Hibernate app retry loop - can't get lambda to compile

// Context: I'm trying to pass a lambda as a higher order function to a `runTransaction` wrapper method.
// It's failing because of "bad return type".  I can't tell if my error is fundamental or syntax or both?

// Version info:

// java version "1.8.0_171"
// Java(TM) SE Runtime Environment (build 1.8.0_171-b11)
// Java HotSpot(TM) 64-Bit Server VM (build 25.171-b11, mixed mode)

// Error message:

// Sample.java:100: error: incompatible types: bad return type in lambda expression
//             int returnValue = runTransaction(session, (Session s) -> {
//                                                       ^
//     missing return value


package com.cockroachlabs;

import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.Transaction;
import org.hibernate.cfg.Configuration;

import java.util.*;
import java.util.function.Function;

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.Table;
import javax.persistence.criteria.CriteriaQuery;

public class Sample {
    // Create a SessionFactory based on our hibernate.cfg.xml configuration
    // file, which defines how to connect to the database.
    private static final SessionFactory sessionFactory =
            new Configuration()
                    .configure("hibernate.cfg.xml")
                    .addAnnotatedClass(Account.class)
                    .buildSessionFactory();

    // Account is our model, which corresponds to the "accounts" database table.
    @Entity
    @Table(name="accounts")
    public static class Account {
        @Id
        @Column(name="id")
        public long id;

        @Column(name="balance")
        public long balance;

        // Convenience constructor.
        public Account(int id, int balance) {
            this.id = id;
            this.balance = balance;
        }

        // Hibernate needs a default (no-arg) constructor to create model objects.
        public Account() {}
    }

    private static Random rand = new Random();

    // static Function addAccounts(Session s1) {
    //     Function f = (Session s) -> {
    //         try {
    //             s.save(new Account(1, 1000));
    //             s.save(new Account(2, 250));
    //             s.save(new Account(3, 314159));
    //             s.getTransaction().commit();
    //         } catch (Exception e) {
    //             throw e;
    //         }
    //     };
    //     return f;
    // }


    static int runTransaction(Session session, Function fn) {

        int returnValue = 0;
        int retryCount = 0;
        int MAX_RETRY_COUNT = 3;
        String RETRY_SQL_STATE = "40001";
        boolean FORCE_RETRY = false;

        try {
            fn.apply(session);
        } catch (org.hibernate.JDBCException e) {

            if (RETRY_SQL_STATE.equals(e.getSQLState())) {
                session.getTransaction().rollback();
                retryCount++;
                int sleepMillis = (int)(Math.pow(2, retryCount) * 100) + rand.nextInt(100);
                System.out.printf("Hit 40001 transaction retry error, sleeping %s milliseconds\n", sleepMillis);
                try {
                    Thread.sleep(sleepMillis);
                } catch (InterruptedException ignored) {
                    // Necessary to allow the retry loop to continue.
                }
                returnValue = -1;
            } else {
                returnValue = -1;
                throw e;
            }
        } finally {
            session.close();
            sessionFactory.close();
        }
    }

    public static void main(String[] args) throws Exception {
        Session session = sessionFactory.openSession();
        try {
// Below lambda `(Session s) -> {` fails with:
// error: incompatible types: bad return type in lambda expression
//             int returnValue = runTransaction(session, (Session s) -> {
//                                                       ^
//     missing return value
            int returnValue = runTransaction(session, (Session s) -> {  // BOOM!!!
                    try {
                        s.save(new Account(1, 1000));
                        s.save(new Account(2, 250));
                        s.save(new Account(3, 314159));
                        s.getTransaction().commit();
                    } catch (Exception e) {
                        throw e;
                    }
                });
            if (returnValue != -1) {
                // Success!
                System.out.printf("transferFunds EXIT_SUCCESS");
            }
        } finally {
            session.close();
            sessionFactory.close();
        }
    }
}

// gradle build

// > Task :compileJava FAILED
// /Users/rloveland/work/code/deemphasize-savepoints/java-hibernate/src/main/java/com/cockroachlabs/Sample.java:100: error: incompatible types: bad return type in lambda expression
//             int returnValue = runTransaction(session, (Session s) -> {
//                                                       ^
//     missing return value
// Note: /Users/rloveland/work/code/deemphasize-savepoints/java-hibernate/src/main/java/com/cockroachlabs/Sample.java uses unchecked or unsafe operations.
// Note: Recompile with -Xlint:unchecked for details.
// Note: Some messages have been simplified; recompile with -Xdiags:verbose to get full output
// 1 error

// FAILURE: Build failed with an exception.

// * What went wrong:
// Execution failed for task ':compileJava'.
// > Compilation failed; see the compiler error output for details.

// * Try:
// Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output. Run with --scan to get full insights.

// * Get more help at https://help.gradle.org

// BUILD FAILED in 564ms
// 1 actionable task: 1 executed

// Compilation exited abnormally with code 1 at Tue Aug 13 13:55:17