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The Art of Java
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The Interpreter
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The interpreter portion of SBasic is the code that actually executes a program. In general, it is quite easy to interpret a Small BASIC program because each statement (except for assignment) begins with a keyword. Thus, the interpreter works by obtaining the keyword at the start of each line and then doing what that keyword specifies. This process repeats until the entire program has been interpreted. The remainder of this section examines each part of the interpreter in detail.
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The InterpreterException Class
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At the start of the interpreter file, you will find the class InterpreterException. This is the type of exception that the interpreter will throw if an error occurs. Code that uses SBasic must handle this exception. Exceptions can be caused by syntax errors, by I/O errors, and by errors in numeric expressions.
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The SBasic Constructor
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The constructor for SBasic is shown here:
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// Constructor for SBasic. public SBasic(String progName) throws InterpreterException { char tempbuf[] = new char[PROG_SIZE]; int size; // Load the program to execute. size = loadProgram(tempbuf, progName); if(size != -1) { // Create a properly sized array to hold the program. prog = new char[size]; // Copy the program into program array. System.arraycopy(tempbuf, 0, prog, 0, size); } }
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The constructor is passed the name of the Small BASIC file that you want to interpret. It then creates a temporary buffer into which this file will be read. The size of this buffer is
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3: Implementing Language Interpreters in Java
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specified by PROG_SIZE, which is arbitrarily set to 10,000. This is the size of the largest program that SBasic can interpret. You can change this size if you want. Next, the constructor calls loadProgram( ), which actually reads the program and returns its size, in characters, or 1 on failure. Then, a new array that is precisely the size of the program is created and a reference to it assigned to prog. Finally, the program is copied into this array. Thus, the size of the array referred to by prog will be exactly the same as the size of the program. The loadProgram( ) method is shown here:
// Load a program. private int loadProgram(char[] p, String fname) throws InterpreterException { int size = 0; try { FileReader fr = new FileReader(fname); BufferedReader br = new BufferedReader(fr); size = br.read(p, 0, PROG_SIZE); fr.close(); } catch(FileNotFoundException exc) { handleErr(FILENOTFOUND); } catch(IOException exc) { handleErr(FILEIOERROR); } // If file ends with an EOF mark, back up. if(p[size-1] == (char) 26) size--; return size; // return size of program }
Most of this method is easily understandable, but pay special attention to these lines:
// If file ends with an EOF mark, back up. if(p[size-1] == (char) 26) size--;
As the comment indicates, this line discards a trailing EOF mark that might end the file. As you may know, some text editors append an end-of-file marker (which is usually the value 26). Others do not. loadProgram( ) handles both cases by removing the mark if it is present.
The Art of Java
The Keywords
The subset of BASIC that Small BASIC interprets is represented by these keywords:
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The internal representation of these keywords plus EOL, for end of line, are declared as final values in SBasic, as shown here:
// Internal representation of the Small BASIC keywords. final int UNKNCOM = 0; final int PRINT = 1; final int INPUT = 2; final int IF = 3; final int THEN = 4; final int FOR = 5; final int NEXT = 6; final int TO = 7; final int GOTO = 8; final int GOSUB = 9; final int RETURN = 10; final int END = 11; final int EOL = 12;
Notice UNKNCOM. This value is used by the lookUp( ) method to indicate an unknown keyword. To facilitate the conversion of a keyword's external representation into its internal representation, both the external and internal forms are held in a table called kwTable comprised of Keyword objects. Both are shown here:
// This class links keywords with their keyword tokens. class Keyword { String keyword; // string form int keywordTok; // internal representation Keyword(String str, int t) { keyword = str; keywordTok = t; } } /* Table of keywords with their internal representation.
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