diff --git a/java/expression/common/Expr.java b/java/expression/common/Expr.java
new file mode 100644
index 0000000..8ce7841
--- /dev/null
+++ b/java/expression/common/Expr.java
@@ -0,0 +1,35 @@
+package expression.common;
+
+import base.Functional;
+import base.Pair;
+import java.util.List;
+import java.util.function.BiFunction;
+import java.util.function.Function;
+
+/**
+ * @author Georgiy Korneev (kgeorgiy@kgeorgiy.info)
+ */
+public record Expr<C, V>(Node<C> node, List<Pair<String, V>> variables) {
+    public <T> List<Pair<String, T>> variables(
+        final BiFunction<String, V, T> f
+    ) {
+        return Functional.map(variables, variable ->
+            variable.second(f.apply(variable.first(), variable.second()))
+        );
+    }
+
+    public <T> Expr<C, T> convert(final BiFunction<String, V, T> f) {
+        return of(node, variables(f));
+    }
+
+    public Expr<C, V> node(final Function<Node<C>, Node<C>> f) {
+        return of(f.apply(node), variables);
+    }
+
+    public static <C, V> Expr<C, V> of(
+        final Node<C> node,
+        final List<Pair<String, V>> variables
+    ) {
+        return new Expr<>(node, variables);
+    }
+}
diff --git a/java/expression/common/ExpressionKind.java b/java/expression/common/ExpressionKind.java
new file mode 100644
index 0000000..5ed7276
--- /dev/null
+++ b/java/expression/common/ExpressionKind.java
@@ -0,0 +1,108 @@
+package expression.common;
+
+import base.ExtendedRandom;
+import base.Functional;
+import base.Pair;
+import expression.ToMiniString;
+import java.util.List;
+
+/**
+ * @author Georgiy Korneev (kgeorgiy@kgeorgiy.info)
+ */
+public class ExpressionKind<E extends ToMiniString, C> {
+
+    private final Type<C> type;
+    private final Class<E> kind;
+    private final Variables<E> variables;
+    private final Evaluator<E, C> evaluator;
+
+    public ExpressionKind(
+        final Type<C> type,
+        final Class<E> kind,
+        final Variables<E> variables,
+        final Evaluator<E, C> evaluator
+    ) {
+        this.type = type;
+        this.kind = kind;
+        this.variables = variables;
+        this.evaluator = evaluator;
+    }
+
+    public ExpressionKind(
+        final Type<C> type,
+        final Class<E> kind,
+        final List<Pair<String, E>> variables,
+        final Evaluator<E, C> evaluator
+    ) {
+        this(type, kind, (r, c) -> variables, evaluator);
+    }
+
+    public C evaluate(
+        final E expression,
+        final List<String> variables,
+        final List<C> values
+    ) throws Exception {
+        return evaluator.evaluate(expression, variables, values);
+    }
+
+    public E cast(final Object expression) {
+        return kind.cast(expression);
+    }
+
+    public String getName() {
+        return kind.getSimpleName();
+    }
+
+    public E constant(final C value) {
+        return cast(type.constant(value));
+    }
+
+    public C randomValue(final ExtendedRandom random) {
+        return type.randomValue(random);
+    }
+
+    public List<List<C>> allValues(
+        final int length,
+        final List<Integer> values
+    ) {
+        return Functional.allValues(fromInts(values), length);
+    }
+
+    public List<C> fromInts(final List<Integer> values) {
+        return Functional.map(values, this::fromInt);
+    }
+
+    public C fromInt(final int value) {
+        return type.fromInt(value);
+    }
+
+    @Override
+    public String toString() {
+        return kind.getName();
+    }
+
+    public ExpressionKind<E, C> withVariables(final Variables<E> variables) {
+        return new ExpressionKind<>(type, kind, variables, evaluator);
+    }
+
+    public Variables<E> variables() {
+        return variables;
+    }
+
+    @FunctionalInterface
+    public interface Variables<E> {
+        List<Pair<String, E>> generate(
+            final ExtendedRandom random,
+            final int count
+        );
+    }
+
+    @FunctionalInterface
+    public interface Evaluator<E, R> {
+        R evaluate(
+            final E expression,
+            final List<String> vars,
+            final List<R> values
+        ) throws Exception;
