java8学习笔记

Maren ·

更新时间:2024-09-20

· 686 次阅读

java8学习笔记基本用法创建测试实体，测试类1、根据字段分组2、根据字段排序3、根据字段去重4、根据条件过滤filter5、stream().map归并 基本用法 创建测试实体，测试类

创建测试对象DemoUser：

public class DemoUser {
   private String name;
   private String pwd;
   private Integer age;
   private Date birthDay;
   //getter/setter...
}

main方法中创建数组，添加数组元素（略），获得stream

List list = new ArrayList();
//...添加DemoUser对象
//转化为流stream()
Stream stream = list.stream();

1、根据字段分组

a、根据pwd分组

//根据pwd分组，返回一个map，collect传入函数Collectors.groupingBy()
Map<String, List> map= stream.collect(Collectors.groupingBy(e -> { return e.getPwd();}));

b、根据自定义的key分组

Map<String, List> collect = stream.collect(Collectors.groupingBy(e -> **fetchGroupKey**(e)));
//定义key规则
String fetchGroupKey(DemoUser demoUser){
        return demoUser.getName()+demoUser.getAge();
}

c、先根据age分组，再根据name分组

Map<Integer, Map<String, List>> collect = stream.collect(Collectors.groupingBy(e -> e.getAge(), Collectors.groupingBy(e -> e.getName())));

d、根据age分组，再统计每组数据总数

Map collect = stream.collect(Collectors.groupingBy(e -> e.getAge(), Collectors.counting()));

e、根据名称分组，再根据age求和

 Map collect = stream.collect(Collectors.groupingBy(e -> e.getName(), Collectors.summarizingInt(e -> e.getAge())));
        System.out.println();

2、根据字段排序

a、根据age从小到大排序

//使用
Collections.sort(list,(e1,e2)->e1.getAge()- e2.getAge());
//或者
list.sort(Comparator.comparing(DemoUser::getAge));

b、多条件组合排序：先根据age，再根据name和pwd排序

list.sort(Comparator.comparing(DemoUser::getAge).thenComparing(DemoUser::getName).thenComparing(DemoUser::getPwd));

c、根据汉字排序

Collections.sort(list, (e1, e2) -> Collator.getInstance(Locale.CHINESE).compare(e1.getName(), e2.getName()));

3、根据字段去重

a、根据名称去重

// 根据name去重
ArrayList collect = stream.collect(Collectors.collectingAndThen(Collectors.toCollection(() -> new TreeSet(Comparator.comparing(DemoUser::getName))), ArrayList::new)
);

b、多条件去重

// 根据name,age两个属性去重
List unique = stream.collect(
    Collectors. collectingAndThen(Collectors.toCollection(() -> new TreeSet(Comparator.comparing(o -> o.getName() + ";" + o.getAge()))), ArrayList::new)
);

c、利用过滤去重

List collect = stream.
	filter(distinctByKey(e -> e.getName())).
		collect(Collectors.toList());
//distinctByKey
static  Predicate distinctByKey(Function keyExtractor) {
        Map seen = new ConcurrentHashMap();
        return t -> seen.putIfAbsent(keyExtractor.apply(t), Boolean.TRUE) == null;
}

4、根据条件过滤filter

a、过滤获取name为liming，并且age大于12岁的user

List collect = stream.
	filter(e -> "liming".equals(e.getName()) && e.getAge() > 12).
		collect(Collectors.toList());

5、stream().map归并

//a 将List通过map重新构造并返回List
List collectVo = stream.map(e -> {
            DemoUserVo demoUserVo = new DemoUserVo();
            demoUserVo.setName(e.getName());
            demoUserVo.setAge(e.getAge());
            return demoUserVo;
}).collect(Collectors.toList());

对于stream.map()，入参为：Function mapper，而public interface Function接口，参数T和R表示：

@param  the type of the input to the function  表示函数的输入
@param  the type of the result of the function 表示函数结果

则上面的代码也可以改写为：

//构建DemoUserVo
public static Function buildDemoUserVo(){
        return e ->{
            DemoUserVo demoUserVo = new DemoUserVo();
            demoUserVo.setName(e.getName());
            demoUserVo.setAge(e.getAge());
            return demoUserVo;
        };
    }
//使用buildDemoUserVo
List collect = stream.map(buildDemoUserVo()).collect(Collectors.toList());

作者：原力与你同在

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