# Data types

Corda provides a large standard library of data types used in financial transactions and contract state objects. These provide a common language for states and contracts.

## Amount

The Amount class is used to represent an amount of some fungible asset. It is a generic class which wraps around a type used to define the underlying product, called the token. For instance it can be the standard JDK type `Currency`, or an `Issued` instance, or this can be a more complex type such as an obligation contract issuance definition (which in turn contains a token definition for whatever the obligation is to be settled in).

Note

Fungible is used here to mean that instances of an asset is interchangeable for any other identical instance, and that they can be split/merged. For example a £5 note can reasonably be exchanged for any other £5 note, and a £10 note can be exchanged for two £5 notes, or vice-versa.

Here are some examples:

    // A quantity of some specific currency like pounds, euros, dollars etc.
    Amount<Currency>
    // A quantity of currency that is issued by a specific issuer, for instance central bank vs other bank dollars
    Amount<Issued<Currency>>
    // A quantity of obligations to deliver currency of any issuer.
    Amount<Obligation.State<Currency>>

`Amount` represents quantities as integers. For currencies the quantity represents pennies, cents or whatever else the smallest integer amount for that currency is. You cannot use `Amount` to represent negative quantities or fractional quantities: if you wish to do this then you must use a different type e.g. `BigDecimal`. `Amount` defines methods to do addition and subtraction and these methods verify that the tokens on both sides of the operator are equal (these are operator overloads in Kotlin and can be used as regular methods from Java). There are also methods to do multiplication and division by integer amounts.

## State

A Corda contract is composed of three parts; the executable code, the legal prose, and the state objects that represent the details of a specific deal or asset (see Data model for further detail). In relational database terms a state is like a row in a database. A reference to a state in the ledger (whether it has been consumed or not) is represented with a `StateRef` object. If the state ref has been looked up from storage, you will have a `StateAndRef` which is simply a `StateRef` plus the data.

The `ContractState` type is an interface that all states must implement. A `TransactionState` is a simple container for a `ContractState` (the custom data used by a contract program) and additional platform-level state information, such as the notary pointer (see Consensus model).

A number of interfaces then extend `ContractState`, representing standardised functionality for common kinds of state:

> `OwnableState`
>   A state which has an owner (represented as a `PublicKey`, discussed later). Exposes the owner and a function for replacing the owner e.g. when an asset is sold.
> `LinearState`
>   A state which links back to its previous state, creating a thread of states over time. A linear state is useful when modelling an indivisible/non-fungible thing like a specific deal, or an asset that can’t be split (like a rare piece of art).
> `DealState`
>   A LinearState representing an agreement between two or more parties. Intended to simplify implementing generic protocols that manipulate many agreement types.
> `FixableDealState`
>   A deal state, with further functions exposed to support fixing of interest rates.

## NamedByHash and UniqueIdentifier

Things which are identified by their hash, like transactions and attachments, should implement the `NamedByHash` interface which standardises how the ID is extracted. Note that a hash is not a globally unique identifier: it is always a derivative summary of the contents of the underlying data. Sometimes this isn’t what you want: two deals that have exactly the same parameters and which are made simultaneously but which are logically different can’t be identified by hash because their contents would be identical. Instead you would use `UniqueIdentifier`. This is a combination of a (Java) `UUID` representing a globally unique 128 bit random number, and an arbitrary string which can be paired with it. For instance the string may represent an existing “weak” (not guaranteed unique) identifier for convenience purposes.

## FungibleAssets and Cash

There is a common `FungibleAsset` superclass for contracts which model fungible assets, which also provides a standard interface for its subclasses’ state objects to implement. The clear use-case is `Cash`, however `FungibleAsset` is intended to be readily extensible to cover other assets, for example commodities could be modelled by using a subclass whose state objects include further details (location of the commodity, origin, grade, etc.) as needed.

## Transaction lifecycle types

The `WireTransaction` class contains the core of a transaction without signatures, and with references to attachments in place of the attachments themselves (see also Data model). Once signed these are encapsulated in the `SignedTransaction` class. For processing a transaction (i.e. to verify it) it is first converted to a `LedgerTransaction`, which involves verifying the signatures and associating them to the relevant command(s), and resolving the attachment references to the attachments. Commands with valid signatures are encapsulated in the `AuthenticatedObject` type.

Note

A `LedgerTransaction` has not necessarily had its contracts be run, and thus could be contract-invalid (but not signature-invalid). You can use the `verify` method as shown below to run the contracts.

When constructing a new transaction from scratch, you use `TransactionBuilder`, which is a mutable transaction that can be signed once modification of the internals is complete. It is typical for contract classes to expose helper methods that can contribute to a `TransactionBuilder`.

Here’s an example of building a transaction that creates an issuance of bananas (note that bananas are not a real contract type in the library):

    val notaryToUse: Party = ...
    val txb = TransactionBuilder(notary = notaryToUse).withItems(BananaState(Amount(20, Bananas), fromCountry = "Elbonia"))
    txb.signWith(myKey)
    txb.setTime(Instant.now(), notaryToUse, 30.seconds)
    // We must disable the check for sufficient signatures, because this transaction is not yet notarised.
    val stx = txb.toSignedTransaction(checkSufficientSignatures = false)
    // Alternatively, let's just check it verifies pretending it was fully signed. To do this, we get
    // a WireTransaction, which is what the SignedTransaction wraps. Thus by verifying that directly we
    // skip signature checking.
    txb.toWireTransaction().toLedgerTransaction(services).verify()

In a unit test, you would typically use a freshly created `MockServices` object, or more realistically, you would write your tests using the domain specific language for writing tests.

## Party and PublicKey

Entities using the network are called parties. Parties can sign structures using keys, and a party may have many keys under their control.

Parties may sometimes be identified pseudonomously, for example, in a transaction sent to your node as part of a chain of custody it is important you can convince yourself of the transaction’s validity, but equally important that you don’t learn anything about who was involved in that transaction. In these cases a public key may be present without any identifying information about who owns it.

Identities of parties involved in signing a transaction can be represented simply by a `PublicKey`, or by further information (such as name) using the `Party` class. An `AuthenticatedObject` represents an object (like a command) that has been signed by a set of parties.

Note

These types are provisional and will change significantly in future as the identity framework becomes more fleshed out.

## Date support

There are a number of supporting interfaces and classes for use by contract which deal with dates (especially in the context of deadlines). As contract negotiation typically deals with deadlines in terms such as “overnight”, “T+3”, etc., it’s desirable to allow conversion of these terms to their equivalent deadline. `Tenor` models the interval before a deadline, such as 3 days, etc., while `DateRollConvention` describes how deadlines are modified to take into account bank holidays or other events that modify normal working days.

Calculating the rollover of a deadline based on working days requires information on the bank holidays involved (and where a contract’s parties are in different countries, for example, this can involve multiple separate sets of bank holidays). The `BusinessCalendar` class models these calendars of business holidays; currently it loads these from files on disk, but in future this is likely to involve reference data oracles in order to ensure consensus on the dates used.

## Cryptography & maths support

The `SecureHash` class represents a secure hash of unknown algorithm. We currently define only a single subclass, `SecureHash.SHA256`. There are utility methods to create them, parse them and so on.

We also provide some mathematical utilities, in particular a set of interpolators and classes for working with splines. These can be found in the maths package.