An example Java application demonstrating National Grid's carbon intensity API:
http://www.v2g.co.uk/projects/national-grid-carbon-intensity-api/
If you have Java and Maven installed then simply:
cd CarbonIntensity
mvn install
mvn exec:java
Failing that install the Java SDK and manually download the dependencies:
gson 2.8.2
jfreechart 1.0.13
jcommon 1.0.16