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neow3j - Implementing a NEP-11 (NFT) Smart Contract in Java

AxLabs ·  · 17 min read · Source
NEP-11JAVANEOW3J
neow3j

neow3j v3.22.1

Neow3j is a development toolkit that provides easy and reliable tools to build Neo dApps and Smart Contracts using the Java platform (Java, Kotlin, Android). Check out neow3j.io for more detailed information on neow3j and the technical documentation.

1. Setup#

If you haven't already set up your environment to use the neow3j library, you can check out our tutorial about setting up a neow3j project here.

2. NEP-11 Overview#

The NEP-11 is the non-fungible token (NFT) standard on Neo N3. Have a look at its official documentation here.

3. Example NEP-11 Contract#

The following example code represents a possible implementation for a token that supports the NEP-11 standard.

info

This example contract supports indivisible NFTs (i.e., decimals is equal to 0).


The NEP-11 standard also describes what methods are required if divisible NTFs should be supported. Some of the methods required for divisible NFTS deviate from the ones discussed here. Check out the documentation of the NEP-11 standard here for more details.

package io.neow3j.examples.contractdevelopment.contracts;
import io.neow3j.devpack.ByteString;import io.neow3j.devpack.Contract;import io.neow3j.devpack.Hash160;import io.neow3j.devpack.Helper;import io.neow3j.devpack.Iterator;import io.neow3j.devpack.Map;import io.neow3j.devpack.Runtime;import io.neow3j.devpack.Storage;import io.neow3j.devpack.StorageContext;import io.neow3j.devpack.StorageMap;import io.neow3j.devpack.StringLiteralHelper;import io.neow3j.devpack.annotations.DisplayName;import io.neow3j.devpack.annotations.ManifestExtra;import io.neow3j.devpack.annotations.OnDeployment;import io.neow3j.devpack.annotations.Permission;import io.neow3j.devpack.annotations.Safe;import io.neow3j.devpack.annotations.SupportedStandard;import io.neow3j.devpack.constants.CallFlags;import io.neow3j.devpack.constants.FindOptions;import io.neow3j.devpack.constants.NativeContract;import io.neow3j.devpack.constants.NeoStandard;import io.neow3j.devpack.contracts.ContractManagement;import io.neow3j.devpack.events.Event3Args;import io.neow3j.devpack.events.Event4Args;
@DisplayName("FurryFriends")@ManifestExtra(key = "author", value = "AxLabs")@SupportedStandard(neoStandard = NeoStandard.NEP_11)@Permission(nativeContract = NativeContract.ContractManagement)public class NonFungibleToken {        // Alice's address    static final Hash160 owner = StringLiteralHelper.addressToScriptHash("NM7Aky765FG8NhhwtxjXRx7jEL1cnw7PBP");
    static final byte contractMapPrefix = 0;    static final byte[] totalSupplyKey = new byte[]{0x00};    static final byte[] tokensOfKey = new byte[]{0x01};
    static final int registryMapPrefix = 1;    static final int ownerOfMapPrefix = 2;    static final int balanceMapPrefix = 3;
    static final int propNameMapPrefix = 8;    static final int propDescriptionMapPrefix = 9;    static final int propImageMapPrefix = 10;    static final int propTokenURIMapPrefix = 11;
    static final String propName = "name";    static final String propDescription = "description";    static final String propImage = "image";    static final String propTokenURI = "tokenURI";
    // endregion keys of key-value pairs in NFT properties    // region deploy, update, destroy
    @OnDeployment    public static void deploy(Object data, boolean update) {        if (!update) {            StorageMap contractMap = new StorageMap(Storage.getStorageContext(), contractMapPrefix);            contractMap.put(totalSupplyKey, 0);        }    }
    public static void update(ByteString script, String manifest) throws Exception {        if (!Runtime.checkWitness(contractOwner())) {            throw new Exception("No authorization");        }        new ContractManagement().update(script, manifest);    }
    public static void destroy() throws Exception {        if (!Runtime.checkWitness(contractOwner())) {            throw new Exception("No authorization");        }        new ContractManagement().destroy();    }
    // endregion deploy, update, destroy    // region NEP-11 methods
