[CHA-4634] feat: add getChannel API

DOCS.md

@@ -367,6 +367,17 @@ User.list()
             FilterCondition.contains("teams", "red"))));
 ```
 
+**Get channel (type,id)**
+
+Returns the channel without creating it. A channel that does not exist yields a 404, so this also works as an existence check.
+
+```java
+Channel.getChannel("messaging", "travel")
+    .state(true)
+    .messagesLimit(20)
+    .request();
+```
+
 **Get or create channel (type,id)**
 
 Standard

src/main/java/io/getstream/chat/java/models/Channel.java

@@ -17,6 +17,7 @@
 import io.getstream.chat.java.models.Channel.ChannelTruncateRequestData.ChannelTruncateRequest;
 import io.getstream.chat.java.models.Channel.ChannelUnMuteRequestData.ChannelUnMuteRequest;
 import io.getstream.chat.java.models.Channel.ChannelUpdateRequestData.ChannelUpdateRequest;
+import io.getstream.chat.java.models.Channel.GetChannelRequestData.GetChannelRequest;
 import io.getstream.chat.java.models.Channel.MarkDeliveredRequestData.MarkDeliveredRequest;
 import io.getstream.chat.java.models.ChannelType.BlocklistBehavior;
 import io.getstream.chat.java.models.ChannelType.ChannelTypeWithCommands;
@@ -551,6 +552,48 @@ protected Call<ChannelGetResponse> generateCall(Client client) {
     }
   }
 
+  @Builder(
+      builderClassName = "GetChannelRequest",
+      builderMethodName = "",
+      buildMethodName = "internalBuild")
+  @Getter
+  @EqualsAndHashCode(callSuper = false)
+  public static class GetChannelRequestData {
+    @Nullable
+    @JsonProperty("state")
+    private Boolean state;
+
+    @Nullable
+    @JsonProperty("messages_limit")
+    private Integer messagesLimit;
+
+    @Nullable
+    @JsonProperty("members_limit")
+    private Integer membersLimit;
+
+    @Nullable
+    @JsonProperty("watchers_limit")
+    private Integer watchersLimit;
+
+    public static class GetChannelRequest extends StreamRequest<ChannelGetResponse> {
+      @NotNull private String channelType;
+
+      @NotNull private String channelId;
+
+      private GetChannelRequest(@NotNull String channelType, @NotNull String channelId) {
+        this.channelType = channelType;
+        this.channelId = channelId;
+      }
+
+      @Override
+      protected Call<ChannelGetResponse> generateCall(Client client) {
+        return client
+            .create(ChannelService.class)
+            .getChannel(this.channelType, this.channelId, this.internalBuild());
+      }
+    }
+  }
+
   @Builder(
       builderClassName = "ChannelUpdateRequest",
       builderMethodName = "",
@@ -1643,6 +1686,19 @@ public static ChannelGetRequest getOrCreate(@NotNull String type) {
     return new ChannelGetRequest(type, null);
   }
 
+  /**
+   * Creates a get channel request. The channel is never created: a missing, deleted or disabled
+   * channel yields a 404, so this request doubles as an existence check.
+   *
+   * @param type the channel type
+   * @param id the channel id
+   * @return the created request
+   */
+  @NotNull
+  public static GetChannelRequest getChannel(@NotNull String type, @NotNull String id) {
+    return new GetChannelRequest(type, id);
+  }
+
   /**
    * Creates an update request
    *

src/main/java/io/getstream/chat/java/services/ChannelService.java

@@ -27,6 +27,14 @@ Call<ChannelGetResponse> getOrCreateWithoutId(
       @NotNull @Path("type") String channelType,
       @Nullable @Body ChannelGetRequestData channelGetRequestData);
 
+  // The endpoint reads its options from a JSON `payload` query parameter and fails with 400 without
+  // one, so the request data goes there instead of into flat query parameters.
