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tags: Software Engineer, programming language, Java,
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# Java Reusable Code Snippets

## Handle Nullable List with Optional.

Optional.ofNullable()
    orElse(Collections.emptyList())
    // do map, filter, functional stuff here ...

Optional.ofNullable(x) 
    .map(...) // if optional of null gets mapped, what happens?
    .orElseGet(...)
    

Optional.of()
.map
.orElse()

Instant, Duration, Period, LocalDate, LocalDateTime

## Spring

injection syntax
- field injection
- setter injection
- constructor injection

lombok `UtilityClass`
toBuilder

retry

## SQL snippets

check tables & columns in Oracle.

time processing snippets

window function
https://www.postgresql.org/docs/current/tutorial-window.html

mapstruct mapper `qualifiedNyName`

What is Generics type.

Use `?`
`Function<A, ?>`

TestNG, validating the flow throws exception

use `expectThrows`, don't use try catch and assert exception.

spring batch concepts