    @Safe    public static String symbol() {        return "NEOW";    }
    @Safe    public static int decimals() {        return 0;    }
    @Safe    public static int totalSupply() {        return new StorageMap(Storage.getReadOnlyContext(), contractMapPrefix).getInt(totalSupplyKey);    }
    @Safe    public static int balanceOf(Hash160 owner) throws Exception {        if (!Hash160.isValid(owner)) {            throw new Exception("The parameter 'owner' must be a 20-byte address.");        }        return getBalance(Storage.getReadOnlyContext(), owner);    }
    @Safe    public static Iterator<ByteString> tokensOf(Hash160 owner) throws Exception {        if (!Hash160.isValid(owner)) {            throw new Exception("The parameter 'owner' must be a 20-byte address.");        }        return (Iterator<ByteString>) Storage.find(Storage.getReadOnlyContext(), createTokensOfPrefix(owner),                (byte) (FindOptions.KeysOnly | FindOptions.RemovePrefix));    }
    public static boolean transfer(Hash160 to, ByteString tokenId, Object data) throws Exception {        if (!Hash160.isValid(to)) {            throw new Exception("The parameter 'to' must be a 20-byte address.");        }        if (tokenId.length() > 64) {            throw new Exception("The parameter 'tokenId' must be a valid NFT ID (64 or less bytes long).");        }        Hash160 owner = ownerOf(tokenId);        if (!Runtime.checkWitness(owner)) {            return false;        }        onTransfer.fire(owner, to, 1, tokenId);        if (owner != to) {            StorageContext ctx = Storage.getStorageContext();            new StorageMap(ctx, ownerOfMapPrefix).put(tokenId, to.toByteArray());
            new StorageMap(ctx, createTokensOfPrefix(owner)).delete(tokenId);            new StorageMap(ctx, createTokensOfPrefix(to)).put(tokenId, 1);
            decreaseBalanceByOne(ctx, owner);            increaseBalanceByOne(ctx, to);        }        if (new ContractManagement().getContract(to) != null) {            Contract.call(to, "onNEP11Payment", CallFlags.All, new Object[]{owner, 1, tokenId, data});        }        return true;    }
    // endregion NEP-11 methods    // region non-divisible NEP-11 methods
    @Safe    public static Hash160 ownerOf(ByteString tokenId) throws Exception {        if (tokenId.length() > 64) {            throw new Exception("The parameter 'tokenId' must be a valid NFT ID (64 or less bytes long).");        }        ByteString owner = new StorageMap(Storage.getReadOnlyContext(), ownerOfMapPrefix).get(tokenId);        if (owner == null) {            throw new Exception("This token id does not exist.");        }        return new Hash160(owner);    }
    // endregion non-divisible NEP-11 methods    // region optional NEP-11 methods
    @Safe    public static Iterator<Iterator.Struct<ByteString, ByteString>> tokens() {        return (Iterator<Iterator.Struct<ByteString, ByteString>>) new StorageMap(Storage.getReadOnlyContext(),                registryMapPrefix).find(FindOptions.RemovePrefix);    }
    @Safe    public static Map<String, String> properties(ByteString tokenId) throws Exception {        if (tokenId.length() > 64) {            throw new Exception("The parameter 'tokenId' must be a valid NFT ID (64 or less bytes long).");        }        Map<String, String> p = new Map<>();        StorageContext ctx = Storage.getReadOnlyContext();        ByteString tokenName = new StorageMap(ctx, propNameMapPrefix).get(tokenId);        if (tokenName == null) {            throw new Exception("This token id does not exist.");        }
        p.put(propName, tokenName.toString());        ByteString tokenDescription = new StorageMap(ctx, propDescriptionMapPrefix).get(tokenId);        if (tokenDescription != null) {            p.put(propDescription, tokenDescription.toString());        }        ByteString tokenImage = new StorageMap(ctx, propImageMapPrefix).get(tokenId);        if (tokenImage != null) {            p.put(propImage, tokenImage.toString());        }        ByteString tokenURI = new StorageMap(ctx, propTokenURIMapPrefix).get(tokenId);        if (tokenURI != null) {            p.put(propTokenURI, tokenURI.toString());        }        return p;    }
    // endregion optional NEP-11 methods    // region events
