1 重点
1.1 对以下方法的应用（这是本人对留的方法的理解，有不对的地方，欢迎大家指正）
Stream.peek: 对流的操作，不操作流，不执行
Returns a stream consisting of the elements of this stream, truncated to be no longer than {@code maxSize} in length.
<p>This is a <a href="package-summary.html#StreamOps">short-circuiting stateful intermediate operation</a>.
Collection.stream 建立流
Returns a sequential
Stream.filter 筛选出结构一致的集合
Returns a stream consisting of the results of applying the given function to the elements of this stream.
Stream.limit（n） 取出前n条
Stream.map 返回新的流（结构可能不一致）
@return the new stream
Stream.collect 返回可操作的集合
@return an {@code Optional} describing the minimum element of this stream, * or an empty {@code Optional} if the stream is empty
2 代码演练
需求：
根据第一章需求，女盆友提出需求
* 1 打印所有商品
* 2 图书类的商品一定给买
* 3 最贵的买两件
* 4 打印最贵的两件商品的名称和总价
测试类：
package com.imooc.zhangxiaoxi.stream; import com.alibaba.fastjson.JSON; import com.imooc.zhangxiaoxi.lambda.cart.CartService; import com.imooc.zhangxiaoxi.lambda.cart.Sku; import com.imooc.zhangxiaoxi.lambda.cart.SkuCategoryEnum; import org.junit.Test; import java.util.ArrayList; import java.util.Arrays; import java.util.Comparator; import java.util.List; import java.util.concurrent.atomic.AtomicReference; import java.util.stream.Collectors; /** * StreamVs * * @author 魏豆豆 * @date 2020/4/7 */ public class StreamVs { /** * 流和普通方法对比，本节应用普通方法 * * 根据第一章需求，女盆友提出需求 * 1 打印所有商品 * 2 图书类的商品一定给买 * 3 最贵的买两件 * 4 打印最贵的两件商品的名称和总价 */ /** * 普通方法 */ /* @Test public void oldCartHandle(){ //1 打印所有商品 List<Sku> skuList = CartService.getSkuList(); System.out.println("===================================================================================================="); for(Sku sku: skuList){ System.out.println(JSON.toJSONString(sku,true)); } //2 排除图书类 List<Sku> skuListNoBooks = new ArrayList<Sku>(); for(Sku sku:skuList){ if(!sku.getSkuCategory().equals(SkuCategoryEnum.BOOKS)){ skuListNoBooks.add(sku); } } //3 最贵的买两件 skuListNoBooks.sort(new Comparator<Sku>() { @Override public int compare(Sku o1, Sku o2) { if(o1.getTotalPrice()> o2.getTotalPrice()){ return -1; }else if(o1.getTotalPrice() < o2.getTotalPrice()){ return 1; }else{ return 0; } } }); List<Sku> skuListPreTwo = new ArrayList<Sku>(); for(int i = 0;i<2;i++){ skuListPreTwo.add(skuListNoBooks.get(i)); } System.out.println("===================================================================================================="); Double totalPrice = 0.0; for(Sku sku:skuListPreTwo){ totalPrice+=sku.getTotalPrice(); } List<String> skuNameList = new ArrayList<String>(); for (Sku sku:skuListPreTwo){ skuNameList.add(sku.getSkuName()); } System.out.println("商品总价为："+totalPrice); System.out.println("商品名称为："+ JSON.toJSONString(skuNameList,true)); }*/ @Test /** * 使用Stream流的方式来实现需求： */ public void newCartHandler(){ //定义 原子引用类，可以保你多线程 安全,防止多线程 计数冲突 AtomicReference<Double> money = new AtomicReference<>(Double.valueOf(0.0)); List<String> skuNameList = CartService.getSkuList() /** * 1 建立stream流 */ .stream() /** * 2 打印所有商品的信息 */ .peek(sku -> System.out.println(JSON.toJSONString(sku,true))) /** * 3 筛选出非图书类 */ .filter(sku -> !sku.getSkuCategory().equals(SkuCategoryEnum.BOOKS)) /** * 4 按照价格排序(使用方法引用) * reversed排序反转 从大到小 */ .sorted(Comparator.comparing(Sku::getTotalPrice).reversed()) /** * 5 取出前两条 */ .limit(3) /** * 6 累加商品总金额 */ .peek(sku -> {money.set(money.get()+sku.getTotalPrice());}) /** * 7 取出商品名称集合 */ .map(sku -> sku.getSkuName()) /** * 8 收集结果 */ .collect(Collectors.toList()); //因为money是get类型，需要get一下值 System.out.println("商品总价为："+money.get()); System.out.println("商品名称为："+ JSON.toJSONString(skuNameList,true)); } }
