流计算中的窗口操作是什么？请解释其作用和使用场景。

import org.apache.flink.api.common.functions.AggregateFunction;
import org.apache.flink.api.java.tuple.Tuple2;
import org.apache.flink.streaming.api.datastream.DataStream;
import org.apache.flink.streaming.api.environment.StreamExecutionEnvironment;
import org.apache.flink.streaming.api.windowing.assigners.TumblingProcessingTimeWindows;
import org.apache.flink.streaming.api.windowing.time.Time;
import org.apache.flink.streaming.api.windowing.windows.TimeWindow;

public class WindowOperationExample {
    public static void main(String[] args) throws Exception {
        // 创建执行环境
        StreamExecutionEnvironment env = StreamExecutionEnvironment.getExecutionEnvironment();

        // 创建数据流
        DataStream<Tuple2<String, Integer>> stream = env.fromElements(
                new Tuple2<>("item1", 10),
                new Tuple2<>("item2", 5),
                new Tuple2<>("item1", 20),
                new Tuple2<>("item2", 15),
                new Tuple2<>("item1", 30),
                new Tuple2<>("item2", 25)
        );

        // 定义窗口操作
        DataStream<Tuple2<String, Integer>> result = stream
                .keyBy(0)
                .window(TumblingProcessingTimeWindows.of(Time.seconds(5)))
                .aggregate(new SumAggregate());

        // 打印结果
        result.print();

        // 执行任务
        env.execute("Window Operation Example");
    }

    // 自定义聚合函数
    public static class SumAggregate implements AggregateFunction<Tuple2<String, Integer>, Tuple2<String, Integer>, Tuple2<String, Integer>> {
        @Override
        public Tuple2<String, Integer> createAccumulator() {
            return new Tuple2<>("", 0);
        }

        @Override
        public Tuple2<String, Integer> add(Tuple2<String, Integer> value, Tuple2<String, Integer> accumulator) {
            return new Tuple2<>(value.f0, accumulator.f1 + value.f1);
        }

        @Override
        public Tuple2<String, Integer> getResult(Tuple2<String, Integer> accumulator) {
            return accumulator;
        }

        @Override
        public Tuple2<String, Integer> merge(Tuple2<String, Integer> a, Tuple2<String, Integer> b) {
            return new Tuple2<>(a.f0, a.f1 + b.f1);
        }
    }
}