# Use an Android SDK generated by API Gateway for a REST API


In this section, we will outline the steps to use an Android SDK generated by API Gateway for a REST API. Before proceeding further, you must have already completed the steps in [Generate SDKs for REST APIs in API Gateway](how-to-generate-sdk.md).

**Note**  
 The generated SDK is not compatible with Android 4.4 and earlier. For more information, see [Amazon API Gateway important notes](api-gateway-known-issues.md). 

**To install and use an Android SDK generated by API Gateway**

1. Extract the contents of the API Gateway-generated .zip file that you downloaded earlier.

1. Download and install [Apache Maven](https://maven.apache.org/) (preferably version 3.x).

1. Download and install [JDK 8](https://docs.oracle.com/javase/8/docs/technotes/guides/install/install_overview.html).

1. Set the `JAVA_HOME` environment variable.

1. Run the **mvn install** command to install the compiled artifact files to your local Maven repository. This creates a `target` folder containing the compiled SDK library.

1. Copy the SDK file (the name of which is derived from the **Artifact Id** and **Artifact Version** you specified when generating the SDK, e.g., `simple-calcsdk-1.0.0.jar`) from the `target` folder, along with all of the other libraries from the `target/lib` folder, into your project's `lib` folder.

   If you use Android Studio, create a `libs` folder under your client app module and copy the required .jar file into this folder. Verify that the dependencies section in the module's gradle file contains the following.

   ```
       compile fileTree(include: ['*.jar'], dir: 'libs')
       compile fileTree(include: ['*.jar'], dir: 'app/libs')
   ```

   Make sure no duplicated .jar files are declared.

1. Use the `ApiClientFactory` class to initialize the API Gateway-generated SDK. For example:

   ```
   ApiClientFactory factory = new ApiClientFactory();
   
   // Create an instance of your SDK. Here, 'SimpleCalcClient.java' is the compiled java class for the SDK generated by API Gateway. 
   final SimpleCalcClient client = factory.build(SimpleCalcClient.class);
   
   // Invoke a method: 
   //   For the 'GET /?a=1&b=2&op=+' method exposed by the API, you can invoke it by calling the following SDK method:
   
   Result output = client.rootGet("1", "2", "+");
   
   //     where the Result class of the SDK corresponds to the Result model of the API.
   //
   
   //   For the 'GET /{a}/{b}/{op}'  method exposed by the API, you can call the following SDK method to invoke the request,
   
   Result output = client.aBOpGet(a, b, c);
   
   //     where a, b, c can be "1", "2", "add", respectively.
   
   //   For the following API method:
   //        POST /
   //        host: ...
   //        Content-Type: application/json
   //    
   //        { "a": 1, "b": 2, "op": "+" }
   // you can call invoke it by calling the rootPost method of the SDK as follows:
   Input body = new Input();
   input.a=1;
   input.b=2;
   input.op="+";
   Result output = client.rootPost(body);
   
   //      where the Input class of the SDK corresponds to the Input model of the API.
   
   // Parse the result:
   //     If the 'Result' object is { "a": 1, "b": 2, "op": "add", "c":3"}, you retrieve the result 'c') as 
   
   String result=output.c;
   ```

1. To use an Amazon Cognito credentials provider to authorize calls to your API, use the `ApiClientFactory` class to pass a set of AWS credentials by using the SDK generated by API Gateway, as shown in the following example.

   ```
   // Use CognitoCachingCredentialsProvider to provide AWS credentials
   // for the ApiClientFactory
   AWSCredentialsProvider credentialsProvider = new CognitoCachingCredentialsProvider(
           context,          // activity context
           "identityPoolId", // Cognito identity pool id
           Regions.US_EAST_1 // region of Cognito identity pool
   );
   
   ApiClientFactory factory = new ApiClientFactory()
     .credentialsProvider(credentialsProvider);
   ```

1. To set an API key by using the API Gateway- generated SDK, use code similar to the following.

   ```
   ApiClientFactory factory = new ApiClientFactory()
     .apiKey("YOUR_API_KEY");
   ```