Java8中的Stream API实战

╰半夏微凉° 2021-07-24 12:08 394阅读 0赞

**一 参考**

[https://www.runoob.com/java/java8-streams.html][https_www.runoob.com_java_java8-streams.html]

**二 实战**

package Stream;
    
    
    import java.util.ArrayList;
    import java.util.Arrays;
    import java.util.IntSummaryStatistics;
    import java.util.List;
    import java.util.Random;
    import java.util.stream.Collectors;
    import java.util.Map;
    
    
    /**
    * Copyright (C), 2020-2020, XXX有限公司
    * FileName: Java8Tester
    * Author:   cakin
    * Date:     2020/3/19 11:46
    * Description: Stream操作
    * 参考
    * https://www.runoob.com/java/java8-streams.html
    */
    
    public class Java8Tester {
        public static void main( String args[] ) {
            System.out.println("使用 Java 7: ");
    
    
            // 计算空字符串
            List<String> strings = Arrays.asList("abc", "", "bc", "efg", "abcd", "", "jkl");
            System.out.println("列表: " + strings);
            long count = getCountEmptyStringUsingJava7(strings);
            System.out.println("空字符数量为: " + count);
    
            count = getCountLength3UsingJava7(strings);
            System.out.println("字符串长度为 3 的数量为: " + count);
    
            // 删除空字符串
            List<String> filtered = deleteEmptyStringsUsingJava7(strings);
            System.out.println("筛选后的列表: " + filtered);
    
            // 删除空字符串，并使用逗号把它们合并起来
            String mergedString = getMergedStringUsingJava7(strings, ", ");
            System.out.println("合并字符串: " + mergedString);
    
            List<Integer> numbers = Arrays.asList(3, 2, 2, 3, 7, 3, 5);
            // 获取列表元素平方数
            List<Integer> squaresList = getSquares(numbers);
            System.out.println("平方数列表: " + squaresList);
    
    
            List<Integer> integers = Arrays.asList(1, 2, 13, 4, 15, 6, 17, 8, 19);
            System.out.println("列表: " + integers);
            System.out.println("列表中最大的数 : " + getMax(integers));
            System.out.println("列表中最小的数 : " + getMin(integers));
            System.out.println("所有数之和 : " + getSum(integers));
            System.out.println("平均数 : " + getAverage(integers));
            System.out.println("随机数: ");
    
            // 输出10个随机数
            Random random = new Random();
    
            for (int i = 0; i < 10; i++) {
                System.out.println(random.nextInt());
            }
    
            System.out.println("使用 Java 8: ");
            System.out.println("列表: " + strings);
    
            // stream()为集合创建串行流
            // filter()通过设置条件过滤出元素，参数是Predicate
            // count()返回流中所有元素的数量
            count = strings.stream().filter(string -> string.isEmpty()).count();
            System.out.println("空字符串数量为: " + count);
    
            count = strings.stream().filter(string -> string.length() == 3).count();
            System.out.println("字符串长度为 3 的数量为: " + count);
    
            // collect()将流转换为集合
            filtered = strings.stream().filter(string -> !string.isEmpty()).collect(Collectors.toList());
            System.out.println("筛选后的列表: " + filtered);
    
            // collect()将流通过,号合并
            mergedString = strings.stream().filter(string -> !string.isEmpty()).collect(Collectors.joining(", "));
            System.out.println("合并字符串: " + mergedString);
    
            // map()方法用于映射到对应的结果
            // distinct()返回由该流的不同元素组成的流,对流去重
            squaresList = numbers.stream().map(i -> i * i).distinct().collect(Collectors.toList());
            System.out.println("Squares List: " + squaresList);
            System.out.println("列表: " + integers);
    
            // summaryStatistics()统计结果收集器
            IntSummaryStatistics stats = integers.stream().mapToInt(( x ) -> x).summaryStatistics();
    
            System.out.println("列表中最大的数 : " + stats.getMax());
            System.out.println("列表中最小的数 : " + stats.getMin());
            System.out.println("所有数之和 : " + stats.getSum());
            System.out.println("平均数 : " + stats.getAverage());
            System.out.println("随机数: ");
    
