> For the complete documentation index, see [llms.txt](https://corman.gitbook.io/evo-api-draft/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://corman.gitbook.io/evo-api-draft/java/explicit-conversions.md).

# Explicit Conversions

Create conversions for custom types

I'm only going to be showing the conversion API in TypeScript for this chapter. There will be a Java API for this, and you should totally use the Java API instead (for speed). However, I prefer to use TypeScript to show how the API will look, it's just nicer. The Java API will have very similar semantics.

## Converters

In both directions, converters work the same way fundamentally.

* Converters have a **predicate** and a **transformer**.
* The predicate determines whether or not the converter will apply. For instance, it could check if the object is a String.
* Then, the transformer will convert that object into the new datatype.

## To Java

```typescript
// Note that this conversion is built-in, don't implement it yourself.

Java.convert({
    type: 'toJava',
    predicate(object, javaType) {
        return Array.isArray(object) && javaType == ArrayList;
    },
    convert(object) {
        const arrayList = new ArrayList();
        for (const el of object) {
            arrayList.add(el);
        }
        return arrayList;
    }
});
```

## From Java

```typescript
// Note that this conversion is built-in, don't implement it yourself.

Java.convert({
    type: 'fromJava',
    predicate(object) {
        return object instanceof ArrayList;
    },
    convert(object) {
        const array = [];
        // See the chapter on "Behaviors" to understand
        // why we can iterate this so naturally.
        for (const el of object) {
            array.push(el);
        }
    }
});
```