Inheritance chapter

src/lox/AstPrinter.java

@@ -47,6 +47,11 @@ public class AstPrinter implements Expr.Visitor<String> {
         return parenthesize(expr.name.lexeme, expr.object, expr.value);
     }
 
+    @Override
+    public String visitSuperExpr(Expr.Super expr) {
+        return parenthesize(expr.method.lexeme);
+    }
+
     @Override
     public String visitThisExpr(Expr.This expr) {
         return parenthesize(expr.keyword.lexeme);

src/lox/Expr.java

@@ -12,6 +12,7 @@ abstract class Expr {
 		R visitLiteralExpr(Literal expr);
 		R visitLogicalExpr(Logical expr);
 		R visitSetExpr(Set expr);
+		R visitSuperExpr(Super expr);
 		R visitThisExpr(This expr);
 		R visitUnaryExpr(Unary expr);
 		R visitVariableExpr(Variable expr);
@@ -132,6 +133,20 @@ abstract class Expr {
 		final Token name;
 		final Expr value;
 	}
+	static class Super extends Expr {
+		Super(Token keyword, Token method) {
+			this.keyword = keyword;
+			this.method = method;
+		}
+
+		@Override
+		<R> R accept(Visitor<R> visitor) {
+			return visitor.visitSuperExpr(this);
+		}
+
+		final Token keyword;
+		final Token method;
+	}
 	static class This extends Expr {
 		This(Token keyword) {
 			this.keyword = keyword;

src/lox/Interpreter.java

@@ -80,7 +80,7 @@ public class Interpreter implements Expr.Visitor<Object>,
                     return (double) left + (double) right;
                 }
                 if (left instanceof String && right instanceof String) {
-                    return (String) left + (String) right;
+                    return left + (String) right;
                 }
 
                 throw new RuntimeError(expr.operator,
@@ -109,12 +109,11 @@ public class Interpreter implements Expr.Visitor<Object>,
             arguments.add(evaluate(argument));
         }
 
-        if (!(callee instanceof LoxCallable)) {
+        if (!(callee instanceof LoxCallable function)) {
             throw new RuntimeError(expr.paren,
                     "Can only call functions and classes.");
         }
 
-        LoxCallable function = (LoxCallable) callee;
         if (arguments.size() != function.arity()) {
             throw new RuntimeError(expr.paren, "Expected " +
                     function.arity() + " arguments but got " +
@@ -172,6 +171,25 @@ public class Interpreter implements Expr.Visitor<Object>,
         return value;
     }
 
+    @Override
+    public Object visitSuperExpr(Expr.Super expr) {
+        int distance = locals.get(expr);
+        LoxClass superclass = (LoxClass)environment.getAt(
+                distance, "super");
+
+        LoxInstance object = (LoxInstance) environment.getAt(
+                distance - 1, "this");
+
+        LoxFunction method = superclass.findMethod(expr.method.lexeme);
+
+        if (method == null) {
+            throw new RuntimeError(expr.method,
+                    "Undefined property '" + expr.method.lexeme + "'.");
+        }
+
+        return  method.bind(object);
+    }
+
     @Override
     public Object visitThisExpr(Expr.This expr) {
         return lookUpVariable(expr.keyword, expr);
@@ -278,8 +296,22 @@ public class Interpreter implements Expr.Visitor<Object>,
 
     @Override
     public Void visitClassStmt(Stmt.Class stmt) {
+        Object superclass = null;
+        if (stmt.superclass != null) {
+            superclass = evaluate(stmt.superclass);
+            if (!(superclass instanceof LoxClass)) {
+                throw new RuntimeError(stmt.superclass.name,
+                        "Superclass must be a class.");
+            }
+        }
+
         environment.define(stmt.name.lexeme, null);
 
+        if (stmt.superclass != null) {
+            environment = new Environment(environment);
+            environment.define("super", superclass);
+        }
+
         Map<String, LoxFunction> methods = new HashMap<>();
         for (Stmt.Function method : stmt.methods) {
             LoxFunction function = new LoxFunction(method, environment,
@@ -287,7 +319,13 @@ public class Interpreter implements Expr.Visitor<Object>,
             methods.put(method.name.lexeme, function);
         }
 
