Expr.java

public abstract class Expr {
    public interface Visitor<R> {
        R visitAssignExpr(AssignExpr expr);
        R visitBinaryExpr(BinaryExpr expr);
        R visitGroupingExpr(GroupingExpr expr);
        R visitLiteralExpr(LiteralExpr expr);
        R visitUnaryExpr(UnaryExpr expr);
        R visitVariableExpr(VariableExpr expr);
    }

    public abstract <R> R accept(Visitor<R> visitor);
}

class AssignExpr extends Expr {
    public final Token name;
    public final Expr value;

    public AssignExpr(Token name, Expr value) {
        this.name = name;
        this.value = value;
    }

    @Override
    public <R> R accept(Visitor<R> visitor) {
        return visitor.visitAssignExpr(this);
    }
}

class BinaryExpr extends Expr {
    public final Expr left;
    public final Token operator;
    public final Expr right;

    public BinaryExpr(Expr left, Token operator, Expr right) {
        this.left = left;
        this.operator = operator;
        this.right = right;
    }

    @Override
    public <R> R accept(Visitor<R> visitor) {
        return visitor.visitBinaryExpr(this);
    }
}

class GroupingExpr extends Expr {
    public final Expr expression;

    public GroupingExpr(Expr expression) {
        this.expression = expression;
    }

    @Override
    public <R> R accept(Visitor<R> visitor) {
        return visitor.visitGroupingExpr(this);
    }
}

class LiteralExpr extends Expr {
    public final Object value;

    public LiteralExpr(Object value) {
        this.value = value;
    }

    @Override
    public <R> R accept(Visitor<R> visitor) {
        return visitor.visitLiteralExpr(this);
    }
}

class UnaryExpr extends Expr {
    public final Token operator;
    public final Expr right;

    public UnaryExpr(Token operator, Expr right) {
        this.operator = operator;
        this.right = right;
    }

    @Override
    public <R> R accept(Visitor<R> visitor) {
        return visitor.visitUnaryExpr(this);
    }
}

class VariableExpr extends Expr {
    public final Token name;

    public VariableExpr(Token name) {
        this.name = name;
    }

    @Override
    public <R> R accept(Visitor<R> visitor) {
        return visitor.visitVariableExpr(this);
    }
}