其他类参考2-3：
打印日志：
D:\java\jdk\jdk9\jdk-9+181_windows-x64_ri\java-se-9-ri\jdk-9\bin\java.exe -ea -Didea.test.cyclic.buffer.size=1048576 "-javaagent:D:\java\devolopKit\idea\anZh\IntelliJ IDEA Community Edition 2018.1.4\lib\idea_rt.jar=38089:D:\java\devolopKit\idea\anZh\IntelliJ IDEA Community Edition 2018.1.4\bin" -Dfile.encoding=UTF-8 -classpath "D:\java\devolopKit\idea\anZh\IntelliJ IDEA Community Edition 2018.1.4\lib\idea_rt.jar;D:\java\devolopKit\idea\anZh\IntelliJ IDEA Community Edition 2018.1.4\plugins\junit\lib\junit-rt.jar;D:\java\devolopKit\idea\anZh\IntelliJ IDEA Community Edition 2018.1.4\plugins\junit\lib\junit5-rt.jar;F:\xiangmu3\Xin\test996\target\test-classes;F:\xiangmu3\Xin\test996\target\classes;F:\xiangmu3\Xin\test996\lib\com\google\guava\guava\28.2-jre\guava-28.2-jre.jar;F:\xiangmu3\Xin\test996\lib\com\google\guava\failureaccess\1.0.1\failureaccess-1.0.1.jar;F:\xiangmu3\Xin\test996\lib\com\google\guava\listenablefuture\9999.0-empty-to-avoid-conflict-with-guava\listenablefuture-9999.0-empty-to-avoid-conflict-with-guava.jar;F:\xiangmu3\Xin\test996\lib\com\google\code\findbugs\jsr305\3.0.2\jsr305-3.0.2.jar;F:\xiangmu3\Xin\test996\lib\org\checkerframework\checker-qual\2.10.0\checker-qual-2.10.0.jar;F:\xiangmu3\Xin\test996\lib\com\google\errorprone\error_prone_annotations\2.3.4\error_prone_annotations-2.3.4.jar;F:\xiangmu3\Xin\test996\lib\com\google\j2objc\j2objc-annotations\1.3\j2objc-annotations-1.3.jar;F:\xiangmu3\Xin\test996\lib\junit\junit\4.12\junit-4.12.jar;F:\xiangmu3\Xin\test996\lib\org\hamcrest\hamcrest-core\1.3\hamcrest-core-1.3.jar;F:\xiangmu3\Xin\test996\lib\com\alibaba\fastjson\1.2.58\fastjson-1.2.58.jar" com.intellij.rt.execution.junit.JUnitStarter -ideVersion5 -junit3 com.imooc.zhangxiaoxi.stream.StreamVs,newCartHandler { "skuCategory":"ELECTRONICS", "skuId":2020001, "skuName":"无人机", "skuPrice":999.0, "totalNum":1, "totalPrice":999.0 } { "skuCategory":"CLOTHING", "skuId":2020002, "skuName":"T-shirt", "skuPrice":50.0, "totalNum":2, "totalPrice":100.0 } { "skuCategory":"BOOKS", "skuId":2020003, "skuName":"人生的枷锁", "skuPrice":30.0, "totalNum":1, "totalPrice":30.0 } { "skuCategory":"BOOKS", "skuId":2020004, "skuName":"老人与海", "skuPrice":20.0, "totalNum":1, "totalPrice":20.0 } { "skuCategory":"BOOKS", "skuId":2020005, "skuName":"剑指高效编程", "skuPrice":288.0, "totalNum":1, "totalPrice":288.0 } { "skuCategory":"CLOTHING", "skuId":2020006, "skuName":"大头皮鞋", "skuPrice":300.0, "totalNum":1, "totalPrice":300.0 } { "skuCategory":"SPROTS", "skuId":2020007, "skuName":"杠铃", "skuPrice":2000.0, "totalNum":1, "totalPrice":2000.0 } { "skuCategory":"ELECTRONICS", "skuId":2020008, "skuName":"ThinkPad", "skuPrice":5000.0, "totalNum":1, "totalPrice":5000.0 } 商品总价为：7999.0 商品名称为：[ "ThinkPad", "杠铃", "无人机" ] Process finished with exit code 0