Add CheckStellarAssetContract example

[lijamie98]

Description

• The CheckStellarAssetContract example shows how to check the existence of the SAC (Stellar Asset Contract) of a class
• asset.

Context

• Add example to help developer experience.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.18%. Comparing base (42733cc) to head (cea7361).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #688   +/-   ##
=========================================
  Coverage     80.18%   80.18%           
  Complexity     1227     1227           
=========================================
  Files           213      213           
  Lines          4820     4820           
  Branches        412      412           
=========================================
  Hits           3865     3865           
  Misses          703      703           
  Partials        252      252           

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[overcat]

The return here is a base64 character; returning the C... address would be better.

[overcat]

Suggested change

	+ assetUSDC
	+ assetUnknown

[lijamie98]

good catch!

[overcat]

Would it be better to change this to boolean isContractDeployed(SorobanServer sorobanServer, String contractId)?

public class CheckStellarAssetContract {
  public static void main(String[] args) throws IOException {
    String rpcServerUrl = "https://soroban-testnet.stellar.org:443";
    Network network = Network.TESTNET;
    Asset assetNative = Asset.createNativeAsset();
    Asset assetUSDC =
        Asset.createNonNativeAsset(
            "USDC", "GDQOE23CFSUMSVQK4Y5JHPPYK73VYCNHZHA7ENKCV37P6SUEO6XQBKPP");
    Asset assetUnknown =
        Asset.createNonNativeAsset(
            "UNKN", "GDQOE23CFSUMSVQK4Y5JHPPYK73VYCNHZHA7ENKCV37P6SUEO6XQBKPP");

    try (SorobanServer sorobanServer = new SorobanServer(rpcServerUrl)) {
      for (Asset asset : List.of(assetNative, assetUSDC, assetUnknown)) {
        String contractId = asset.getContractId(network);
        System.out.printf(
            "The contract id for %s is %s, it is %s\n",
            asset,
            contractId,
            isContractDeployed(sorobanServer, contractId) ? "deployed" : "not deployed");
      }
    }
  }

  public static boolean isContractDeployed(SorobanServer sorobanServer, String contractId) {
    List<LedgerKey> ledgerKeys =
        List.of(
            LedgerKey.builder()
                .discriminant(LedgerEntryType.CONTRACT_DATA)
                .contractData(
                    LedgerKeyContractData.builder()
                        .contract(new Address(contractId).toSCAddress())
                        .key(Scv.toLedgerKeyContractInstance())
                        .durability(ContractDataDurability.PERSISTENT)
                        .build())
                .build());
    GetLedgerEntriesResponse response = sorobanServer.getLedgerEntries(ledgerKeys);
    return response.getEntries() != null && !response.getEntries().isEmpty();
  }
}

[lijamie98]

sure