java stream的常用例子

Posted 2020-11-24 轻语~

一、标题
　　java stream的常用例子

二、描述
　　stream在当前互联网技术社区传播的已经很广泛了，且有阿里11.11用stream很好的完成数据处理案例，为此迎着互联网技术风口细细地学习一下stream，说不定能让代码飘起来

三、常用的例子
　　以下内容均使用该代码作为前提：

 1 public class AppTest {
 2 
 3          public static void main(String[] args){
 4             List<StreamDemo> list = new ArrayList<>();
 5             list.add(StreamDemo.builder().dateValue(LocalDate.now()).intValue(10).strValue("str1").descValue("str1").timeValue(LocalTime.now()).dateTimeValue(LocalDateTime.now()).bigDecimal(BigDecimal.TEN).build());
 6             list.add(StreamDemo.builder().dateValue(LocalDate.now()).intValue(15).strValue("str3").descValue("str2").timeValue(LocalTime.now()).dateTimeValue(LocalDateTime.now()).bigDecimal(BigDecimal.ONE).build());
 7             list.add(StreamDemo.builder().dateValue(LocalDate.now()).intValue(11).strValue("str6").descValue("str3").timeValue(LocalTime.now()).dateTimeValue(LocalDateTime.now()).bigDecimal(BigDecimal.ZERO).build());
 8             list.add(StreamDemo.builder().dateValue(LocalDate.now()).intValue(16).strValue("str8").descValue("str4").timeValue(LocalTime.now()).dateTimeValue(LocalDateTime.now()).bigDecimal(BigDecimal.TEN).build());
 9         }
10     }
11     @Builder
12     @AllArgsConstructor
13     @Data
14     class StreamDemo{
15         private String strValue;
16         private String descValue;
17         private Integer intValue=2;
18         private LocalDate dateValue;
19         private LocalTime timeValue;
20         private LocalDateTime dateTimeValue;
21         private BigDecimal bigDecimal;
22 
23         public String toString() {
24             return "StreamDemo{" +
25                     "strValue=‘" + strValue + ‘‘‘ +
26                     ", descValue=‘" + descValue + ‘‘‘ +
27                     ", intValue=" + intValue +
28                     ", dateValue=" + dateValue +
29                     ", timeValue=" + timeValue +
30                     ", dateTimeValue=" + dateTimeValue +
31                     ", bigDecimal=" + bigDecimal +
32                     ‘}‘;
33         }
34     }

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　　1、操作数值

　　　　1.1、获取累积指

System.out.println(Stream.of(list.get(0),list.get(1),list.get(2)).map(StreamDemo::getIntValue).reduce(10,(n,m)->n*m));
　　　　System.out.println(Stream.of(list.get(0),list.get(1),list.get(2)).map(StreamDemo::getIntValue).collect(Collectors.reducing((n,m)->n+m)).get());

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　　　　1.2、获取最大值

System.out.println(Stream.of(list.get(0),list.get(1),list.get(2)).map(StreamDemo::getIntValue).max((n,m)->n.compareTo(m)).get());
    System.out.println(Stream.of(list.get(0),list.get(1),list.get(2)).collect(Collectors.maxBy((n,m)->n.getIntValue().compareTo(m.getIntValue()))).get());

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　　　　1.3、获取最小值

System.out.println(Stream.of(list.get(0),list.get(1),list.get(2)).map(StreamDemo::getIntValue).min((n,m)->n.compareTo(m)).get());
    System.out.println(Stream.of(list.get(0),list.get(1),list.get(2)).collect(Collectors.minBy((n,m)->n.getIntValue().compareTo(m.getIntValue()))).get());

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　　　　1.4、获取数值集合

Stream.of(list.get(0),list.get(1),list.get(2)).mapToInt(StreamDemo::getIntValue).forEach(System.out::println);
    Stream.of(list.get(0),list.get(1),list.get(2)).collect(Collectors.mapping(StreamDemo::getIntValue,Collectors.toList())).forEach(System.out::println);

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　　　　1.5、将多个数值集合合并为一个数值集合

Stream.of(list.get(0),list.get(1),list.get(2)).flatMapToInt(f-> IntStream.concat(IntStream.of(f.getIntValue()),IntStream.of(f.getBigDecimal().intValue()))).sorted().forEachOrdered(System.out::println);

