# Part 1 — ObjectMapper: reading and writing

Post: <https://ankurm.com/jackson-objectmapper-guide/>

## B01 — writing

[`B01WriteJson.java`](../src/main/java/com/ankurm/jackson3/part1objectmapper/B01WriteJson.java)

String, File and pretty-printed output.

```
{"articleId":1,"title":"Jackson Deep Dive","tags":["java","json"]}
file     : {"articleId":1,"title":"Jackson Deep Dive","tags":["java","json"]}
pretty   :
{
  "articleId" : 1,
  "title" : "Jackson Deep Dive",
  "tags" : [ "java", "json" ]
}
```

## B02 — reading

[`B02ReadJson.java`](../src/main/java/com/ankurm/jackson3/part1objectmapper/B02ReadJson.java)

String, File and InputStream. The post's `readValue(new URL(...), ...)` example is
deliberately not reproduced — it makes a live network call, so it could not be part of
a reproducible run. The `InputStream` form is what an HTTP client hands you in practice.

```
from String : Jackson Deep Dive
from File   : 1
from Stream : [java, json]
```

## B03 — generic collections

[`B03GenericCollections.java`](../src/main/java/com/ankurm/jackson3/part1objectmapper/B03GenericCollections.java)

`TypeReference` lives in `tools.jackson.core.type` in Jackson 3. The example also runs
the wrong version — `readValue(json, List.class)` — to show what it actually produces.

```
size          : 2
element class : Article
first title   : First
raw element   : LinkedHashMap  <- not Article
cast fails   : ClassCastException, as expected
map value     : Nine
```

The failure is deferred, not immediate: the call succeeds and returns a
`List<LinkedHashMap>`. You find out at the first cast, often far from the parse site.

## B04 — property ordering

[`B04PropertyOrdering.java`](../src/main/java/com/ankurm/jackson3/part1objectmapper/B04PropertyOrdering.java)

Beyond the post, but it explains why your first output does not look like the article's.

```
POJO    : {"listPrice":79.99,"productId":1,"productName":"Mechanical Keyboard"}
record  : {"productId":1,"productName":"Mechanical Keyboard","listPrice":79.99}
ordered : {"productId":1,"productName":"Mechanical Keyboard","listPrice":79.99}
```

Getter-based POJOs serialise alphabetically; records follow declaration order;
`@JsonPropertyOrder` overrides both. Worth pinning explicitly if the JSON is
cached, signed, or diffed against a fixture.