+    }
+}
diff --git a/java/expression/common/Generator.java b/java/expression/common/Generator.java
new file mode 100644
index 0000000..ebf0707
--- /dev/null
+++ b/java/expression/common/Generator.java
@@ -0,0 +1,233 @@
+package expression.common;
+
+import base.*;
+import java.util.ArrayList;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Set;
+import java.util.function.BiFunction;
+import java.util.function.Consumer;
+import java.util.function.Function;
+import java.util.function.Supplier;
+import java.util.stream.Stream;
+
+public class Generator<C, E> {
+
+    private final Supplier<C> constant;
+    private final List<Named<Integer>> ops;
+    private final ExpressionKind.Variables<E> variables;
+    private final Set<String> forbidden;
+    private final ExtendedRandom random;
+    private final List<Function<List<Node<C>>, Stream<Node<C>>>> basicTests;
+
+    public Generator(
+        final Supplier<C> constant,
+        final List<Named<Integer>> ops,
+        final ExpressionKind.Variables<E> variables,
+        final Set<String> forbidden,
+        final ExtendedRandom random,
+        final List<Function<List<Node<C>>, Stream<Node<C>>>> basicTests
+    ) {
+        this.constant = constant;
+        this.ops = List.copyOf(ops);
+        this.variables = variables;
+        this.forbidden = Set.copyOf(forbidden);
+        this.random = random;
+        this.basicTests = List.copyOf(basicTests);
+    }
+
+    public static <C> Builder<C> builder(
+        final Supplier<C> constant,
+        final ExtendedRandom random
+    ) {
+        return new Builder<>(random, constant);
+    }
+
+    public void testRandom(
+        final TestCounter counter,
+        final int denominator,
+        final Consumer<Expr<C, E>> consumer
+    ) {
+        final int d = Math.max(TestCounter.DENOMINATOR, denominator);
+        testRandom(counter, consumer, 1, 100, 100 / d, (vars, depth) ->
+            generateFullDepth(vars, Math.min(depth, 3))
+        );
+        testRandom(counter, consumer, 2, 1000 / d, 1, this::generateSize);
+        testRandom(counter, consumer, 3, 12, 100 / d, this::generateFullDepth);
+        testRandom(
+            counter,
+            consumer,
+            4,
+            777 / d,
+            1,
+            this::generatePartialDepth
+        );
+    }
+
+    private void testRandom(
+        final TestCounter counter,
+        final Consumer<Expr<C, E>> consumer,
+        final int seq,
+        final int levels,
+        final int perLevel,
+        final BiFunction<List<Node<C>>, Integer, Node<C>> generator
+    ) {
+        counter.scope("Random tests #" + seq, () -> {
+            final int total = levels * perLevel;
+            int generated = 0;
+            for (int level = 0; level < levels; level++) {
+                for (int j = 0; j < perLevel; j++) {
+                    if (generated % 100 == 0) {
+                        progress(counter, total, generated);
+                    }
+                    generated++;
+
+                    final List<Pair<String, E>> vars = variables(
+                        random.nextInt(10) + 1
+                    );
+                    consumer.accept(
+                        Expr.of(
+                            generator.apply(
+                                Functional.map(vars, v -> Node.op(v.first())),
+                                level
+                            ),
+                            vars
+                        )
+                    );
+                }
+            }
+            progress(counter, generated, total);
+        });
+    }
+
+    private static void progress(
+        final TestCounter counter,
+        final int total,
+        final int generated
+    ) {
+        counter.format("Completed %4d out of %d%n", generated, total);
+    }
+
+    private Node<C> generate(
+        final List<Node<C>> variables,
+        final boolean nullary,
+        final Supplier<Node<C>> unary,
+        final Supplier<Pair<Node<C>, Node<C>>> binary
+    ) {
+        if (nullary || ops.isEmpty()) {
+            return random.nextBoolean()
