过滤filter()
获取最大最小值
根据条件统计数量
list转map
- key值唯一
- key值不唯一
distinct去重
groupingBy分组
map遍历
取list中某元素组成新的list
list转数组
- String
- 基本数据类型数组转换
数组转list
- Arrays.asList(）
- Collections.addAll
- 基本数据类型数组转list
源码和执行结果参考
- 执行源码
- 执行结果

过滤filter()

List<DemoEntity> filterList = initDatas.stream().filter(demo -> (StringUtils.isNotEmpty(demo.getBrief()))).collect(Collectors.toList());
// 循环
filterList.forEach(demo->System.out.println(demo.toString()));

获取最大最小值

List<DemoEntity> filterAgeList = initDatas.stream().filter(demo -> demo.getAge() != null).collect(Collectors.toList());
Integer maxAge = filterAgeList.stream().max(Comparator.comparing(DemoEntity::getAge)).get().getAge();
Integer minAge = filterAgeList.stream().min(Comparator.comparing(DemoEntity::getAge)).get().getAge();

根据条件统计数量

long count = initDatas.stream().distinct().filter(DemoEntity -> DemoEntity.getAge() != null &&  DemoEntity.getAge() > 30).count();
System.out.println("去重后统计大于30岁的人数="+count);

list转map

key值唯一

Map<Integer, DemoEntity> initMap = initDatas.stream().collect(Collectors.toMap(DemoEntity::getId, demo -> demo));

key值不唯一

key值冲突会报错，可以设置重复覆盖操作，后面数据覆盖前面的指定key2,指定key1则前面的覆盖后续重复的key的值

Exception in thread "main" java.lang.IllegalStateException: Duplicate key DemoEntity(id=6, name=Monica, sex=F, age=59, brief=球花, birthDate=Wed Sep 30 00:00:00 CST 1964)
	at java.util.stream.Collectors.lambda$throwingMerger$0(Collectors.java:133)
	at java.util.HashMap.merge(HashMap.java:1254)
	at java.util.stream.Collectors.lambda$toMap$58(Collectors.java:1320)
	at java.util.stream.ReduceOps$3ReducingSink.accept(ReduceOps.java:169)
	at java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1380)
	at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:481)
	at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:471)
	at java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:708)
	at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
	at java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:499)
	at com.renxiaozhao.service.demo.stream.StreamUtilDemo.main(StreamUtilDemo.java:32)

Map<Integer, DemoEntity> initMap = initDatas.stream().collect(Collectors.toMap(DemoEntity::getId, Function.identity(), (key1, key2) -> key2));

distinct去重

List<DemoEntity> distincts = initDatas.stream().distinct().collect(Collectors.toList());

groupingBy分组

Map<Integer, List<DemoEntity>> groupMap = distincts.stream().collect(Collectors.groupingBy(demo -> demo.getId()));

map遍历

groupMap.forEach((key, value) -> {
    System.out.println("key = " + key + ", value = " + value);
});

取list中某元素组成新的list

List<String> names = initDatas.stream().map(DemoEntity::getName).collect(Collectors.toList());
List<Integer> ids = initDatas.stream().map(DemoEntity::getId).collect(Collectors.toList());

list转数组

String

String[] arr = names.toArray(new String[names.size()]);

基本数据类型数组转换

int[] intArr = ids.stream().mapToInt(Integer::intValue).toArray();

数组转list

Arrays.asList(）

String[] arr = {"aa","bb","cc"};
List<String> list = Arrays.asList(arr);

list不能增删改操作

Collections.addAll

String[] arr = {"aa","bb","cc"};
List<String> list1= new ArrayList<>();
Collections.addAll(list1, arr);

list可以增删改操作

基本数据类型数组转list

int[] intArr= {1,2,3};
List<Integer> intList = Arrays.stream(intArr).mapToObj(Integer::new).collect(Collectors.toList());

源码和执行结果参考

执行源码

package com.renxiaozhao.service.demo.stream;

import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.Comparator;
import java.util.Date;
import java.util.List;
import java.util.Map;
import java.util.function.Function;
import java.util.stream.Collectors;

import com.baomidou.mybatisplus.core.toolkit.StringUtils;

