How to write a junit Test Case of a function which is reading a file?

Here is my code :

 public String getFile() throws IOException {
    StringBuilder fileURI = new StringBuilder(locationName);
    fileURI.append(keyName);
    S3ObjectInputStream ts = null;
    ts = (S3ObjectInputStream) s3Services.getResourceStream(fileURI.toString());
    String time;
    try (BufferedReader reader = new BufferedReader(new InputStreamReader(ts))) {
        time = reader.readLine();
    }
    return time;
}

I am trying to mock this function, I am not sure how to Mock the InputStreamReader object :

 public void testgetTs() throws IOException {
    Mockito.mock(Service.class);
    StringBuilder fileURI = new StringBuilder("test");
    fileURI.append("/");
    fileURI.append("testFile.txt");
    S3ObjectInputStream ts = null;
    Mockito.when(s3Services.getResourceStream(fileURI.toString())).thenReturn(ts);
    service.getTime();

}

You need to mock out any external resources, in your case that's hard because you are creating all your resources internally and therefore your method does not have a single responsibility, but many. To properly unit test you would need to break it into units, inject parameters and mock out resources:

Unit 1, Building the file name

StringBuilder fileURI = new StringBuilder(locationName);
fileURI.append(keyName);

Here you want to be able to test what the result of setting locationName and keyName to different things is. i.e. something like FileUriBuilder(locationName, keyName).getString()

Unit 2, Getting the resource stream

S3ObjectInputStream ts = null;
ts = (S3ObjectInputStream) s3Services.getResourceStream(fileURI.toString());

Here you want to mock out s3Services and test that getResourceStream() is called with the expected value. If you are injecting FileUriBuilder you'll want to mock that.

Unit 3, Reading the stream

String time;
try (BufferedReader reader = new BufferedReader(new InputStreamReader(ts))) {
    time = reader.readLine();
}
return time;

Here you want to mock out S3ObjectInputStream and make sure the correct methods on it are called and that when it returns something that this unit reports it as expected.