Recall that internally, as in all previous “… Lanes” cookbooks to date, swim.structure.Value is the type of every lane, regardless of its parametrized type.

More formally, any type – even a custom type – can serve as a type within a lane. The only requirement is to define a transformation for that type from Java’s built-in nominal type system to Swim’s swim.structure.Value-defined structural type system.

This offers a handful of advantages, primarily that swim.structure.Value types have general, well-defined serializations to Recon (this also applies to JSON, XML, and more, though that is less pertinent to this guide).

In essence, this means that clients can connect to Lanes directly, and authors of Web Agents don’t have to worry about explicit (de)serializations in the server-side code.

Transforming Values

An instance of swim.structure.Form<T> handles transformations between Value and T.

When declaring a lane parametrized with a type T, the Swim core always uses some Form<T> to manage conversions under the hood.

Several methods implemented in the BarType class handle these transformations manually.

Types of Forms

The swim.structure.form package contains many out-of-the-box Forms already for a variety of common types, such as LongForm, CharacterForm, and StringForm. This is why one can declare, for example, a MapLane<String, Long> that works without any additional effort.

For other types (e.g. custom pojos), you need to implement your own Form for that type, and tag the static accessor with @Kind.

There are two methods to achieve this: the “no-magic” (explicit) way exercised in Bartype and through annotations as exercised in FooType.

The BarType Class

There are several methods responsible for handling transformations between Value types and a custom Java object such as BarType:

- mold()

The mold() method is used to convert the fields of some custom object into a Swim structure of type Value.

// swim/basic/
public Value mold(BarType barType) {
  return Record.create(3)
    .slot("i", barType.getNumber1())
    .slot("s", barType.getString())
    .slot("j", barType.getNumber2());
- cast()

The cast() method is responsible for the inverse operation, parsing a Value into a Java object of the class type, e.g. BarType

// swim/basic/
public BarType cast(Item value) {
  try {
    final Attr attr = (Attr) value.head();
    final String barType = attr.getKey().stringValue("");
    if (!tag().equals(barType)) {
      return null;
    BarType b = new BarType(value.get("i").intValue(0),
    return b;
  } catch (Exception e) {
    return null;
- tag()

By default, the Attr associated with a Form, such as that in Record.create(3).attr("fooType").slot("i", 5).slot("s", "potato");, defaults to a camelCase string of the class name. The tag() method can be used to explicitly set a unique id for objects of a pojo.

// swim/basic/
class BarTypeForm extends Form<BarType> {
  public String tag() {
      return "barType";
- type()

The type() method is used to return the type of an object in the form of its defined class.

public Class<?> type() {
    return BarType.class;

The FooType Class

The FooType class in this project sets out a method of streamlining the process of defining a custom Form through more succinct annotations.

- @Kind

The @Kind annotation is used to tag a static accessor method for a custom Form.

// swim/basic/
private static Form<FooType> form;
- @Tag

A shorthand way of overriding the default classname Attr picked up by the reflection api.

// swim/basic/
- @Member

By default, the variable name of a class member will get picked up as the default attribute. However, as in @Tag, this string can be overriden using the @Member annotation.

This is by no means required, though when used @Member will change the attribute used in Recon serialization. This can prove helpful in Forms when one wishes to use shorthand value names internally but more explicit names in serialization (or vise versa).

// swim/basic/
public class FooType {
  private int i = 0;
  private String s = "";

As an exercise, imagine what would need to happen to achieve the same thing in

- form()

The form() method is used to simply return the form. In this example, it is best practice to check whether the form does not exist before returning, due to potentially JVM-dependent class-loading behavior.

// swim/basic/
public static Form<FooType> form() {
    if(form == null) {
      form = Form.forClass(FooType.class);
    return form;

Advanced Topic: Composability

Consider the problem of creating Forms for some type TrickyType, whose fields are other pojos that each have their own Forms.

To manually build a Form for this type by explictly implementing some Form<TrickyType> (e.g. like what was done with BarType), we can expect a clean implementation to call the other Forms’ mold() and cast() methods, following a fairly predictable pattern.

Because it’s so predictable, the annotation style Form generation can still be used, provided that these field pojos themselves have properly staged Forms. This is exercised via BazType in the companion app.

// swim/basic/
public class BazType {
  private FooType f = new FooType();
  private BarType b = new BarType();
  public BazType(FooType f, BarType b) {
    this.f = f;
    this.b = b;

This clean composability also enables some of the more complicated forms that were linked earlier. For example, one can get a working Form<List<FooType> handle by simply doing Form<List<FooType> = Form.forList(FooType.form()) (see here). Even something like a Form<List<List<BazType> could be created similarly!

Try It Yourself

A standalone project that demonstrates each method of defining a Form for a custom Java class can be found here.