    @DisplayName("Transfer")    private static Event4Args<Hash160, Hash160, Integer, ByteString> onTransfer;
    // endregion events    // region custom methods
    @Safe    public static Hash160 contractOwner() {        return owner;    }
    public static void mint(Hash160 to, ByteString tokenId, Map<String, String> properties) throws Exception {        if (!Runtime.checkWitness(contractOwner())) {            throw new Exception("No authorization");        }        StorageContext ctx = Storage.getStorageContext();        StorageMap registryMap = new StorageMap(ctx, registryMapPrefix);        if (registryMap.get(tokenId) != null) {            throw new Exception("This token id already exists.");        }        if (!properties.containsKey(propName)) {            throw new Exception("The properties must contain a value for the key 'name'.");        }        String tokenName = properties.get(propName);        new StorageMap(ctx, propNameMapPrefix).put(tokenId, tokenName);        if (properties.containsKey(propDescription)) {            String description = properties.get(propDescription);            new StorageMap(ctx, propDescriptionMapPrefix).put(tokenId, description);        }        if (properties.containsKey(propImage)) {            String image = properties.get(propImage);            new StorageMap(ctx, propImageMapPrefix).put(tokenId, image);        }        if (properties.containsKey(propTokenURI)) {            String tokenURI = properties.get(propTokenURI);            new StorageMap(ctx, propTokenURIMapPrefix).put(tokenId, tokenURI);        }
        registryMap.put(tokenId, tokenId);        new StorageMap(ctx, ownerOfMapPrefix).put(tokenId, to.toByteArray());        new StorageMap(ctx, createTokensOfPrefix(to)).put(tokenId, 1);
        increaseBalanceByOne(ctx, to);        incrementTotalSupplyByOne(ctx);        onTransfer.fire(null, to, 1, tokenId);    }
    public static void burn(ByteString tokenId) throws Exception {        Hash160 owner;        try {            owner = ownerOf(tokenId);        } catch (Exception e) {            throw new Exception(e.getMessage());        }        if (!Runtime.checkWitness(owner)) {            throw new Exception("No authorization.");        }
        StorageContext ctx = Storage.getStorageContext();
        new StorageMap(ctx, registryMapPrefix).delete(tokenId);        new StorageMap(ctx, propNameMapPrefix).delete(tokenId);        new StorageMap(ctx, propDescriptionMapPrefix).delete(tokenId);        new StorageMap(ctx, propImageMapPrefix).delete(tokenId);        new StorageMap(ctx, propTokenURIMapPrefix).delete(tokenId);        new StorageMap(ctx, ownerOfMapPrefix).delete(tokenId);
        new StorageMap(ctx, createTokensOfPrefix(owner)).delete(tokenId);        decreaseBalanceByOne(ctx, owner);        decrementTotalSupplyByOne(ctx);        onTransfer.fire(owner, null, 1, tokenId);    }
    // endregion custom methods    // region private helper methods
    private static int getBalance(StorageContext ctx, Hash160 owner) {        return new StorageMap(ctx, balanceMapPrefix).getIntOrZero(owner.toByteArray());    }
    private static void increaseBalanceByOne(StorageContext ctx, Hash160 owner) {        new StorageMap(ctx, balanceMapPrefix).put(owner.toByteArray(), getBalance(ctx, owner) + 1);    }
    private static void decreaseBalanceByOne(StorageContext ctx, Hash160 owner) {        new StorageMap(ctx, balanceMapPrefix).put(owner.toByteArray(), getBalance(ctx, owner) - 1);    }
    private static void incrementTotalSupplyByOne(StorageContext ctx) {        StorageMap contractMap = new StorageMap(ctx, contractMapPrefix);        int updatedTotalSupply = contractMap.getInt(totalSupplyKey) + 1;        contractMap.put(totalSupplyKey, updatedTotalSupply);    }
    private static void decrementTotalSupplyByOne(StorageContext ctx) {        StorageMap contractMap = new StorageMap(ctx, contractMapPrefix);        int updatedTotalSupply = contractMap.getInt(totalSupplyKey) - 1;        contractMap.put(totalSupplyKey, updatedTotalSupply);    }
    private static byte[] createTokensOfPrefix(Hash160 owner) {        return Helper.concat(tokensOfKey, owner.toByteArray());    }
    // endregion private helper methods
}

