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CHAPTER
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Implementing Language Interpreters in Java
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ave you ever wanted to create your own computer language If you're like most programmers, you probably have. Frankly, the idea of being able to create, control, enhance, and modify your own computer language is very appealing. However, few programmers realize how easy, and enjoyable, it can be. Be assured that the development of a full-featured compiler, such as Java, is a major undertaking, but the creation of a language interpreter is a much simpler task. Although both interpreters and compilers take as input the source code for a program, what they do with that source code differs significantly. A compiler converts the source code of a program into an executable form. Often, as is the case with a language like C++, this executable form consists of actual CPU instructions that are directly executed by the computer. In other cases, the output of a compiler is a portable intermediate form, which is then executed by a runtime system. This is the way Java works. In Java, this intermediate code is called bytecode. An interpreter works in a completely different way. It reads the source code to a program, executing each statement as it is encountered. Thus, an interpreter does not translate the source code into object code. Instead, the interpreter directly executes the program. Although program execution via an interpreter is slower than when the same program is executed in its compiled form, interpreters are still commonly used in programming for the following reasons. First, they can provide a truly interactive environment in which program execution can be paused and resumed through user interaction. Such an interactive environment is helpful in robotics, for example. Second, because of the nature of language interpreters, they are especially well suited for interactive debugging. Third, interpreters are excellent for script languages, such as query languages for databases. Fourth, they allow the same program to run on a variety of different platforms. Only the interpreter's runtime package must be implemented for each new environment. Sometimes the term interpreter is used in situations other than those just described. For example, the original Java runtime system was called a bytecode interpreter. This is not the same type of interpreter developed in this chapter. The Java runtime system provides an execution environment for bytecode, which is a highly optimized set of portable machine instructions. Thus, the Java runtime system does not operate on source code, but on portable machine code. This is why the Java runtime system is called the Java Virtual Machine. In addition to being an interesting and useful piece of code, the interpreter developed in this chapter serves a second purpose: it demonstrates the streamlined elegance of the Java language. Like the parser in 2, the language interpreter is a pure code example. It is also a fairly sophisticated program. The ease by which the interpreter can be implemented in Java gives testimony to Java's versatility. Moreover, the transparency of the code shows the expressive power of the Java syntax and libraries.
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Before we can build an interpreter, it is necessary to choose the language that we want to interpret. Although Java might seem an obvious choice, it is too large and sophisticated a language. The source code for an interpreter for even a small subset of the Java language
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3: Implementing Language Interpreters in Java
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would be far too big to fit into a chapter of this book! Moreover, you won't normally need to write an interpreter for a language as powerful as Java. Most likely, you will be writing interpreters for relatively simple languages. Thus, a better choice for the interpreter is a compact language that is readily adapted to interpretation. A language that fits these criteria is the original version of BASIC, and the interpreter developed in this chapter will accept a subset of this language. This subset is hereafter referred to as Small BASIC. A BASIC-like language was chosen for three reasons. First, BASIC was originally designed to be interpreted. As such, it is relatively easy to implement an interpreter for BASIC. For example, the original version of BASIC did not support local variables, recursive methods, blocks, classes, overloading, and so on all of which increase the complexity of the language. However, the same principles used to interpret a subset of BASIC will also apply to other languages, and you can use the code developed here as a starting point. The second reason for selecting BASIC is that a reasonable subset can be implemented in a relatively small amount of code. Finally, the original BASIC syntax is easy to master, requiring nearly no time to learn. Thus, even if you have no familiarity with traditional BASIC, you will have no trouble using Small BASIC. The following example of a Small BASIC program illustrates just how easy the language is. Even if you have never seen a traditional-style BASIC program before, you will probably find its operation clear.
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