+  @GET("channels/{type}/{id}")
+  Call<ChannelGetResponse> getChannel(
+      @NotNull @Path("type") String channelType,
+      @NotNull @Path("id") String channelId,
+      @NotNull @ToJson @Query("payload") GetChannelRequestData getChannelRequestData);
+
   @DELETE("channels/{type}/{id}")
   Call<ChannelDeleteResponse> delete(
       @NotNull @Path("type") String channelType, @NotNull @Path("id") String channelId);

src/test/java/io/getstream/chat/java/ChannelTest.java

@@ -1,5 +1,6 @@
 package io.getstream.chat.java;
 
+import io.getstream.chat.java.exceptions.StreamException;
 import io.getstream.chat.java.models.Channel;
 import io.getstream.chat.java.models.Channel.*;
 import io.getstream.chat.java.models.DeleteStrategy;
@@ -37,6 +38,31 @@ void whenRetrievingChannel_thenNoException() {
                 .request());
   }
 
+  @DisplayName("Can get an existing channel")
+  @Test
+  void whenGettingExistingChannel_thenReturnsChannel() {
+    var response =
+        Assertions.assertDoesNotThrow(
+            () ->
+                Channel.getChannel(testChannel.getType(), testChannel.getId())
+                    .state(true)
+                    .request());
+    Assertions.assertNotNull(response.getChannel());
+    Assertions.assertEquals(testChannel.getId(), response.getChannel().getId());
+  }
+
+  @DisplayName("Get channel does not create a missing channel")
+  @Test
+  void whenGettingMissingChannel_thenNotFound() {
+    var exception =
+        Assertions.assertThrows(
+            StreamException.class,
+            () ->
+                Channel.getChannel(testChannel.getType(), RandomStringUtils.randomAlphabetic(12))
+                    .request());
+    Assertions.assertEquals(404, exception.getResponseData().getStatusCode());
+  }
+
   @DisplayName("Can create channel with invites")
   @Test
   void whenCreatingChannelWithInvites_thenNoException() {

src/test/java/io/getstream/chat/java/GetChannelTest.java

@@ -0,0 +1,47 @@
+package io.getstream.chat.java;
+
+import io.getstream.chat.java.models.Channel;
+import io.getstream.chat.java.services.ChannelService;
+import io.getstream.chat.java.services.framework.DefaultClient;
+import java.util.Properties;
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.DisplayName;
+import org.junit.jupiter.api.Test;
+
+public class GetChannelTest {
+
+  @Test
+  @DisplayName("Get channel sends a GET with its options in the payload query parameter")
+  void whenBuildingGetChannelRequest_thenOptionsGoIntoPayloadQueryParameter() {
+    var data = Channel.getChannel("messaging", "abc").state(true).messagesLimit(5).internalBuild();
+
+    var request =
+        client().create(ChannelService.class).getChannel("messaging", "abc", data).request();
+
+    Assertions.assertEquals("GET", request.method());
+    Assertions.assertEquals("/channels/messaging/abc", request.url().encodedPath());
+
+    var payload = request.url().queryParameter("payload");
+    Assertions.assertNotNull(payload);
+    Assertions.assertTrue(payload.contains("\"state\":true"), payload);
+    Assertions.assertTrue(payload.contains("\"messages_limit\":5"), payload);
+  }
+
+  @Test
+  @DisplayName("Get channel sends a payload query parameter even without options")
+  void whenBuildingGetChannelRequestWithoutOptions_thenPayloadQueryParameterIsStillSent() {
+    var data = Channel.getChannel("messaging", "abc").internalBuild();
+
+    var request =
+        client().create(ChannelService.class).getChannel("messaging", "abc", data).request();
+
+    Assertions.assertNotNull(request.url().queryParameter("payload"));
+  }
+
+  private DefaultClient client() {
+    var properties = new Properties();
+    properties.put(DefaultClient.API_KEY_PROP_NAME, "test-key");
+    properties.put(DefaultClient.API_SECRET_PROP_NAME, "test-secret");
+    return new DefaultClient(properties);
+  }
+}