4. Contract Breakdown#

Imports#

The imports show the neow3j devpack classes that are used in the example contract. Check out neow3j devpack's javadoc for a full overview of classes and methods that are supported.

package io.neow3j.examples.contractdevelopment.contracts;
import io.neow3j.devpack.ByteString;import io.neow3j.devpack.Contract;import io.neow3j.devpack.Hash160;import io.neow3j.devpack.Helper;import io.neow3j.devpack.Iterator;import io.neow3j.devpack.Map;import io.neow3j.devpack.Runtime;import io.neow3j.devpack.Storage;import io.neow3j.devpack.StorageContext;import io.neow3j.devpack.StorageMap;import io.neow3j.devpack.StringLiteralHelper;import io.neow3j.devpack.annotations.DisplayName;import io.neow3j.devpack.annotations.ManifestExtra;import io.neow3j.devpack.annotations.OnDeployment;import io.neow3j.devpack.annotations.Permission;import io.neow3j.devpack.annotations.Safe;import io.neow3j.devpack.annotations.SupportedStandard;import io.neow3j.devpack.constants.CallFlags;import io.neow3j.devpack.constants.FindOptions;import io.neow3j.devpack.constants.NativeContract;import io.neow3j.devpack.constants.NeoStandard;import io.neow3j.devpack.contracts.ContractManagement;import io.neow3j.devpack.events.Event3Args;import io.neow3j.devpack.events.Event4Args;

Contract-specific Information#

Annotations on top of the smart contract's class represent contract-specific information. The following annotations are used in the example contract:

@DisplayName

Specifies the contract's name. If this annotation is not present, the class name is used for the contract's name.

@ManifestExtra

Adds the provided key-value pair information in the manifest's extra field. You can also use @ManifestsExtras to gather multiple @ManifestExtra annotations (results in the same as when using single @ManifestExtra annotations).

@SupportedStandard

Sets the supportedStandards field in the manifest. You can use neoStandard = with the enum NeoStandard to use an official standard (see here), or customStandard = with a custom string value.

Permission Specifies, which third-party contracts and methods the smart contract is allowed to call. By default (i.e., if no permission annotation is set), the contract is not allowed to call any contract. Use contract = and methods = to specify, respectively, which contracts and methods are allowed.

For example, if you want to allow transferring NEO tokens from the contract, you can add the annotation @Permission(nativeContract = NativeContract.NeoToken, methods = "transfer").

@DisplayName("FurryFriends")@ManifestExtra(key = "author", value = "AxLabs")@SupportedStandard(neoStandard = NeoStandard.NEP_11)@Permission(nativeContract = NativeContract.ContractManagement)public class NonFungibleToken {

Constants#

You can set a constant value for the contract by using final variables. These values are always loaded when the contract is called and cannot be changed once the contract is deployed. If a final value does not include a method call (e.g., raw types, or a final String value, such as "name"), then these values are inlined during compilation.

note

All contract constants and all methods must be static (since the object-orientation of the JVM is different on the NeoVM).

tip

The contract owner of this example contract is fixed (i.e., it is a final variable). If you intend to provide a way to change such a variable, you should not store it as a final variable. Rather, you would store it as a value in the storage, which provides the possibility to be modified through a method.

// Alice's addressstatic final Hash160 owner = StringLiteralHelper.addressToScriptHash("NM7Aky765FG8NhhwtxjXRx7jEL1cnw7PBP");
static final byte contractMapPrefix = 0;static final byte[] totalSupplyKey = new byte[]{0x00};static final byte[] tokensOfKey = new byte[]{0x01};
static final int registryMapPrefix = 1;static final int ownerOfMapPrefix = 2;static final int balanceMapPrefix = 3;
static final int propNameMapPrefix = 8;static final int propDescriptionMapPrefix = 9;static final int propImageMapPrefix = 10;static final int propTokenURIMapPrefix = 11;
static final String propName = "name";static final String propDescription = "description";static final String propImage = "image";static final String propTokenURI = "tokenURI";

Deploy#

Once a deployment transaction is made (containing the contract and other parameters), the contract data is first stored on the blockchain and then the native contract ContractManagement calls the smart contract's deploy() method. In neow3j, that method is marked with the annotation @OnDeployment.

