Hot to smart-cast a Java class variable in Kotlin

Emoloyee.java

import javax.annotation.Nullable;

public class Employee {
    private String firstName;
    private String lastName;
    private Integer age;

    public Employee(String firstName, String lastName, Integer age){
        this.firstName = firstName;
        this.lastName = lastName;
        this.age = age;
    }


    @Nullable
    public String getFirstName() {
        return firstName;
    }

    @Nullable
    public String getLastName() {
        return lastName;
    }

    @Nullable
    public Integer getAge() {
        return age;
    }
}

main.kt

/*
 * This Kotlin source file was generated by the Gradle 'init' task.
 */
package com.github.serihiro

fun main() {
//    val e = Employee("a", "b", 3)
//    val canNotCompile1 = e.age + 3 // can not call nullable age
//    val canNotCompile2 = if (e.age != null) {
//         e.age + 3 // can not call nullable age even if after null checked.
//    } else {
//        0
//    }


    // Approach 1: Assign a Java variable to a Kotlin variable
    val e1 = Employee("a", "b", 3)
    val tmpAge = e1.age
    val age = if (tmpAge != null){
        tmpAge + 3
    } else {
        0
    }
    println(age)

    // Approach 2: Create a kotlin wrapper class
    val e2 = KtEmployee("a", "b", null)
    val tmpAge2 = e2.age()?.plus(3) ?: 0
    println(tmpAge2)
}

class KtEmployee {
    private val e: Employee
    constructor(fistName: String?, lastName: String?, age: Int?) {
        this.e = Employee(fistName, lastName, age)
    }

    fun fistName(): String? = e.firstName
    fun lastName(): String? = e.lastName
    fun age(): Int? = e.age
}