Mini Java Compiler Front-End

A Mini Java Compiler Front-End built in Python using PLY (Python Lex-Yacc).

This project performs multiple compiler front-end phases:

Source Code → Lexical Analysis → Syntax Analysis → Abstract Syntax Tree (AST) Generation → Semantic Analysis → Error Reporting

The goal of this project is to understand how real compilers process source code by implementing key compiler-design concepts from scratch.

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Features

Lexical Analysis

• Tokenization using PLY Lex
• Recognition of keywords, identifiers, literals, operators, and delimiters

Syntax Analysis

• LALR(1) parser implemented using PLY Yacc
• Grammar-based validation of Mini Java constructs
• Error recovery with multiple syntax error reporting

Abstract Syntax Tree (AST)

• Generates an AST for valid source code
• Pretty-prints the AST in a tree-like structure
• Preserves source line information for diagnostics

Semantic Analysis

• Symbol table implementation with nested scopes
• Semantic validation after parsing
• Separate semantic error reporting

Supported Language Constructs

Enums

enum Color {
    RED,
    GREEN,
    BLUE
}

Generic Classes

public class Container<T> {
}

Method Declarations

public int add(int a, int b) {
    return a;
}

Method Overloading Detection

public int add(int a) {
    return a;
}

public int add(int a) {
    return a;
}

Duplicate definitions are detected and reported.

Lambda Expressions

var handler = (int x) -> {
    return;
};

Exception Handling

try {
    count = 10;
}
catch(Exception e) {
    return;
}

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Semantic Checks

The semantic analyzer performs:

Duplicate Declaration Detection

• Duplicate class names
• Duplicate enum names
• Duplicate parameter names
• Duplicate variable declarations

Scope Analysis

• Global scope
• Class scope
• Method scope
• Lambda scope
• Try-Catch scope

Undefined Symbol Detection

• Undeclared variable usage
• Assignment to undeclared variables
• Calls to undefined methods

Type Checking

Detects type mismatches such as:

int x;
x = "hello";

Variable Shadowing Warnings

Warns when an inner-scope variable hides an outer-scope declaration.

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Compiler Pipeline

Source Code
    ↓
Lexer
    ↓
Parser
    ↓
AST Generation
    ↓
Semantic Analysis
    ↓
Error Summary

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Project Structure

Mini-Java-Syntax-Analyzer/
│
├── ast_nodes.py
├── lexer.py
├── parser.py
├── semantic_analyzer.py
├── main.py
│
├── test_input/
│   ├── Container.java
│   ├── ErrorTest.java
│   └── test_input.txt
│
└── README.md

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Requirements

Install PLY:

pip install ply

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Running the Project

Interactive Mode

python main.py

Enter code and finish with:

END

Parse a File

python main.py test_input/Container.java

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Example Output

AST Generation

Program
└── ClassDecl [Test]
    └── MethodDecl [hello]
        ├── ReturnType [void]
        ├── Params
        └── Body
            └── ReturnStmt

Semantic Analysis

✓ Semantic analysis passed — no errors or warnings.

Error Reporting

[E-SEM-007] Assignment to undeclared variable: 'x'
[E-SEM-006] Call to undefined method: 'foo'

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Concepts Demonstrated

• Lexical Analysis
• Context-Free Grammars
• LALR(1) Parsing
• Abstract Syntax Trees
• Symbol Tables
• Scope Management
• Semantic Analysis
• Compiler Front-End Design
• Error Recovery Techniques

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Future Improvements

• Local variable declarations
• Expression parsing
• Type inference
• Intermediate Code Generation (Three Address Code)
• Constant Folding Optimization
• Graphviz AST Visualization
• JVM Bytecode Generation

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Educational Purpose

This project is designed as a learning-oriented compiler front-end that demonstrates how programming languages are analyzed before execution. It combines concepts from Compiler Design, Automata & Formal Languages, and Programming Language Theory into a working implementation.