Choosing a Language for Use with Cocoa
Cocoa can be used with many different languages, so how does a programmer choose the language to use? As always, the answer depends on many factors, including the programmer's familiarity with the languages and the special features of different languages that are applicable to the problem's solution. The pros and cons of the most common language choices for using Cocoa are described in this section. The bottom line is that any of the languages presented here can be used, and different programmers will make different choices.
Java Pros and Cons
Java is a powerful and modern language that emphasizes portability and security before performance and flexibility. Java is ideal for writing long-running server processes that heavily use network resources and databases. Java is well-suited for use in Web browsers where security is a concern. Java's use of automatic garbage collection simplifies application development and avoids whole classes of potential dynamic memory management errors. Java's standard libraries contain a broad range of features that provide a head start when developing many types of applications.
Java supports object orientation in a way that is compatible with Cocoa. Java applications can use Cocoa if portability is not important. Cocoa and the standard Java libraries have many features in common, but each contains objects that the other does not. Cocoa and standard Java libraries can be used well together.
Unlike Objective-C, Java is widely taught at universities and elsewhere. Many programmers who are learning Cocoa are already experienced with Java. For an experienced Java programmer, using Java with Cocoa might seem like less work than using Objective-C with Cocoa, but learning Objective-C takes little time for most programmers experienced with C and at least one object-oriented language. Because Cocoa is written in Objective-C, Cocoa programmers inevitably encounter Objective-C code even if that encounter is limited to example programs and documentation. Learning Objective-C makes learning Cocoa easier in the long run.
Java has several disadvantages for desktop applications. The cross-platform support and security features that make Java ideal for some applications can get in the way of others. The Java Virtual Machine that provides cross-platform support and security is large and takes a long time to load. When a Java desktop application is first started, the Java Virtual Machine must also be started and initialized. The JVM slows application start-up and consumes resources. For long-running server applications, the startup cost is negligible when averaged over the months or years that the program runs. Desktop applications are started and quit much more often.
Java Cocoa applications pay the price for the JVM, but they don't reap the benefits. When Cocoa is used, the Java application no longer has cross-platform support. Cocoa provides access to features that circumvent Java's security restrictions, which are probably inappropriate for a desktop application anyway.
Java is a popular language. Many libraries of Java objects are available, but it is inconvenient to use Java with most existing libraries that were written in different languages. To maximize the benefits of Java's portability and security, it is necessary to avoid existing non-Java libraries. The fact that it is inconvenient to reuse the millions of lines of existing C code and libraries from Java is a disadvantage.
Objective-C Pros and Cons
An advantage of Objective-C is its easy integration with existing C code. As a superset of ANSI C, Objective-C can be used with existing C libraries including traditional Mac and Unix libraries. The Objective-C++ compiler provided by Apple makes integration with existing C++ code convenient.
Objective-C is the implementation language of Cocoa, and some features unique to Objective-C are used by Cocoa. When Cocoa is used with different languages, small incompatibilities and features that do not translate well are exposed. Objective-C provides the most natural interface to Cocoa.
Objective-C is one of the most dynamic and flexible object-oriented languages, and Cocoa programming often benefits from these advantages. When Cocoa is used by less flexible languages, features and benefits of Cocoa are compromised to some degree. Java is also a flexible and dynamic language but not quite as flexible and dynamic as Objective-C.
Most existing Cocoa sample code and training resources use Objective-C. Familiarity with Objective-C maximizes the resources available to programmers who are learning to use Cocoa. Some of the features of Cocoa are based on unique features of Objective-C. Understanding how and why to use such Cocoa features is easier with an understanding of Objective-C.
Objective-C is a small extension of ANSI standard C. Unlike C++, which attempted to create a better C while adding a certain type of static strongly typed object support to C, Objective-C adds the minimum features necessary to support dynamic weak typed object support. Objective-C makes no attempt to improve the underlying C language. Objective-C is usually easy for C and C++ programmers to learn. The essential additions that Objective-C makes to C can be described in minutes.
Compared to existing implementations of the Java Virtual Machine, Objective-C's runtime makes more efficient use of system resources. For desktop applications and applications that occasionally need to resort to low-level system features and even assembly language, Objective-C is a better choice than Java. When performance is critical, the C subset of Objective-C can always be used to maximize performance.
There are two general types of applications: closed world and open world. Objective-C's flexibility and power are generally inappropriate for closed-world applications, but much more suited for open-world applications, as described in the following sections.
The engine compartment of an automobile is analogous to closed-world applications. It is desirable to know in advance every component that will be inside the engine compartment and how they will fit together. Engines are carefully designed and their design is seldom modified after they leave the factory. Any variation in the connections between engine components is probably a manufacturing error. Languages such as C++ and to a lesser extent Java provide language-level features that are well-suited to solving closed-world problems. The static strong typing used by C++ and Java enables the compiler to verify that all components fit together as planned at the cost of making variation, redesign, and modification of existing applications more difficult. Some applications require the verifiability of static strong typing and can overcome the reduction in flexibility. Some programmers are just more comfortable solving closed-world style problems and might never be satisfied with Cocoa because it is designed to solve open-world problems.
The passenger compartment of an automobile is analogous to open-world applications. Any constraints on the type or number of people and things that can be placed in the passenger compartment detract from the utility of the automobile. Some constraints are inevitable, but the designer of a passenger compartment must strive for maximum flexibility. The price of that flexibility is that the designer cannot anticipate everything that might be put in the passenger compartment. The designer must work with incomplete information. Objective-C provides language-level support for solving open-world problems. Objective-C objects can operate with anonymous objects in different applications. It is possible to send messages to objects even though the receiver might not understand the message. It is possible to add behaviors to existing compiled objects without recompiling them. The flexibility provided by Objective-C aids the development and life-cycle modification of most desktop applications, but the cost is that the compiler cannot always verify that the components fit together. In some cases, errors that might have been caught by a compiler with a different language cannot be caught until an Objective-C application is running.
Most graphical user interfaces are examples of open-world applications. Restrictions on the type and number of graphical components available reduce the utility of user interfaces. Sometimes it is necessary to create new user interface components that were not anticipated by the original designers and still be able to integrate the new components with the old components. Plug-ins and context menus are other examples of open-world applications.
It is certainly possible to create open-world applications with static strongly typed languages, but it is more difficult. It is also possible to use strong static typing with Objective-C and gain many of the benefits at the cost of flexibility. Cocoa and Objective-C emphasize flexibility at the expense of compile time verifiability. Much of the increased programmer productivity attributed to using Cocoa results from Objective-C's flexibility.
Scripting Language Pros and Cons
Scripting languages are usually interpreted rather than compiled. Even scripting languages that can be compiled often also operate in an interpreted mode. In most cases, scripting languages promote rapid application development and programmer productivity before runtime performance and compile time verifiability. Scripting languages are often easy to learn and accessible to programming novices.
To access system resources that are only available from compiled languages, scripting languages almost always provide a mechanism to extend the language for use with compiled software written in other languages.
The extensibility of scripting languages combined with the power of Objective-C's runtime makes using Objective-C objects from within scripting languages possible. Details about allocating Objective-C objects and sending messages to them are provided in Appendix A. As long as a scripting language can call a small number of Objective-C runtime functions, Cocoa can be used in its entirety from the scripting language.
Scripting languages usually exhibit inferior performance and make producing large applications difficult in comparison to compiled languages. When the performance and scalability of scripting languages are acceptable, scripting languages can be an ideal way to use Cocoa.