@OnDeploymentpublic static void deploy(Object data, boolean update) {    if (!update) {        StorageMap contractMap = new StorageMap(Storage.getStorageContext(), contractMapPrefix);        contractMap.put(totalSupplyKey, 0);    }}

Update and Destroy#

In order to update the contract, the following method first checks that the contract owner witnessed the transaction and then the native ContractManagement.update() method is called. When updating a smart contract, you can change the smart contract's code and its manifest. This means that you can update how the contract programmatically manages its storage context.

note

Additionally to changing the smart contract's script and manifest, the method ContractManagement.update() eventually calls the smart contract's deploy() method (shown above) with the boolean update set to true.

public static void update(ByteString script, String manifest) throws Exception {    if (!Runtime.checkWitness(contractOwner())) {        throw new Exception("No authorization");    }    new ContractManagement().update(script, manifest);}

The example contract also provides the option to destroy the smart contract. As well as the update() method, it first verifies that the contract owner witnessed the transaction and then calls the method ContractManagement.destroy() method.

caution

When the native method ContractManagement.destroy() is called from a smart contract, the whole smart contract's storage context is erased, and the contract can no longer be used.

public static void destroy() throws Exception {    if (!Runtime.checkWitness(contractOwner())) {        throw new Exception("No authorization");    }    new ContractManagement().destroy();}

NEP-11 Methods#

The required NEP-11 methods are implemented as follows. If a method does not change the state of the contract (i.e., it is just used for reading), it can be annotated with the @Safe annotation. Out of the required NEP-11 methods, only the transfer() method should be writing to the contract's storage and, thus, is not annotated as safe.

@Safepublic static String symbol() {    return "NEOW";}
@Safepublic static int decimals() {    return 0;}
@Safepublic static int totalSupply() {    return new StorageMap(Storage.getReadOnlyContext(), contractMapPrefix).getInt(totalSupplyKey);}
@Safepublic static int balanceOf(Hash160 owner) throws Exception {    if (!Hash160.isValid(owner)) {        throw new Exception("The parameter 'owner' must be a 20-byte address.");    }    return getBalance(Storage.getReadOnlyContext(), owner);}
@Safepublic static Iterator<ByteString> tokensOf(Hash160 owner) throws Exception {    if (!Hash160.isValid(owner)) {        throw new Exception("The parameter 'owner' must be a 20-byte address.");    }    return (Iterator<ByteString>) Storage.find(Storage.getReadOnlyContext(), createTokensOfPrefix(owner),            (byte) (FindOptions.KeysOnly | FindOptions.RemovePrefix));}
public static boolean transfer(Hash160 to, ByteString tokenId, Object data) throws Exception {    if (!Hash160.isValid(to)) {        throw new Exception("The parameter 'to' must be a 20-byte address.");    }    if (tokenId.length() > 64) {        throw new Exception("The parameter 'tokenId' must be a valid NFT ID (64 or less bytes long).");    }    Hash160 owner = ownerOf(tokenId);    if (!Runtime.checkWitness(owner)) {        return false;    }    onTransfer.fire(owner, to, 1, tokenId);    if (owner != to) {        StorageContext ctx = Storage.getStorageContext();        new StorageMap(ctx, ownerOfMapPrefix).put(tokenId, to.toByteArray());
        new StorageMap(ctx, createTokensOfPrefix(owner)).delete(tokenId);        new StorageMap(ctx, createTokensOfPrefix(to)).put(tokenId, 1);
        decreaseBalanceByOne(ctx, owner);        increaseBalanceByOne(ctx, to);    }    if (new ContractManagement().getContract(to) != null) {        Contract.call(to, "onNEP11Payment", CallFlags.All, new Object[]{owner, 1, tokenId, data});    }    return true;}

Non-divisible NEP-11 Methods#

The NEP-11 standard specifies non-divisible as well as divisible NFT smart contracts. Since this smart contract is indivisible (i.e., its decimals are 0), it is required to implement a specific method ownerOf for it. It returns the script hash of the owner the token with the specified token id.