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　　　　1.6、获取平均值

System.out.println(Stream.of(list.get(0),list.get(1),list.get(2)).collect(Collectors.averagingInt(StreamDemo::getIntValue)).intValue());

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　　　　1.7、获取数据统计

System.out.println(Stream.of(list.get(0),list.get(1),list.get(2)).collect(Collectors.summarizingInt(StreamDemo::getIntValue)).toString());

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　　2、操作集合

　　　　2.1:、获取集合长度

System.out.println(Stream.of(list.get(0),list.get(1),list.get(2)).count());
    System.out.println(Stream.of(list.get(0),list.get(1),list.get(2)).collect(Collectors.counting()));

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　　　　2.2、限制集合长度

Stream.of(list.get(0),list.get(1),list.get(2)).limit(1).forEach(System.out::println);

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　　　　2.3、跳过集合前几项

Stream.of(list.get(0),list.get(1),list.get(2)).skip(1).forEach(System.out::println);

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　　　　2.4、集合排序

Stream.of(list.get(0),list.get(1),list.get(2)).sorted((n,m)->n.getIntValue().compareTo(m.getIntValue())).forEach(System.out::println);

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　　　　2.5、去重

Stream.of(list.get(0),list.get(1),list.get(2)).distinct().forEach(System.out::println);

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　　　　2.6、集合过滤

Stream.of(list.get(0),list.get(1),list.get(2)).filter(streamDemo -> StringUtils.isEmpty(streamDemo.getStrValue())).forEach(System.out::println);
    System.out.println(Stream.of(list.get(0),list.get(1),list.get(2)).noneMatch(streamDemo -> StringUtils.isEmpty(streamDemo.getStrValue())));
    System.out.println(Stream.of(list.get(0),list.get(1),list.get(2)).anyMatch(streamDemo -> StringUtils.isEmpty(streamDemo.getStrValue())));
    System.out.println(Stream.of(list.get(0),list.get(1),list.get(2)).allMatch(streamDemo -> StringUtils.isEmpty(streamDemo.getStrValue())));
    Stream.of(list.get(0),list.get(1),list.get(2)).collect(Collectors.partitioningBy(streamDemo -> StringUtils.isEmpty(streamDemo.getStrValue()))).forEach((k,v)->System.out.printf("k:%s,v:%s
",k,v));

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　　　　2.7、给集合每项添加内容

Stream.of(list.get(0),list.get(1),list.get(2)).peek(streamDemo -> streamDemo.setDescValue(streamDemo.getStrValue())).forEach(System.out::println);

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　　　　2.8、集合分组

Stream.of(list.get(0),list.get(1),list.get(2)).collect(Collectors.groupingBy(StreamDemo::getStrValue,Collectors.toList())).forEach((k,v)->System.out.printf("k:%s,v:%s
",k,v));
    Stream.of(list.get(0),list.get(1),list.get(2)).collect(Collectors.groupingBy(StreamDemo::getStrValue,Collectors.groupingBy(StreamDemo::getStrValue))).forEach((k,v)->System.out.printf("k:%s,v:%s
",k,v));
    System.out.println(Stream.of(list.get(0),list.get(1),list.get(2)).map(StreamDemo::getStrValue).collect(Collectors.joining(";")));
    Stream.of(list.get(0),list.get(1),list.get(2)).collect(Collectors.toMap(StreamDemo::getStrValue,StreamDemo::getDescValue)).forEach((k,v)->System.out.printf("k:%s,v:%s
",k,v));
    System.out.println(Stream.of(list.get(0),list.get(1),list.get(2)).collect(Collectors.mapping(StreamDemo::getStrValue,Collectors.joining(";"))));

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　　　　2.9、合并集合

Stream.of(list.get(0),list.get(1),list.get(2)).flatMap(f->Stream.concat(Stream.of(f.getIntValue()),Stream.of(f.getBigDecimal()))).forEach(System.out::println);

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　　　　2.10、将集合转为数组

System.out.println(Arrays.toString(Stream.of(list.get(0),list.get(1),list.get(2)).toArray(t->new StreamDemo[t])));

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