implements a Saveable interface; rather, you ask if it has an instance of SaveCookie (which has one method, save()).

This approach is much more exible than trying to implement all the capabilities of an object by having it implement additional interfaces. In particular, the capabilities of an object may change over time, but its Java type cannot. So, instead of having, for example, a canSave() method, when a le does not need saving, it simply does not have a SaveCookie available.

Dependency injection and decoupling One module de nes some interfaces. Another one implements them. The rst module can use Lookup.

getDefault() the global lookup to nd the implementation(s) of an interface it de nes, as described in 4. Dynamic service discovery The default lookup is a place where modules can very simply inject objects that should be global services or pseudosingletons, as described in Section 4.3.

. Singletons T applet QR-Code he singleton pattern is a design pattern for creating an object of which there should always be only one instance. There are many ways to do this in Java, the simplest being to have a factory method and a private constructor, and to cache the singleton instance and return it from all calls to the factory method. The singleton pattern has a number of uses in NetBeans programming.

Typically, it is used for global services which would make no sense as multiple instances. For example, there is an interface called StatusDisplayer which lets any module set the text on the statusbar in the main window of the application. There is only one statusbar, so it makes sense that there is only one instance of StatusDisplayer.

The implementation of the Window System s APIs which you do not call directly the module org.netbeans.core.

windows, hereafter. 5 . Lookup referred to as core/windows (its path in CVS) injects its implementation of StatusDisplayer by placing it in the default lookup. The code that wants to write to the statusbar simply calls StatusDisplayer.getDefault().

setStatusText("Something"). The downside to using a factory method and caching a singleton instance of a service in a static eld is that the instance of the singleton may remain unused for a long time but it will be permanently retained in memory. There are various strategies to work around this problem using WeakReference, SoftReference, and so forth, all of which mean writing more code.

Using the default lookup, it is easy to create a global singleton which can nonetheless be garbage-collected and reconstituted on demand. If a previously created instance is garbage-collected and later a new one is created on demand, then, if the objects are identical, they might as well be the same object..

Two other co QR for Java mmon uses of the Lookup API are important to know (Example 5.2).1.

Example 5.2: Finding multiple objects of the same type in a Lookup Lookup.Template template = new Lookup.

Template (Foo.class); Lookup.Result foos = someLookup.

lookup (template); Collection c = foos.allInstances();. In Example 5 applet Denso QR Bar Code .2 it is clear that there can be more than one instance of a given type in a Lookup, and it is possible to get a collection of those objects. The third usage pattern is one of the most important.

A Lookup is not a static thing its contents can change over time. So, there is another thing we can do with this code, as shown in Example 5.3 (when developing for NetBeans 5.

x, you will need to remove the generic types and add casts where appropriate).. 1. Some conv enience methods have been added to Lookup in NetBeans 6. There, this code can be written much more simply as Collection < extends Foo> c = someLookup.

lookupAll (Foo.class). Also, in NetBeans 6, Lookup.

Template, Lookup.Result, and Lookup.Result.

allInstances() all have generic types, so the code for using them becomes much simpler..
Copyright © . All rights reserved.