feat: add support for /accounts end-point with ?signer and ?asset f…

…ilters.

src/main/java/org/stellar/sdk/requests/AccountsRequestBuilder.java

@@ -5,6 +5,7 @@
 import okhttp3.OkHttpClient;
 import okhttp3.Request;
 import okhttp3.Response;
+import org.stellar.sdk.Asset;
 import org.stellar.sdk.responses.AccountResponse;
 import org.stellar.sdk.responses.Page;
 
@@ -14,6 +15,9 @@
  * Builds requests connected to accounts.
  */
 public class AccountsRequestBuilder extends RequestBuilder {
+  private static final String ASSET_PARAMETER_NAME = "asset";
+  private static final String SIGNER_PARAMETER_NAME = "signer";
+
   public AccountsRequestBuilder(OkHttpClient httpClient, HttpUrl serverURI) {
     super(httpClient, serverURI, "accounts");
   }
@@ -44,6 +48,36 @@ public AccountResponse account(String account) throws IOException {
     return this.account(this.buildUri());
   }
 
+  /**
+   * Returns all accounts that contain a specific signer.
+   *
+   * @param signer Account ID
+   * @return current {@link AccountsRequestBuilder} instance
+   * @see <a href="https://www.stellar.org/developers/horizon/reference/endpoints/accounts.html">Accounts</a>
+   */
+  public AccountsRequestBuilder forSigner(String signer) {
+    if (uriBuilder.build().queryParameter(ASSET_PARAMETER_NAME) != null) {
+      throw new RuntimeException("cannot set both signer and asset");
+    }
+    uriBuilder.setQueryParameter(SIGNER_PARAMETER_NAME, signer);
+    return this;
+  }
+
+  /**
+   * Returns all accounts who are trustees to a specific asset.
+   *
+   * @param asset An issued asset
+   * @return current {@link AccountsRequestBuilder} instance
+   * @see <a href="https://www.stellar.org/developers/horizon/reference/endpoints/accounts.html">Accounts</a>
+   */
+  public AccountsRequestBuilder forAsset(Asset asset) {
+    if (uriBuilder.build().queryParameter(SIGNER_PARAMETER_NAME) != null) {
+      throw new RuntimeException("cannot set both signer and asset");
+    }
+    setAssetParameter(ASSET_PARAMETER_NAME, asset);
+    return this;
+  }
+
   /**
    * Requests specific <code>uri</code> and returns {@link Page} of {@link AccountResponse}.
    * This method is helpful for getting the next set of results.

src/main/java/org/stellar/sdk/requests/RequestBuilder.java

@@ -79,25 +79,41 @@ public RequestBuilder order(Order direction) {
 
   /**
    * Sets a parameter consisting of a comma separated list of assets on the request.
-   * @param name the name of the query parameter
+   *
+   * @param name   the name of the query parameter
    * @param assets the list of assets to be serialized into the query parameter value
    */
   public RequestBuilder setAssetsParameter(String name, List<Asset> assets) {
     List<String> assetStrings = Lists.newArrayList();
     for (Asset asset : assets) {
-      if (asset instanceof AssetTypeNative) {
-        assetStrings.add("native");
-      } else if (asset instanceof AssetTypeCreditAlphaNum) {
-        AssetTypeCreditAlphaNum creditAsset = (AssetTypeCreditAlphaNum) asset;
-        assetStrings.add(creditAsset.getCode()+":"+creditAsset.getIssuer());
-      } else {
-        throw new RuntimeException("unsupported asset "+ asset.getType());
-      }
+      assetStrings.add(buildAssetParameter(asset));
     }
     uriBuilder.setQueryParameter(name, Joiner.on(",").join(assetStrings));
     return this;
   }
 
+  /**
+   * Sets a parameter consisting of an asset represented as "assetCode:assetIssue" on the request.
+   *
+   * @param name  the name of the query parameter
+   * @param asset the asset to be serialized into the query parameter value
+   */
+  public RequestBuilder setAssetParameter(String name, Asset asset) {
+    uriBuilder.setQueryParameter(name, buildAssetParameter(asset));
+    return this;
+  }
+
+  private String buildAssetParameter(Asset asset) {
+    if (asset instanceof AssetTypeNative) {
+      return "native";
+    } else if (asset instanceof AssetTypeCreditAlphaNum) {
+      AssetTypeCreditAlphaNum creditAsset = (AssetTypeCreditAlphaNum) asset;
+      return creditAsset.getCode() + ":" + creditAsset.getIssuer();
+    } else {
+      throw new RuntimeException("unsupported asset " + asset.getType());
+    }
+  }
+
   HttpUrl buildUri() {
     if (segments.size() > 0) {
       for (String segment : segments) {

src/test/java/org/stellar/sdk/requests/AccountsRequestBuilderTest.java

@@ -2,7 +2,8 @@
 
 import okhttp3.HttpUrl;
 import org.junit.Test;
-import org.stellar.sdk.Network;
+import org.stellar.sdk.Asset;
+import org.stellar.sdk.AssetTypeCreditAlphaNum4;
 import org.stellar.sdk.Server;
 
 import static org.junit.Assert.assertEquals;
@@ -18,4 +19,23 @@ public void testAccounts() {
             .buildUri();
     assertEquals("https://horizon-testnet.stellar.org/accounts?cursor=13537736921089&limit=200&order=asc", uri.toString());
   }
+
+  @Test
+  public void testForSigner() {
+    Server server = new Server("https://horizon-testnet.stellar.org");
+    HttpUrl uri = server.accounts()
+            .forSigner("GDSBCQO34HWPGUGQSP3QBFEXVTSR2PW46UIGTHVWGWJGQKH3AFNHXHXN")
+            .buildUri();
+    assertEquals("https://horizon-testnet.stellar.org/accounts?signer=GDSBCQO34HWPGUGQSP3QBFEXVTSR2PW46UIGTHVWGWJGQKH3AFNHXHXN", uri.toString());
+  }
+
+  @Test
+  public void testForAsset() {
+    Server server = new Server("https://horizon-testnet.stellar.org");
+    Asset asset = new AssetTypeCreditAlphaNum4("USD", "GDVDKQFP665JAO7A2LSHNLQIUNYNAAIGJ6FYJVMG4DT3YJQQJSRBLQDG");
+    HttpUrl uri = server.accounts()
+            .forAsset(asset)
+            .buildUri();
+    assertEquals("https://horizon-testnet.stellar.org/accounts?asset=USD%3AGDVDKQFP665JAO7A2LSHNLQIUNYNAAIGJ6FYJVMG4DT3YJQQJSRBLQDG", uri.toString());
+  }
 }