忘了在哪看的，挺好用
import lombok.Data;
/**
* Description: 流测试类
* Author: WangZW
* CreateDate: 2018/12/24
*/
@Data
public class TestStreamModel {
private int id;
private String name;
private int grade;
private int classes;
private double score;
}
import com.alibaba.acm.shaded.com.google.common.collect.Lists
import java.util.Comparator
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;
/**
* Description: TODO
* Author: WangZW
*/
public class StreamTest {
private List<TestStreamModel> getList() {
List<TestStreamModel> list = Lists.newArrayList();
TestStreamModel testStreamModel = new TestStreamModel();
testStreamModel.setId(2);/*主键*/
testStreamModel.setName("张三");/*姓名*/
testStreamModel.setClasses(1);/*班级*/
testStreamModel.setGrade(1);/*年级*/
testStreamModel.setScore(80);/*成绩*/
list.add(testStreamModel);
TestStreamModel testStreamModel1 = new TestStreamModel();
testStreamModel1.setId(1);
testStreamModel1.setName("李四");
testStreamModel1.setClasses(1);
testStreamModel1.setGrade(1);
testStreamModel1.setScore(60);
list.add(testStreamModel1);
TestStreamModel testStreamModel2 = new TestStreamModel();
testStreamModel2.setId(3);
testStreamModel2.setName("王二麻子");
testStreamModel2.setClasses(2);
testStreamModel2.setGrade(1);
testStreamModel2.setScore(90);
list.add(testStreamModel2);
TestStreamModel testStreamModel3 = new TestStreamModel();
testStreamModel3.setId(4);
testStreamModel3.setName("王五");
testStreamModel3.setClasses(2);
testStreamModel3.setGrade(1);
testStreamModel3.setScore(59.5);
list.add(testStreamModel3);
TestStreamModel testStreamModel4 = new TestStreamModel();
testStreamModel4.setId(8);
testStreamModel4.setName("小明");
testStreamModel4.setClasses(1);
testStreamModel4.setGrade(2);
testStreamModel4.setScore(79.5);
list.add(testStreamModel4);
TestStreamModel testStreamModel5 = new TestStreamModel();
testStreamModel5.setId(5);
testStreamModel5.setName("小红");
testStreamModel5.setClasses(2);
testStreamModel5.setGrade(2);
testStreamModel5.setScore(99);
list.add(testStreamModel5);
TestStreamModel testStreamModel6 = new TestStreamModel();
testStreamModel6.setId(7);
testStreamModel6.setName("小黑");
testStreamModel6.setClasses(2);
testStreamModel6.setGrade(2);
testStreamModel6.setScore(45);
list.add(testStreamModel6);
TestStreamModel testStreamModel7 = new TestStreamModel();
testStreamModel7.setId(6);
testStreamModel7.setName("小白");
testStreamModel7.setClasses(1);
testStreamModel7.setGrade(2);
testStreamModel7.setScore(88.8);
list.add(testStreamModel7);
TestStreamModel testStreamModel8 = new TestStreamModel();
testStreamModel8.setId(6);
testStreamModel8.setName("小白");
testStreamModel8.setClasses(1);
testStreamModel8.setGrade(2);
testStreamModel8.setScore(88.8);
list.add(testStreamModel8);
return list;
}
public static void main(String[] args) {
StreamTest streamTest = new StreamTest();
List<TestStreamModel> list = streamTest.getList();
/*去重，去除重复对象（每个属性的值都一样的），需要注意的是要先重写对象TestStreamModel的equals和hashCode方法*/
System.out.println("去重前：" + list);
List<TestStreamModel> distinctList = list.stream().distinct().collect(Collectors.toList());
