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# Custom Java Aggregate Function Sample

## Introduction

The `nth.sbapp` sample application demonstrates a custom Java aggregate function, `calljava`, that performs an aggregation on three tuples. The `calljava` function for aggregate expressions was built by extending the `AggregateWindow` class, which is documented in the StreamBase Java Client library.

In the application, each input tuple contains a single integer field. The Aggregate operator has a window size of 3, so it It waits until three tuples are input before emitting a tuple. The output tuple contains four fields, each generated by an expression in the Aggregate operator:

- firstval(val)

- calljava('com.streambase.sb.sample.Nth', 2, val)

- calljava('com.streambase.sb.sample.Nth', 3, val)

- calljava('com.streambase.sb.sample.Nth', 4, val)

In the first expression, the built-in function `firstval` is used to return the value in the first tuple.

In the remaining expressions `calljava` is used to do the same thing: it returns the value of a specified tuple.

Notice that the last expression references a fourth tuple. However, only three tuples should be in the window when it closes, because the aggregate window size is 3. This reference to a non-existent tuple is a deliberate mistake — read on and run the sample to find out the result.

## Importing This Sample into StreamBase Studio

In StreamBase Studio, import this sample with the following steps:

- From the top-level menu, click File → Load StreamBase Sample.

- Enter `custom` to narrow the list of options.

- Select the Custom Java Aggregate Function sample from the Extending StreamBase list.

- Click OK.

StreamBase Studio creates a project for the sample.

## Running This Sample in StreamBase Studio

1. In the Project Explorer view, double-click to open the `nth.sbapp` application. Make sure the application is the currently active tab in the EventFlow Editor.

2. Click the Run button. This opens the SB Test/Debug perspective and starts the module.

3. When the server starts, StreamBase Studio switches to the SB Test/Debug perspective.

4. On the Manual Input view, enter the following values in the val field, pressing Send Data after each entry:

   `1
   2
   3`

5. Observe the tuples in the Output Streams view. No tuples are output until three input tuples are sent, because the aggregate's window size is set to 3. The output you should observe is:

   `first=1.0, second=2.0, third=3.0, fourth=null`

   These values are returned by the `calljava` functions for each field. Notice that the fourth field is always null because there are only three tuples in the window.

6. When done, press F9 or click the Terminate EventFlow Fragment button.

## This Sample's Files

The sample has the following files:

- The source code for the function, `Nth.java`

- A sample configuration file, `sbd.sbconf`, which tells StreamBase Server to load the custom simple function.

- A sample application, `nth.sbapp`, which uses the function.

## Sample Location

When you load the sample into StreamBase Studio, Studio copies the sample project's files to your Studio workspace, which is normally part of your home directory, with full access rights.

### Important

Load this sample in StreamBase Studio, and thereafter use the Studio workspace copy of the sample to run and test it, even when running from the command prompt.

Using the workspace copy of the sample avoids permission problems. The default workspace location for this sample is:

    studio-workspace/sample_custom-java-aggregate

See Default Installation Directories for the default location of `studio-workspace` on your system.