-        LoxClass klass = new LoxClass(stmt.name.lexeme, methods);
+        LoxClass klass = new LoxClass(stmt.name.lexeme,
+                (LoxClass)superclass, methods);
+
+        if (superclass != null) {
+            environment = environment.enclosing;
+        }
+
         environment.assign(stmt.name, klass);
         return null;
     }

src/lox/LoxClass.java

@@ -5,10 +5,13 @@ import java.util.Map;
 
 public class LoxClass implements LoxCallable {
     final String name;
+    final LoxClass superclass;
     private final Map<String, LoxFunction> methods;
 
-    public LoxClass(String name, Map<String, LoxFunction> methods) {
+    public LoxClass(String name, LoxClass superclass,
+                    Map<String, LoxFunction> methods) {
         this.name = name;
+        this.superclass = superclass;
         this.methods = methods;
     }
 
@@ -17,6 +20,10 @@ public class LoxClass implements LoxCallable {
             return methods.get(name);
         }
 
+        if (superclass != null) {
+            return superclass.findMethod(name);
+        }
+
         return null;
     }
 

src/lox/Parser.java

@@ -45,6 +45,13 @@ public class Parser {
 
     private Stmt classDeclaration() {
         Token name = consume(IDENTIFIER, "Expect class name.");
+
+        Expr.Variable superclass = null;
+        if (match(LESS)) {
+            consume(IDENTIFIER, "Expect superclass name.");
+            superclass = new Expr.Variable(previous());
+        }
+
         consume(LEFT_BRACE, "Expect '{' before class body.");
 
         List<Stmt.Function> methods = new ArrayList<>();
@@ -54,7 +61,7 @@ public class Parser {
 
         consume(RIGHT_BRACE, "Expect '}' after class body.");
 
-        return new Stmt.Class(name, methods);
+        return new Stmt.Class(name, superclass, methods);
     }
 
     private Stmt statement() {
@@ -210,8 +217,7 @@ public class Parser {
             if (expr instanceof Expr.Variable) {
                 Token name = ((Expr.Variable) expr).name;
                 return new Expr.Assign(name, value);
-            } else if (expr instanceof Expr.Get) {
+            } else if (expr instanceof Expr.Get get) {
-                Expr.Get get = (Expr.Get)expr;
                 return new Expr.Set(get.object, get.name, value);
             }
 
@@ -347,6 +353,14 @@ public class Parser {
             return new Expr.Literal(previous().literal);
         }
 
+        if (match(SUPER)) {
+            Token keyword = previous();
+            consume(DOT, "Expect '.' after 'super'.");
+            Token method = consume(IDENTIFIER,
+                    "Expect superclass method name.");
+            return new Expr.Super(keyword, method);
+        }
+
         if (match(THIS)) return new Expr.This(previous());
 
         if (match(IDENTIFIER)) {

src/lox/Resolver.java

@@ -23,7 +23,8 @@ public class Resolver implements Expr.Visitor<Void>, Stmt.Visitor<Void> {
 
     private enum ClassType {
         NONE,
-        CLASS
+        CLASS,
+        SUBCLASS
     }
 
     private ClassType currentClass = ClassType.NONE;
@@ -90,6 +91,20 @@ public class Resolver implements Expr.Visitor<Void>, Stmt.Visitor<Void> {
         return null;
     }
 
+    @Override
+    public Void visitSuperExpr(Expr.Super expr) {
+        if (currentClass == ClassType.NONE) {
+            Lox.error(expr.keyword,
+                    "Can't use 'super' outside of a class.");
+        } else if (currentClass != ClassType.SUBCLASS) {
+            Lox.error(expr.keyword,
+                    "Can't use 'super' in a class with no superclass.");
+        }
+
+        resolveLocal(expr, expr.keyword);
+        return null;
+    }
+
     @Override
     public Void visitThisExpr(Expr.This expr) {
         if (currentClass == ClassType.NONE) {
@@ -136,6 +151,22 @@ public class Resolver implements Expr.Visitor<Void>, Stmt.Visitor<Void> {
         declare(stmt.name);
         define(stmt.name);
 
