This demo page displays a simple Hexbin layer example with using gwt-leaflet-d3 library which lets you use Java to deal with LeafletJs, d3.js and Leaflet D3 extension. For more details please take a look at the Documentation.

perm_data_settingGenerate new data

<head>
    <!-- Leaflet Javascript and Styles - a base map library stuff -->
    <script src="./leaflet/leaflet.js" type="text/javascript" language="javascript"></script>
    <link href="./leaflet/leaflet.css" rel="stylesheet">

    <!-- Simple styles for this demo's map -->
    <style>
        #map {
            height: 280px;
            width: 100%;
            max-width: 660px;
            margin-top: 7%;
        }

        .hexbin-hexagon {
            stroke: #000;
            stroke-width: 1px;
        }
    </style>
</head>
<body>
    <div id="map"></div>
</body>

@Override
public void onModuleLoad() {
    // the remaining D3 related JS files are injected to HTML document from here
    LeafletD3.init(() -> {
        createAndAttachMap();
    });
}

private void createAndAttachMap() {
        MapOptions mapOptions = new MapOptions.Builder(
            L.latLng(52.233333, 21.016667), 6.0, 5.0).dragging(true).build();

    final com.gwidgets.api.leaflet.Map map = L.map("map", mapOptions);

    TileLayerOptions tileLayerOptions =
            new TileLayerOptions.Builder().minZoom(3).maxZoom(8).build();

    L.tileLayer(MAP_URL, tileLayerOptions).addTo(map);

    HexbinLayer<Coordinate> hexbinLayer = new HexbinLayer<>(
            HexbinLayer.Config.create()
                    .withDuration(500d).withOpacity(0.5).withRadius(12d).build());

    hexbinLayer
            .withColorRange(new String[]{"white", "orange", "red"})
            .withColorValue(value -> value.length)
            .withRadiusValue(value -> value.length)
            .withLng(value -> value.longitude)
            .withLat(value -> value.latitude)
            .withData(getCoordinates()); // a data have to be set as the last

    hexbinLayer.addTo(map);
}

For complete code please see here.