@Safepublic static Hash160 ownerOf(ByteString tokenId) throws Exception {    if (tokenId.length() > 64) {        throw new Exception("The parameter 'tokenId' must be a valid NFT ID (64 or less bytes long).");    }    ByteString owner = new StorageMap(Storage.getReadOnlyContext(), ownerOfMapPrefix).get(tokenId);    if (owner == null) {        throw new Exception("This token id does not exist.");    }    return new Hash160(owner);}

NEP-11 Optional Methods#

The NEP-11 standard describes two optional methods called tokens() and properties(). Meaning that if methods with these names and parameters are implemented, they must follow the standard. Below you can see the implementation of these two methods. The tokens() method iterates through the registryMap and returns an Iterator based on the key-value pairs that are found in the registry. The properties() method returns a map of the provided token's properties stored in the contract's storage. This includes its name, and if present its description, image, and URI.

@Safepublic static Iterator<Iterator.Struct<ByteString, ByteString>> tokens() {    return (Iterator<Iterator.Struct<ByteString, ByteString>>) new StorageMap(Storage.getReadOnlyContext(),            registryMapPrefix).find(FindOptions.RemovePrefix);}
@Safepublic static Map<String, String> properties(ByteString tokenId) throws Exception {    if (tokenId.length() > 64) {        throw new Exception("The parameter 'tokenId' must be a valid NFT ID (64 or less bytes long).");    }    Map<String, String> p = new Map<>();    StorageContext ctx = Storage.getReadOnlyContext();    ByteString tokenName = new StorageMap(ctx, propNameMapPrefix).get(tokenId);    if (tokenName == null) {        throw new Exception("This token id does not exist.");    }
    p.put(propName, tokenName.toString());    ByteString tokenDescription = new StorageMap(ctx, propDescriptionMapPrefix).get(tokenId);    if (tokenDescription != null) {        p.put(propDescription, tokenDescription.toString());    }    ByteString tokenImage = new StorageMap(ctx, propImageMapPrefix).get(tokenId);    if (tokenImage != null) {        p.put(propImage, tokenImage.toString());    }    ByteString tokenURI = new StorageMap(ctx, propTokenURIMapPrefix).get(tokenId);    if (tokenURI != null) {        p.put(propTokenURI, tokenURI.toString());    }    return p;}

Events#

The NEP-11 standard requires an event Transfer that contains the values from, to, amount, and tokenId. For this, the class Event4Args can be used with the annotation @DisplayName to set the event's name that will be shown in the manifest and notifications when it has been fired.

@DisplayName("Transfer")private static Event4Args<Hash160, Hash160, Integer, ByteString> onTransfer;

An event variable can effectively fire an event by using the fire() method with the corresponding arguments. For example, the Transfer event (represented by the onTransfer variable) should be fired whenever a transfer happens.

onTransfer.fire(owner, to, 1, tokenId);

Custom Methods#

The example contract contains some custom methods, that are not specified in the NEP-11 standard.

The method contractOwner() simply returns the script hash of the contract owner.

The method mint() can be invoked by the contract owner in order to mint new NFT tokens. It stores the tokenId in the registryMap, its properties in the propertiesMap, and its owner in the ownerMap. Further, it increases the owner's balance, and the total supply by 1, before it fires the Transfer event.

The method burn() can be invoked by the owner of a token. It deletes all information about the token and updates the balance and total supply accordingly. If the intent of burning a token need not require the storage to be freed, the token could also just be sent to a burner address.