public class StreamUtilDemo {

    public static void main(String[] args) throws ParseException {
        //初始化数据
        List<DemoEntity> initDatas = initData();
        //获取去除简介brief为空的数据后的list
        List<DemoEntity> filterList = initDatas.stream().filter(demo -> (StringUtils.isNotEmpty(demo.getBrief()))).collect(Collectors.toList());
        System.out.println("过滤brief为空后的数据-----------------------------------");
        filterList.forEach(demo->System.out.println(demo.toString()));
        //获取最成熟的年龄,先过滤年龄为空的数据
        List<DemoEntity> filterAgeList = initDatas.stream().filter(demo -> demo.getAge() != null).collect(Collectors.toList());
        Integer maxAge = filterAgeList.stream().max(Comparator.comparing(DemoEntity::getAge)).get().getAge();
        Integer minAge = filterAgeList.stream().min(Comparator.comparing(DemoEntity::getAge)).get().getAge();
        System.out.println("最大年龄="+maxAge+",最小年龄="+minAge);
        //获取去重后大于30岁的人数
        long count = initDatas.stream().distinct().filter(DemoEntity -> DemoEntity.getAge() != null &&  DemoEntity.getAge() > 30).count();
        System.out.println("去重后统计大于30岁的人数="+count);
        //list转map key值冲突会报错，可以设置重复覆盖操作
        //Map<Integer, DemoEntity> initMap = initDatas.stream().collect(Collectors.toMap(DemoEntity::getId, demo -> demo));
        //后面数据覆盖前面的指定key2,指定key1则前面的覆盖后续重复的key的值
        System.out.println("list转map key冲突时覆盖前面的key值设置，详细数据如下:");
        Map<Integer, DemoEntity> initMap = initDatas.stream().collect(Collectors.toMap(DemoEntity::getId, Function.identity(), (key1, key2) -> key2));
        initMap.forEach((key, value) -> {
            System.out.println("key = " + key + ", value = " + value);
        });
        //分组
        System.out.println("distinct去重");
        List<DemoEntity> distincts = initDatas.stream().distinct().collect(Collectors.toList());
        System.out.println("list转Map,按照id分组");
        Map<Integer, List<DemoEntity>> groupMap = distincts.stream().collect(Collectors.groupingBy(demo -> demo.getId()));
        groupMap.forEach((key, value) -> {
            System.out.println("key = " + key + ", value = " + value);
        });
        //取list中name组成新的list
        List<String> names = initDatas.stream().map(DemoEntity::getName).collect(Collectors.toList());
        List<Integer> ids = initDatas.stream().map(DemoEntity::getId).collect(Collectors.toList());
        System.out.println("新list:"+names.toString());
        System.out.println("新list去重:"+names.stream().distinct().collect(Collectors.toList()).toString());
        //list转数组
        String[] arr = names.toArray(new String[names.size()]);
        System.out.println("list.toarray转数组"+Arrays.toString(arr));
        int[] intArr = ids.stream().mapToInt(Integer::intValue).toArray();
        System.out.println("list通过stream转数组"+Arrays.toString(intArr));
        //数组转list
        List<String> list = Arrays.asList(arr);
        System.out.println("Arrays.asList转List,只运行查看操作"+list.toString());
        List<String> list1 = new ArrayList<>();
        Collections.addAll(list1, arr);
        System.out.println("Collections.addAll，通过new一个List，可以进行增删改操作"+list1.toString());
        //基本数据数组转list
        List<Integer> intList = Arrays.stream(intArr).mapToObj(Integer::new).collect(Collectors.toList());
        System.out.println("mapToObj基本数据数组转list"+intList.toString());
    }
    
    private static List<DemoEntity> initData() throws ParseException{
        List<DemoEntity> initDatas = new ArrayList<>();
        for (int i = 0;i < 5;i++) {
            DemoEntity demo = new DemoEntity();
            demo.setId(i+1);
            demo.setName("TOM_" + i);
            demo.setBirthDate(new Date());
            demo.setAge(5*(i+1));
            demo.setSex('M');
            initDatas.add(demo);
        }
        //重复数据
        String dateString = "1964-09-30";
        Date date= new SimpleDateFormat("yyyy-MM-dd").parse(dateString);
        DemoEntity demoF1 = new DemoEntity();
        demoF1.setId(6);
        demoF1.setName("Monica");
        demoF1.setBirthDate(date);
        demoF1.setBrief("球花");
        demoF1.setAge(59);
        demoF1.setSex('F');
        initDatas.add(demoF1);
        DemoEntity demoF2 = new DemoEntity();
        demoF2.setId(6);
        demoF2.setName("Monica");
        demoF2.setBirthDate(date);
        demoF2.setBrief("球花");
        demoF2.setAge(59);
        demoF2.setSex('F');
        initDatas.add(demoF2);
        //年龄为空
        DemoEntity demoF3 = new DemoEntity();
        demoF3.setId(6);
        demoF3.setName("Monica");
        demoF3.setBirthDate(date);
        demoF3.setBrief("球花");
        demoF3.setSex('F');
        initDatas.add(demoF3);
        System.out.println("初始数据-----------------------------------");
        initDatas.forEach(demo->System.out.println(demo.toString()));
        return initDatas;
    }
}

package com.renxiaozhao.service.demo.stream;

import java.util.Date;

import org.springframework.format.annotation.DateTimeFormat;

import io.swagger.annotations.ApiModelProperty;
import lombok.Data;

@Data
public class DemoEntity {
    @ApiModelProperty(value = "主键")
    private Integer id;
    
    @ApiModelProperty(value = "姓名")
    private String name;
    
    @ApiModelProperty(value = "性别:男-M 女-F")
    private char sex;
    
    @ApiModelProperty(value = "年龄")
    private Integer age;
    
    @ApiModelProperty(value = "简介")
    private String brief;
    
