mport static java.util.stream.Collectors.groupingBy;public class stream {//比如说 你需要在一个列表中 筛选出所以大于15的苹果，然后按照颜色分组//按常规的筛选写法 就是在循环里面 迭代筛选public static void main(String[] args) {List<Apple> appleList = new ArrayList<>();for (int i = 0; i < 20; i++) {Random random = new Random();switch (random.nextInt(4)) {case 0:appleList.add(new Apple("red", random.nextInt(30), random.nextBoolean()));break;case 1:appleList.add(new Apple("green", random.nextInt(30), random.nextBoolean()));break;case 2:appleList.add(new Apple("white", random.nextInt(30), random.nextBoolean()));break;default:appleList.add(new Apple("yello", random.nextInt(30), random.nextBoolean()));}}// //常规写法
// Map<String, List<Apple>> AppMap = new HashMap<>();
// for (Apple apple : appleList) {
// if (apple.getWeight() > 15) { //如果大于15
// if (AppMap.get(apple.getColor()) == null) { //该颜色还没分类
// List<Apple> list = new ArrayList<>(); //新建该颜色的列表
// list.add(apple);//将苹果放进去列表
// AppMap.put(apple.getColor(), list);//将列表放到map中
// } else { //该颜色分类已存在
// AppMap.get(apple.getColor()).add(apple);//该颜色分类已存在，则直接放进去即可
// }
// }
// }
//
// System.out.println(AppMap);
// System.out.println(AppMap.size());
// showList(AppMap);//如上方式 就可以筛选出来所有的15大小以上的苹果，并按颜色分类//方式二 使用java8提供的流api实现 这种叫内部迭代
// Map<String, List<Apple>> AppMap2 = appleList.stream().filter((Apple a) -> a.getWeight() > 15) //筛选出大于150的
// .collect(groupingBy(Apple::getColor)); //按颜色分组 最后得到map
// showList(AppMap2);// 也可以条件 true flase 分组查询Map<Boolean, List<Apple>> AppMap2 = appleList.stream().filter((Apple a) -> a.getWeight() > 15) //筛选出大于150的.collect(groupingBy(Apple::getSale)); //按颜色分组 最后得到mapshowList2(AppMap2);}public static void showList2(Map<Boolean, List<Apple>> AppMap) {System.out.println(AppMap);//如上方式 就可以筛选出来所有的15大小以上的苹果，并按颜色分类List<Apple> saleApple = AppMap.get(Boolean.TRUE);List<Apple> unsaleApple = AppMap.get(Boolean.FALSE);for (Apple apple : saleApple) {System.out.println(apple);}for (Apple apple : unsaleApple) {System.out.println(apple);}}public static void showList(Map<String, List<Apple>> AppMap) {System.out.println(AppMap);//如上方式 就可以筛选出来所有的15大小以上的苹果，并按颜色分类List<Apple> redApple = AppMap.get("red");List<Apple> greenApple = AppMap.get("green");List<Apple> whiteApple = AppMap.get("white");List<Apple> yelloApple = AppMap.get("yello");for (Apple apple : redApple) {System.out.println(apple);}for (Apple apple : greenApple) {System.out.println(apple);}for (Apple apple : whiteApple) {System.out.println(apple);}for (Apple apple : yelloApple) {System.out.println(apple);}}}
package cn.itcast.article.po;import java.io.Serializable;/****/
public class Apple implements Serializable {private String color;//颜色private Integer weight; //重量private Boolean sale;//public Apple(String color, Integer weight) {this.color = color;this.weight = weight;}public Apple(String color) {this.color = color;}public Apple(String color, Integer weight, Boolean isSale) {this.color = color;this.weight = weight;this.sale = isSale;}public String getColor() {return color;}public void setColor(String color) {this.color = color;}public Integer getWeight() {return weight;}public void setWeight(Integer weight) {this.weight = weight;}public Boolean getSale() {return this.sale;}public void setSale(Boolean sale) {this.sale = sale;}@Overridepublic String toString() {return "Apple{" +"color='" + color + '\'' +", weight=" + weight +", isSale=" + sale +'}';}
}