+                ? random.randomItem(variables)
+                : Node.constant(constant.get());
+        } else {
+            final Named<Integer> op = random.randomItem(ops);
+            if (Math.abs(op.value()) == 1) {
+                return Node.op(op.name(), (op.value() + 1) >> 1, unary.get());
+            } else {
+                final Pair<Node<C>, Node<C>> pair = binary.get();
+                return Node.op(op.name(), pair.first(), pair.second());
+            }
+        }
+    }
+
+    private Node<C> generate(
+        final List<Node<C>> variables,
+        final boolean nullary,
+        final Supplier<Node<C>> child
+    ) {
+        return generate(variables, nullary, child, () ->
+            Pair.of(child.get(), child.get())
+        );
+    }
+
+    private Node<C> generateFullDepth(
+        final List<Node<C>> variables,
+        final int depth
+    ) {
+        return generate(variables, depth == 0, () ->
+            generateFullDepth(variables, depth - 1)
+        );
+    }
+
+    private Node<C> generatePartialDepth(
+        final List<Node<C>> variables,
+        final int depth
+    ) {
+        return generate(variables, depth == 0, () ->
+            generatePartialDepth(variables, random.nextInt(depth))
+        );
+    }
+
+    private Node<C> generateSize(
+        final List<Node<C>> variables,
+        final int size
+    ) {
+        final int first = size <= 1 ? 0 : random.nextInt(size);
+        return generate(
+            variables,
+            size == 0,
+            () -> generateSize(variables, size - 1),
+            () ->
+                Pair.of(
+                    generateSize(variables, first),
+                    generateSize(variables, size - 1 - first)
+                )
+        );
+    }
+
+    public void testBasic(final Consumer<Expr<C, E>> consumer) {
+        basicTests.forEach(test -> {
+            final List<Pair<String, E>> vars = variables(random.nextInt(5) + 3);
+            test
+                .apply(Functional.map(vars, v -> Node.op(v.first())))
+                .map(node -> Expr.of(node, vars))
+                .forEachOrdered(consumer);
+        });
+    }
+
+    public List<Pair<String, E>> variables(final int count) {
+        List<Pair<String, E>> vars;
+        do {
+            vars = variables.generate(random, count);
+        } while (vars.stream().map(Pair::first).anyMatch(forbidden::contains));
+        return vars;
+    }
+
+    /**
+     * @author Georgiy Korneev (kgeorgiy@kgeorgiy.info)
+     */
+    public static final class Builder<C> {
+
+        private final ExtendedRandom random;
+        private final Supplier<C> constant;
+
+        private final List<Named<Integer>> ops = new ArrayList<>();
+        private final Set<String> forbidden = new HashSet<>();
+
+        private Builder(
+            final ExtendedRandom random,
+            final Supplier<C> constant
+        ) {
+            this.random = random;
+            this.constant = constant;
+        }
+
+        public void add(final String name, final int arity) {
+            ops.add(Named.of(name, arity));
+            forbidden.add(name);
+        }
+
+        public <E> Generator<C, E> build(
+            final ExpressionKind.Variables<E> variables,
+            final List<Function<List<Node<C>>, Stream<Node<C>>>> basicTests
+        ) {
+            return new Generator<>(
+                constant,
+                ops,
+                variables,
+                forbidden,
+                random,
+                basicTests
+            );
+        }
+    }
+}
diff --git a/java/expression/common/Node.java b/java/expression/common/Node.java
new file mode 100644
index 0000000..5b062ba
--- /dev/null
+++ b/java/expression/common/Node.java
@@ -0,0 +1,173 @@
+package expression.common;
+
+import java.util.function.Function;
+
+/**
+ * @author Georgiy Korneev (kgeorgiy@kgeorgiy.info)
+ */
+public abstract class Node<T> {
+
+    private Node() {}
+
+    public abstract <R> R get(
+        Const<T, R> con,
+        Nullary<R> nul,
+        Unary<Node<T>, R> un,
+        Binary<Node<T>, R> bin
+    );
+
+    public abstract <R> R cata(
