@Override
      public void init(Context context) {
        String leftStoreName = joinOpSpec.getLeftOpId();
        leftStreamState =
            (KeyValueStore<Object, TimestampedValue<Object>>) context.getTaskContext().getStore(leftStoreName);

        // user-defined joinFn should only be initialized once, so we do it only in left partial join function.
        joinFn.init(context);
      }
Creates a new {@link OperatorImpl} instance for the provided {@link OperatorSpec}.

@param prevOperatorSpec the original {@link OperatorSpec} that produces output for {@code operatorSpec} from {@link OperatorSpecGraph}
@param operatorSpec  the original {@link OperatorSpec} from {@link OperatorSpecGraph}
@param context  the {@link Context} required to instantiate operators
@return  the {@link OperatorImpl} implementation instance
samza-core/src/main/java/org/apache/samza/operators/impl/OperatorImplGraph.java