+        if (stmt.superclass != null &&
+            stmt.name.lexeme.equals(stmt.superclass.name.lexeme)) {
+            Lox.error(stmt.superclass.name,
+                    "A class can't inherit from itself.");
+        }
+
+        if (stmt.superclass != null) {
+            currentClass = ClassType.SUBCLASS;
+            resolve(stmt.superclass);
+        }
+
+        if (stmt.superclass != null) {
+            beginScope();
+            scopes.peek().put("super", true);
+        }
+
         beginScope();
         scopes.peek().put("this", true);
 
@@ -149,6 +180,8 @@ public class Resolver implements Expr.Visitor<Void>, Stmt.Visitor<Void> {
 
         endScope();
 
+        if (stmt.superclass != null) endScope();
+
         currentClass = enclosingClass;
         return null;
     }
@@ -176,7 +209,7 @@ public class Resolver implements Expr.Visitor<Void>, Stmt.Visitor<Void> {
     }
 
     private void beginScope() {
-        scopes.push(new HashMap<String, Boolean>());
+        scopes.push(new HashMap<>());
     }
 
     private void endScope() {

src/lox/Stmt.java

@@ -27,8 +27,9 @@ abstract class Stmt {
 		final List<Stmt> statements;
 	}
 	static class Class extends Stmt {
-		Class(Token name, List<Stmt.Function> methods) {
+		Class(Token name, Expr.Variable superclass, List<Stmt.Function> methods) {
 			this.name = name;
+			this.superclass = superclass;
 			this.methods = methods;
 		}
 
@@ -38,6 +39,7 @@ abstract class Stmt {
 		}
 
 		final Token name;
+		final Expr.Variable superclass;
 		final List<Stmt.Function> methods;
 	}
 	static class Expression extends Stmt {

src/tool/GenerateAst.java

@@ -1,13 +1,13 @@
 package tool;
 
-import java.io.FileNotFoundException;
+import java.io.IOException;
 import java.io.PrintWriter;
-import java.io.UnsupportedEncodingException;
+import java.nio.charset.StandardCharsets;
 import java.util.Arrays;
 import java.util.List;
 
 public class GenerateAst {
-    public static void main(String[] args) throws FileNotFoundException, UnsupportedEncodingException {
+    public static void main(String[] args) throws IOException {
         if (args.length != 1) {
             System.err.println("Usage: generate_ast <output directory>");
             System.exit(64);
@@ -22,6 +22,7 @@ public class GenerateAst {
                 "Literal  : Object value",
                 "Logical  : Expr left, Token operator, Expr right",
                 "Set      : Expr object, Token name, Expr value",
+                "Super    : Token keyword, Token method",
                 "This     : Token keyword",
                 "Unary    : Token operator, Expr right",
                 "Variable : Token name"
@@ -29,7 +30,8 @@ public class GenerateAst {
 
         defineAst(outputDir, "Stmt", Arrays.asList(
                 "Block      : List<Stmt> statements",
-                "Class      : Token name, List<Stmt.Function> methods",
+                "Class      : Token name, Expr.Variable superclass," +
+                            " List<Stmt.Function> methods",
                 "Expression : Expr expression",
                 "Function   : Token name, List<Token> params," +
                             " List<Stmt> body",
@@ -43,9 +45,9 @@ public class GenerateAst {
     }
 
     private static void defineAst(
-            String outputDir, String baseName, List<String> types) throws FileNotFoundException, UnsupportedEncodingException {
+            String outputDir, String baseName, List<String> types) throws IOException {
         String path = outputDir + "/" + baseName + ".java";
-        PrintWriter writer = new PrintWriter(path, "UTF-8");
+        PrintWriter writer = new PrintWriter(path, StandardCharsets.UTF_8);
 
         writer.println("package lox;");
         writer.println();