+        Const<T, R> con,
+        Nullary<R> nul,
+        Unary<R, R> un,
+        Binary<R, R> bin
+    );
+
+    public final String toPolish() {
+        return cata(
+            T::toString,
+            name -> name,
+            (name, priority, a) -> a + " " + name + ":1",
+            (name, a1, a2) -> a1 + " " + a2 + " " + name + ":2"
+        );
+    }
+
+    @Override
+    public final String toString() {
+        return cata(
+            T::toString,
+            name -> name,
+            (name, priority, a) ->
+                name.equals("[")
+                    ? "[" + a + "]"
+                    : (priority & 1) == 1
+                        ? "(" + name + " " + a + ")"
+                        : "(" + a + " " + name + ")",
+            (name, a1, a2) -> "(" + a1 + " " + name + " " + a2 + ")"
+        );
+    }
+
+    public static <T> Node<T> constant(final T value) {
+        return new Node<>() {
+            @Override
+            public <R> R get(
+                final Const<T, R> con,
+                final Nullary<R> nul,
+                final Unary<Node<T>, R> un,
+                final Binary<Node<T>, R> bin
+            ) {
+                return con.apply(value);
+            }
+
+            @Override
+            public <R> R cata(
+                final Const<T, R> con,
+                final Nullary<R> nul,
+                final Unary<R, R> un,
+                final Binary<R, R> bin
+            ) {
+                return con.apply(value);
+            }
+        };
+    }
+
+    public static <T> Node<T> op(final String name) {
+        return new Node<>() {
+            @Override
+            public <R> R get(
+                final Const<T, R> con,
+                final Nullary<R> nul,
+                final Unary<Node<T>, R> un,
+                final Binary<Node<T>, R> bin
+            ) {
+                return nul.apply(name);
+            }
+
+            @Override
+            public <R> R cata(
+                final Const<T, R> con,
+                final Nullary<R> nul,
+                final Unary<R, R> un,
+                final Binary<R, R> bin
+            ) {
+                return nul.apply(name);
+            }
+        };
+    }
+
+    public static <T> Node<T> op(
+        final String name,
+        final int priority,
+        final Node<T> arg
+    ) {
+        return new Node<>() {
+            @Override
+            public <R> R get(
+                final Const<T, R> con,
+                final Nullary<R> nul,
+                final Unary<Node<T>, R> un,
+                final Binary<Node<T>, R> bin
+            ) {
+                return un.apply(name, priority, arg);
+            }
+
+            @Override
+            public <R> R cata(
+                final Const<T, R> con,
+                final Nullary<R> nul,
+                final Unary<R, R> un,
+                final Binary<R, R> bin
+            ) {
+                return un.apply(name, priority, arg.cata(con, nul, un, bin));
+            }
+        };
+    }
+
+    public static <T> Node<T> op(
+        final String name,
+        final Node<T> arg1,
+        final Node<T> arg2
+    ) {
+        return new Node<>() {
+            @Override
+            public <R> R get(
+                final Const<T, R> con,
+                final Nullary<R> nul,
+                final Unary<Node<T>, R> un,
+                final Binary<Node<T>, R> bin
+            ) {
+                return bin.apply(name, arg1, arg2);
+            }
+
+            @Override
+            public <R> R cata(
+                final Const<T, R> con,
+                final Nullary<R> nul,
+                final Unary<R, R> un,
+                final Binary<R, R> bin
+            ) {
+                return bin.apply(
+                    name,
+                    arg1.cata(con, nul, un, bin),
+                    arg2.cata(con, nul, un, bin)
+                );
+            }
+        };
+    }
+
+    @FunctionalInterface
+    public interface Const<T, R> extends Function<T, R> {}
+
+    @FunctionalInterface
+    public interface Nullary<R> extends Function<String, R> {}
