The document discusses nullable and non-nullable variables in Kotlin, a data class with String and Int properties, and two database query methods - one in Java using a StringBuilder and the other in Kotlin using a multi-line string. The Kotlin method returns an optional Account while the Java method returns a non-optional Account.

6.
//NonNull
val hoge: String = "aaa"
//Nullable
val hogehoge: String? = null

7. data class Foo(val name: String, val age: Int)

11.
@Select("SELECT * "
+ "FROM `account` "
+ "WHERE `id` = #{id}”)
public Account get(long id);

12.
@Select("""
SELECT *
FROM `account`
WHERE `id` = #{id}
""")
fun get(id: Long): Account?