CL Compiler   _{^{JavaCC · JJTree · JavaFX}}

A from-scratch compiler front end and three-address-code generator for the Classroom Language (CL), shipped with a JavaFX playground that lets you watch the source flow through every phase live -- tokens, AST, symbol table, semantic errors and quadruples -- as you type.

Built for the CSC323 Compiler Construction course at Bahria University Islamabad, Spring 2026, and then dressed up as a proper desktop app.

Authors ·  Sameer Hassan (01-134232-168)  ·  Haroon Mazhar (01-134232-121)  ·  BSCS 6-C  ·  Sir Abdul Raheem Aleem

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What it does

CL is a small imperative language with int, float, string and char data types, assignment, outString, a conditional loop (loopif) and a multi-way branch (switchFor). The compiler:

Phase	Implementation	Output you can see in the GUI
Lexical	JavaCC token rules in CL.jjt	Tokens tab (line / col / lexeme / kind)
Syntax	Top-down LL(1) parser generated by JavaCC + JJTree	AST tab (indented tree)
Symbol table	Hand-written SymbolTable.java	Symbol Table tab
Semantic	Hand-written SemanticAnalyzer.java	Errors tab (line-numbered)
Intermediate code	Hand-written IntermediateCodeGenerator.java	Quadruples tab + TAC tab

The IR is the textbook three-address-code in quadruple form (op, arg1, arg2, result) stored in an ArrayList<Quad>. Loops and switches lower to labels + conditional jumps in the style of the Dragon Book.

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Screenshots

JavaFX playground

The Tokens tab, showing the lexer output for the addition sample (line, column, lexeme, token kind for every token the scanner emitted):

The AST tab on the factorial program (12 quadruples emitted, indented tree of LoopStmt, CondExpr, AssignStmt, Expr, Term, Factor...):

The Errors tab when the semantic phase rejects a program (here, y is used in the code block but never declared):

CLI mode

The same compiler core also runs as a classic stdin tool via run-cli.bat:

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Quick start

You need a JDK 17 or newer and (optionally) Apache Maven. The included build.bat will fall back to a bundled Maven install at C:\Users\samee\apache-maven-3.9.9 if mvn is not on the PATH.

build.bat        :: generates parser, compiles, builds the fat jar
run.bat          :: launches the JavaFX playground
run-cli.bat      :: stdin-only mode (paste a CL program, end with 'endProgram')

Or directly with Maven:

mvn package           # build target/cl-compiler-1.0.0.jar
mvn javafx:run        # launch GUI
java -jar target/cl-compiler-1.0.0.jar    # also launches GUI

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Project layout

src/main/jjtree/CL.jjt                       grammar (input to javacc)
src/main/java/cl/
    CLCompiler.java                          orchestrates every phase
    CompileResult.java                       structured result data class
    SymbolTable.java                         hand-written data structure
    SemanticAnalyzer.java                    hand-written checks
    IntermediateCodeGenerator.java           hand-written TAC emitter
    ASTPrinter.java                          tree -> indented string
src/main/java/clgui/
    App.java          MainController.java    JavaFX entry + controller
    Launcher.java                            shim for fat-jar main class
    TokenLister.java  Samples.java           lexer view + sample programs
src/main/resources/clgui/
    main.fxml         styles.css             UI layout + dark theme
report/report.tex     report/report.pdf      academic project report
PIPELINE.md                                  build + runtime pipeline diagram
.github/workflows/ci.yml                     GitHub Actions build

See PIPELINE.md for the full build and runtime diagrams.

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CL by example

A factorial program. Paste it into the playground (or pipe it to run-cli.bat):

startProgram
  variables:
    int n    = 5;
    int fact = 1;
  code:
    loopif n > 0 holds
      fact = fact * n;
      n    = n - 1;
    endloop
    outString(fact);
endProgram

A program the semantic analyser rejects (a = b + c where b is a string and c is an integer):

startProgram
  variables:
    string b = "hi";
    int    c = 5;
    string a = "";
  code:
    a = b + c;
endProgram

Semantic Error at line 7: type mismatch in expression: '+' between string and int

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Implementation notes worth reading

• Grammar trick. Expr -> Term (+|-) Term* and Term -> Factor (*|/) Factor* give * and / higher precedence than + and - without needing any explicit precedence table -- it falls out of how the parser nests productions. This is the same shape used in the Dragon Book.
• No nested control flow. The CL spec forbids loopif or switchFor inside another loopif / switchFor. We enforce this at the grammar level: the body of loopif only accepts AssignStmt | OutStmt, so illegal nesting is rejected by the parser, not by the semantic phase.
• Two error-collection styles. Lexical and syntax errors short-circuit immediately (they tend to cascade). Semantic errors are accumulated into a list so the user sees every problem in one go.
• JavaFX Launcher trick. When JavaFX is bundled inside a shaded fat jar, the JVM refuses to run a class that extends Application as the Main-Class. A trivial Launcher that forwards to App.main sidesteps it.

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License

MIT