Building a Mini Java Compiler: Step-by-Step Guide for Beginners

Mini Java Compiler Tutorial: Writing a Simple Compiler in 10 Lessons

This 10-lesson tutorial walks you through building a minimal Java-like compiler that parses a small subset of Java, generates an abstract syntax tree (AST), performs basic semantic checks, and emits simple bytecode-like instructions. Each lesson includes objectives, key concepts, and short code examples in Java. Assume Java 11+ and a basic familiarity with parsing and data structures.

Overview: what this compiler supports

• Source: a tiny Java-like language with:
  • class with a single static method main
  • primitive ints, arithmetic (+, -,, /)
  • variable declarations and assignments
  • if statements and while loops
  • return statement
  • method calls (to built-in print)
• No objects, inheritance, or types beyond int and void.
• Output: a simple stack-based bytecode (text form) executed by a small VM.

Lesson 1 — Project scaffolding and tokenization (lexer)

Objective

Set up project and implement a lexer that converts source text into tokens: identifiers, numbers, symbols, keywords.

Key points

• Token types: IDENT, NUMBER, KEYWORD, SYMBOL, EOF
• Keep token positions for error messages.

Example (sketch)

java
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enum TokenType { IDENT, NUMBER, IF, WHILE, RETURN, INT, CLASS, STATIC, VOID, PRINT, LPAREN, RPAREN, LBRACE, RBRACE, SEMI, PLUS, MINUS, STAR, SLASH, ASSIGN, EOF }
class Token { TokenType type; String text; int line, col; }

Lesson 2 — Parser: building the AST

Objective

Write a recursive-descent parser that produces an AST representing program structure.

Key points

• Grammar (simplified):
  • program -> classDecl
  • classDecl -> ‘class’ IDENT ‘{’ methodDecl ‘}’
  • methodDecl -> ‘static’ ‘void’ IDENT ‘(’ ‘)’ block
  • block -> ‘{’ stmt* ‘}’
  • stmt -> varDecl | ifStmt | whileStmt | exprStmt | returnStmt
  • expr -> assignment
  • assignment -> IDENT ‘=’ expr | equality
  • equality -> additive ((‘==’| ‘!=’) additive)
  • additive -> multiplicative ((‘+’|‘-’) multiplicative)
  • multiplicative -> primary ((’’|‘/’) primary)
  • primary -> NUMBER | IDENT | ‘(’ expr ‘)’ | call
• Create AST node classes: Program, ClassDecl, MethodDecl, Stmt (and subclasses), Expr (and subclasses).

Example (sketch)

java
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abstract class Expr {}
class Binary extends Expr { Expr left; String op; Expr right; }
class Literal extends Expr { int value; }
class Var extends Expr { String name; }

Lesson 3 — AST printing and debugging

Objective

Implement a pretty-printer or tree walker to visualize ASTs for debugging.

Key points

• Visitor pattern helps separate operations from AST structure.
• Print indentation per node depth.

Example (sketch)

java
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void printExpr(Expr e, int indent) { if (e instanceof Binary) { printBinary(...); } ... }

Lesson 4 — Semantic analysis: symbol table and scope

Objective

Add symbol table to track variable declarations and simple checks: undefined variables, duplicate declarations