    @ApiModelProperty(value = "生日")
    @DateTimeFormat(pattern = "yyyy-MM-dd HH:mm:ss")
    private Date birthDate;
}

执行结果

初始数据-----------------------------------
DemoEntity(id=1, name=TOM_0, sex=M, age=5, brief=null, birthDate=Fri Jul 07 13:23:24 CST 2023)
DemoEntity(id=2, name=TOM_1, sex=M, age=10, brief=null, birthDate=Fri Jul 07 13:23:24 CST 2023)
DemoEntity(id=3, name=TOM_2, sex=M, age=15, brief=null, birthDate=Fri Jul 07 13:23:24 CST 2023)
DemoEntity(id=4, name=TOM_3, sex=M, age=20, brief=null, birthDate=Fri Jul 07 13:23:24 CST 2023)
DemoEntity(id=5, name=TOM_4, sex=M, age=25, brief=null, birthDate=Fri Jul 07 13:23:24 CST 2023)
DemoEntity(id=6, name=Monica, sex=F, age=59, brief=球花, birthDate=Wed Sep 30 00:00:00 CST 1964)
DemoEntity(id=6, name=Monica, sex=F, age=59, brief=球花, birthDate=Wed Sep 30 00:00:00 CST 1964)
DemoEntity(id=6, name=Monica, sex=F, age=null, brief=球花, birthDate=Wed Sep 30 00:00:00 CST 1964)
过滤brief为空后的数据-----------------------------------
DemoEntity(id=6, name=Monica, sex=F, age=59, brief=球花, birthDate=Wed Sep 30 00:00:00 CST 1964)
DemoEntity(id=6, name=Monica, sex=F, age=59, brief=球花, birthDate=Wed Sep 30 00:00:00 CST 1964)
DemoEntity(id=6, name=Monica, sex=F, age=null, brief=球花, birthDate=Wed Sep 30 00:00:00 CST 1964)
最大年龄=59,最小年龄=5
去重后统计大于30岁的人数=1
list转map key冲突时覆盖前面的key值设置，详细数据如下:
key = 1, value = DemoEntity(id=1, name=TOM_0, sex=M, age=5, brief=null, birthDate=Fri Jul 07 13:23:24 CST 2023)
key = 2, value = DemoEntity(id=2, name=TOM_1, sex=M, age=10, brief=null, birthDate=Fri Jul 07 13:23:24 CST 2023)
key = 3, value = DemoEntity(id=3, name=TOM_2, sex=M, age=15, brief=null, birthDate=Fri Jul 07 13:23:24 CST 2023)
key = 4, value = DemoEntity(id=4, name=TOM_3, sex=M, age=20, brief=null, birthDate=Fri Jul 07 13:23:24 CST 2023)
key = 5, value = DemoEntity(id=5, name=TOM_4, sex=M, age=25, brief=null, birthDate=Fri Jul 07 13:23:24 CST 2023)
key = 6, value = DemoEntity(id=6, name=Monica, sex=F, age=null, brief=球花, birthDate=Wed Sep 30 00:00:00 CST 1964)
distinct去重
list转Map,按照id分组
key = 1, value = [DemoEntity(id=1, name=TOM_0, sex=M, age=5, brief=null, birthDate=Fri Jul 07 13:23:24 CST 2023)]
key = 2, value = [DemoEntity(id=2, name=TOM_1, sex=M, age=10, brief=null, birthDate=Fri Jul 07 13:23:24 CST 2023)]
key = 3, value = [DemoEntity(id=3, name=TOM_2, sex=M, age=15, brief=null, birthDate=Fri Jul 07 13:23:24 CST 2023)]
key = 4, value = [DemoEntity(id=4, name=TOM_3, sex=M, age=20, brief=null, birthDate=Fri Jul 07 13:23:24 CST 2023)]
key = 5, value = [DemoEntity(id=5, name=TOM_4, sex=M, age=25, brief=null, birthDate=Fri Jul 07 13:23:24 CST 2023)]
key = 6, value = [DemoEntity(id=6, name=Monica, sex=F, age=59, brief=球花, birthDate=Wed Sep 30 00:00:00 CST 1964), DemoEntity(id=6, name=Monica, sex=F, age=null, brief=球花, birthDate=Wed Sep 30 00:00:00 CST 1964)]
新list:[TOM_0, TOM_1, TOM_2, TOM_3, TOM_4, Monica, Monica, Monica]
新list去重:[TOM_0, TOM_1, TOM_2, TOM_3, TOM_4, Monica]
list.toarray转数组[TOM_0, TOM_1, TOM_2, TOM_3, TOM_4, Monica, Monica, Monica]
list通过stream转数组[1, 2, 3, 4, 5, 6, 6, 6]
Arrays.asList转List,只运行查看操作[TOM_0, TOM_1, TOM_2, TOM_3, TOM_4, Monica, Monica, Monica]
Collections.addAll，通过new一个List，可以进行增删改操作[TOM_0, TOM_1, TOM_2, TOM_3, TOM_4, Monica, Monica, Monica]
mapToObj基本数据数组转list[1, 2, 3, 4, 5, 6, 6, 6]