+
+    @FunctionalInterface
+    public interface Unary<T, R> {
+        R apply(String name, int priority, T arg);
+    }
+
+    @FunctionalInterface
+    public interface Binary<T, R> {
+        R apply(String name, T arg1, T arg2);
+    }
+}
diff --git a/java/expression/common/NodeRenderer.java b/java/expression/common/NodeRenderer.java
new file mode 100644
index 0000000..02f2bf1
--- /dev/null
+++ b/java/expression/common/NodeRenderer.java
@@ -0,0 +1,114 @@
+package expression.common;
+
+import base.ExtendedRandom;
+import java.util.List;
+import java.util.Map;
+
+/**
+ * @author Georgiy Korneev (kgeorgiy@kgeorgiy.info)
+ */
+public class NodeRenderer<C> {
+
+    public static final String PAREN = "[";
+    public static final List<Paren> DEFAULT_PARENS = List.of(paren("(", ")"));
+
+    public static final Mode MINI_MODE = Mode.SIMPLE_MINI; // Replace by TRUE_MINI for some challenge;
+    public static final Settings FULL = Mode.FULL.settings(0);
+    public static final Settings FULL_EXTRA = Mode.FULL.settings(
+        Integer.MAX_VALUE / 4
+    );
+    public static final Settings SAME = Mode.SAME.settings(0);
+    public static final Settings MINI = MINI_MODE.settings(0);
+    public static final Settings TRUE_MINI = Mode.TRUE_MINI.settings(0);
+
+    private final Renderer<C, Settings, Node<C>> renderer;
+    private final Map<String, String> brackets;
+    private final ExtendedRandom random;
+
+    public NodeRenderer(
+        final Renderer<C, Settings, Node<C>> renderer,
+        final Map<String, String> brackets,
+        final ExtendedRandom random
+    ) {
+        this.renderer = renderer;
+        this.brackets = Map.copyOf(brackets);
+        this.random = random;
+    }
+
+    public static <C> Node<C> paren(
+        final boolean condition,
+        final Node<C> node
+    ) {
+        return condition ? Node.op(PAREN, 1, node) : node;
+    }
+
+    public static Paren paren(final String open, final String close) {
+        return new Paren(open, close);
+    }
+
+    public Node<C> renderToNode(
+        final Settings settings,
+        final Expr<C, ?> expr
+    ) {
+        final Expr<C, Node<C>> convert = expr.convert((name, variable) ->
+            Node.op(name)
+        );
+        return renderer.render(convert, settings);
+    }
+
+    public String render(final Node<C> node, final List<Paren> parens) {
+        return node.cata(
+            String::valueOf,
+            name -> name,
+            (name, priority, arg) ->
+                name == PAREN
+                    ? random.randomItem(parens).apply(arg)
+                    : priority == Integer.MAX_VALUE
+                        ? name + arg + brackets.get(name)
+                        : (priority & 1) == 1
+                            ? name + arg
+                            : arg + name,
+            (name, a, b) -> a + " " + name + " " + b
+        );
+    }
+
+    public String render(final Expr<C, ?> expr, final Settings settings) {
+        return render(renderToNode(settings, expr), settings.parens());
+    }
+
+    public enum Mode {
+        FULL,
+        SAME,
+        TRUE_MINI,
+        SIMPLE_MINI;
+
+        public Settings settings(final int limit) {
+            return new Settings(this, limit);
+        }
+    }
+
+    public record Paren(String open, String close) {
+        String apply(final String expression) {
+            return open() + expression + close();
+        }
+    }
+
+    public record Settings(Mode mode, int limit, List<Paren> parens) {
+        public Settings(final Mode mode, final int limit) {
+            this(mode, limit, DEFAULT_PARENS);
+        }
+
+        public <C> Node<C> extra(Node<C> node, final ExtendedRandom random) {
+            while (random.nextInt(Integer.MAX_VALUE) < limit) {
+                node = paren(true, node);
+            }
+            return node;
+        }
+
+        public Settings withParens(final List<Paren> parens) {
+            return this.parens.equals(parens)
+                ? this
+                : new Settings(mode, limit, List.copyOf(parens));
+        }
+    }
+}