System.out.println("去重后：" + distinctList);
/*排序，按id升续排列，如果要降续则改成：(a, b) -> b.getId() - a.getId(); a和b都是变量名（可以按自己意愿取名字），都是list中的对象的实例*/
// 下面代码以自然序排序一个list
// list.stream().sorted();
// 自然序逆序元素，使用Comparator 提供的reverseOrder() 方法
// list.stream().sorted(Comparator.reverseOrder());
// 例如
// 使用Comparator 来排序一个list
list.stream().sorted(Comparator.comparing(TestStreamModel::getId));
// 把上面的元素逆序
list.stream().sorted(Comparator.comparing(TestStreamModel::getId).reversed());
System.out.println("排序前：" + list);
List<TestStreamModel> sortList = list.stream().sorted((a, b) -> a.getId() - b.getId()).collect(Collectors.toList());
List<TestStreamModel> sortList2 = list.stream().sorted(Comparator.comparing(TestStreamModel::getId).reversed()).collect(Collectors.toList());
System.out.println("排序后" + sortList);
System.out.println("排序后" + sortList2);
/*过滤，按照自己的需求来筛选list中的数据，比如我筛选出不及格的（小于60分）的人,t为实例*/
System.out.println("过滤后：" + list);
List<TestStreamModel> filterList = list.stream().filter(t -> t.getScore() < 60).collect(Collectors.toList());
System.out.println("过滤后" + filterList);
/*map, 提取对象中的某一元素，例子中我取的是每个人的name，注意list中类型对应，如果取的是id或者班级，就应该是integer类型*/
System.out.println("提取前：" + list);
List<String> mapList = list.stream().map(t -> t.getName()).collect(Collectors.toList());
//另一种写法
List<String> mapList2 = list.stream().map(TestStreamModel::getName).collect(Collectors.toList());
System.out.println("提取后：" + mapList);
/*统计，统计所有人分数的和, 主要我设置的分数属性是double类型的，所以用mapToDouble，如果是int类型的，则需要用mapToInt*/
double sum = list.stream().mapToDouble(t -> t.getScore()).sum();
double sum2 = list.stream().mapToDouble(TestStreamModel::getScore).sum();
int count = list.stream().mapToInt(t -> t.getId()).sum();
/*分组， 按照字段中某个属性将list分组*/
Map<Integer, List<TestStreamModel>> map = list.stream().collect(Collectors.groupingBy(t -> t.getGrade()));
System.out.println("按年级分组" + map);
/*然后再对map处理，这样就方便取出自己要的数据*/
for (Map.Entry<Integer, List<TestStreamModel>> entry : map.entrySet()) {
System.out.println("key:" + entry.getKey());
System.out.println("value:" + entry.getValue());
}
/*多重分组，先按年级分组，再按班级分组*/
Map<Integer/*年级id*/, Map<Integer/*班级id*/, List<TestStreamModel>>> groupMap = list.stream().collect(Collectors.groupingBy(t -> t.getGrade(), Collectors.groupingBy(t -> t.getClasses())));
System.out.println("按照年级再按班级分组：" + groupMap);
System.out.println("取出一年级一班的list：" + groupMap.get(1).get(1));
/*多重分组，一般多重分组后都是为了统计，比如说统计每个年级，每个班的总分数*/
Map<Integer/*年级id*/, Map<Integer/*班级id*/, Double>> sumMap = list.stream().collect(Collectors.groupingBy(t -> t.getGrade(), Collectors.groupingBy(t -> t.getClasses(), Collectors.summingDouble(t -> t.getScore()))));
System.out.println(sumMap);
System.out.println("取出一年级一班的总分：" + sumMap.get(1).get(1));
/*stream是链式的，这些功能是可以一起使用的，例如：计算每个年级每个班的及格人数*/
Map<Integer/*年级*/, Map<Integer/*班级*/, Long/*人数*/>> integerMap = list.stream().filter(t -> t.getScore() >= 60).collect(Collectors.groupingBy(t -> t.getGrade(), Collectors.groupingBy(t -> t.getClasses(), Collectors.counting())));
System.out.println("取出一年级一班及格人数：" + integerMap.get(1).get(1));
}
}
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