flink - Working with State example

[Ramanan, Buvana (Nokia - US)]

Hello,

I am utilizing the code snippet in: 
https://ci.apache.org/projects/flink/flink-docs-master/apis/streaming/state.html
 and particularly ‘open’ function in my code:
@Override
    public void open(Configuration config) {
        ValueStateDescriptor<Tuple2<Long, Long>> descriptor =
                new ValueStateDescriptor<>(
                        "average", // the state name
                        TypeInformation.of(new TypeHint<Tuple2<Long, Long>>() 
{}), // type information
                        Tuple2.of(0L, 0L)); // default value of the state, if 
nothing was set
        sum = getRuntimeContext().getState(descriptor);
    }

When I run, I get the following error:
Caused by: java.lang.RuntimeException: Error while getting state
               at 
org.apache.flink.streaming.api.operators.StreamingRuntimeContext.getState(StreamingRuntimeContext.java:120)
               at wikiedits.stateful$Rec2Tuple2.open(stateful.java:103)
               at 
org.apache.flink.api.common.functions.util.FunctionUtils.openFunction(FunctionUtils.java:38)
               at 
org.apache.flink.streaming.api.operators.AbstractUdfStreamOperator.open(AbstractUdfStreamOperator.java:91)
               at 
org.apache.flink.streaming.api.operators.StreamFlatMap.open(StreamFlatMap.java:41)
               at 
org.apache.flink.streaming.runtime.tasks.StreamTask.openAllOperators(StreamTask.java:314)
               at 
org.apache.flink.streaming.runtime.tasks.StreamTask.invoke(StreamTask.java:214)
               at org.apache.flink.runtime.taskmanager.Task.run(Task.java:559)
               at java.lang.Thread.run(Thread.java:745)
Caused by: java.lang.Exception: State key serializer has not been configured in 
the config. This operation cannot use partitioned state.
               at 
org.apache.flink.runtime.state.AbstractStateBackend.getPartitionedState(AbstractStateBackend.java:199)
               at 
org.apache.flink.streaming.api.operators.AbstractStreamOperator.getPartitionedState(AbstractStreamOperator.java:260)
               at 
org.apache.flink.streaming.api.operators.StreamingRuntimeContext.getState(StreamingRuntimeContext.java:118)
               ... 8 more

Where do I define the key & value serializer for state?

Thanks,
Buvana