@Safepublic static Hash160 contractOwner() {    return owner;}
public static void mint(Hash160 to, ByteString tokenId, Map<String, String> properties) throws Exception {    if (!Runtime.checkWitness(contractOwner())) {        throw new Exception("No authorization");    }    StorageContext ctx = Storage.getStorageContext();    StorageMap registryMap = new StorageMap(ctx, registryMapPrefix);    if (registryMap.get(tokenId) != null) {        throw new Exception("This token id already exists.");    }    if (!properties.containsKey(propName)) {        throw new Exception("The properties must contain a value for the key 'name'.");    }    String tokenName = properties.get(propName);    new StorageMap(ctx, propNameMapPrefix).put(tokenId, tokenName);    if (properties.containsKey(propDescription)) {        String description = properties.get(propDescription);        new StorageMap(ctx, propDescriptionMapPrefix).put(tokenId, description);    }    if (properties.containsKey(propImage)) {        String image = properties.get(propImage);        new StorageMap(ctx, propImageMapPrefix).put(tokenId, image);    }    if (properties.containsKey(propTokenURI)) {        String tokenURI = properties.get(propTokenURI);        new StorageMap(ctx, propTokenURIMapPrefix).put(tokenId, tokenURI);    }
    registryMap.put(tokenId, tokenId);    new StorageMap(ctx, ownerOfMapPrefix).put(tokenId, to.toByteArray());    new StorageMap(ctx, createTokensOfPrefix(to)).put(tokenId, 1);
    increaseBalanceByOne(ctx, to);    incrementTotalSupplyByOne(ctx);    onTransfer.fire(null, to, 1, tokenId);}
public static void burn(ByteString tokenId) throws Exception {    Hash160 owner;    try {        owner = ownerOf(tokenId);    } catch (Exception e) {        throw new Exception(e.getMessage());    }    if (!Runtime.checkWitness(owner)) {        throw new Exception("No authorization.");    }
    StorageContext ctx = Storage.getStorageContext();
    new StorageMap(ctx, registryMapPrefix).delete(tokenId);    new StorageMap(ctx, propNameMapPrefix).delete(tokenId);    new StorageMap(ctx, propDescriptionMapPrefix).delete(tokenId);    new StorageMap(ctx, propImageMapPrefix).delete(tokenId);    new StorageMap(ctx, propTokenURIMapPrefix).delete(tokenId);    new StorageMap(ctx, ownerOfMapPrefix).delete(tokenId);
    new StorageMap(ctx, createTokensOfPrefix(owner)).delete(tokenId);    decreaseBalanceByOne(ctx, owner);    decrementTotalSupplyByOne(ctx);    onTransfer.fire(owner, null, 1, tokenId);}

Private Helper Methods#

Private methods can be used to simplify and make the smart contract more readable. The following private methods are used in the NEP-11 example contract.

private static int getBalance(StorageContext ctx, Hash160 owner) {    return new StorageMap(ctx, balanceMapPrefix).getIntOrZero(owner.toByteArray());}
private static void increaseBalanceByOne(StorageContext ctx, Hash160 owner) {    new StorageMap(ctx, balanceMapPrefix).put(owner.toByteArray(), getBalance(ctx, owner) + 1);}
private static void decreaseBalanceByOne(StorageContext ctx, Hash160 owner) {    new StorageMap(ctx, balanceMapPrefix).put(owner.toByteArray(), getBalance(ctx, owner) - 1);}
private static void incrementTotalSupplyByOne(StorageContext ctx) {    StorageMap contractMap = new StorageMap(ctx, contractMapPrefix);    int updatedTotalSupply = contractMap.getInt(totalSupplyKey) + 1;    contractMap.put(totalSupplyKey, updatedTotalSupply);}
private static void decrementTotalSupplyByOne(StorageContext ctx) {    StorageMap contractMap = new StorageMap(ctx, contractMapPrefix);    int updatedTotalSupply = contractMap.getInt(totalSupplyKey) - 1;    contractMap.put(totalSupplyKey, updatedTotalSupply);}
private static byte[] createTokensOfPrefix(Hash160 owner) {    return Helper.concat(tokensOfKey, owner.toByteArray());}

5. Compile the Contract#

The contract can be compiled using the gradle plugin. First, set the className in the file gradle.build to the contract's class name. Then, the gradle task neow3jCompile can be executed from the project's root path to compile the contract.

./gradlew neow3jCompile

The output is then accessible in the folder ./build/neow3j, and should contain the following three files:

FurryFriends.manifest.jsonFurryFriends.nefFurryFriends.nefdbgnfo
note

The filenames can deviate according to what the contract's name is. See here.

Now, the contract's .manifest.json and .nef files can be used to deploy the contract. Neow3j's SDK can be used to do so. Check out the example here about how to deploy a contract with its manifest and nef files.

About#

Feel free to report any issues that might arise. Open an issue here to help us directly including it in our backlog.