            // limit()用于获取指定数量的流
            // sorted()用于对流进行排序
            // forEach(Consumer<? super T> action)对流中每个元素指向action操作
            random.ints().limit(10).sorted().forEach(System.out::println);
    
            // 创建并行处理流
            count = strings.parallelStream().filter(string -> string.isEmpty()).count();
            System.out.println("空字符串的数量为: " + count);
        }
    
        private static int getCountEmptyStringUsingJava7( List<String> strings ) {
            int count = 0;
    
    
            for (String string : strings) {
    
    
                if (string.isEmpty()) {
                    count++;
                }
            }
            return count;
        }
    
        private static int getCountLength3UsingJava7( List<String> strings ) {
            int count = 0;
    
    
            for (String string : strings) {
    
    
                if (string.length() == 3) {
                    count++;
                }
            }
            return count;
        }
    
        private static List<String> deleteEmptyStringsUsingJava7( List<String> strings ) {
            List<String> filteredList = new ArrayList<String>();
    
    
            for (String string : strings) {
    
    
                if (!string.isEmpty()) {
                    filteredList.add(string);
                }
            }
            return filteredList;
        }
    
        private static String getMergedStringUsingJava7( List<String> strings, String separator ) {
            StringBuilder stringBuilder = new StringBuilder();
    
    
            for (String string : strings) {
    
    
                if (!string.isEmpty()) {
                    stringBuilder.append(string);
                    stringBuilder.append(separator);
                }
            }
            String mergedString = stringBuilder.toString();
            return mergedString.substring(0, mergedString.length() - 2);
        }
    
        private static List<Integer> getSquares( List<Integer> numbers ) {
            List<Integer> squaresList = new ArrayList<Integer>();
    
    
            for (Integer number : numbers) {
                Integer square = new Integer(number.intValue() * number.intValue());
    
    
                if (!squaresList.contains(square)) {
                    squaresList.add(square);
                }
            }
            return squaresList;
        }
    
        private static int getMax( List<Integer> numbers ) {
            int max = numbers.get(0);
    
    
            for (int i = 1; i < numbers.size(); i++) {
    
    
                Integer number = numbers.get(i);
    
    
                if (number.intValue() > max) {
                    max = number.intValue();
                }
            }
            return max;
        }
    
        private static int getMin( List<Integer> numbers ) {
            int min = numbers.get(0);
    
    
            for (int i = 1; i < numbers.size(); i++) {
                Integer number = numbers.get(i);
    
    
                if (number.intValue() < min) {
                    min = number.intValue();
                }
            }
            return min;
        }
    
        private static int getSum( List numbers ) {
            int sum = (int) (numbers.get(0));
    
    
            for (int i = 1; i < numbers.size(); i++) {
                sum += (int) numbers.get(i);
            }
            return sum;
        }
    
        private static int getAverage( List<Integer> numbers ) {
            return getSum(numbers) / numbers.size();
        }
    }

**三 测试结果**

使用 Java 7:
    列表: [abc, , bc, efg, abcd, , jkl]
    空字符数量为: 2
    字符串长度为 3 的数量为: 3
    筛选后的列表: [abc, bc, efg, abcd, jkl]
    合并字符串: abc, bc, efg, abcd, jkl
    平方数列表: [9, 4, 49, 25]
    列表: [1, 2, 13, 4, 15, 6, 17, 8, 19]
    列表中最大的数 : 19
    列表中最小的数 : 1
    所有数之和 : 85
    平均数 : 9
    随机数:
    1171775117
    610148801
    -828054550
    -2111770640
    -1896363502
    -1808328212
    -2024920079
    -1385788610
    -1006060100
    801927238
    使用 Java 8:
    列表: [abc, , bc, efg, abcd, , jkl]
    空字符串数量为: 2
    字符串长度为 3 的数量为: 3
    筛选后的列表: [abc, bc, efg, abcd, jkl]
    合并字符串: abc, bc, efg, abcd, jkl
    Squares List: [9, 4, 49, 25]
    列表: [1, 2, 13, 4, 15, 6, 17, 8, 19]
    列表中最大的数 : 19
    列表中最小的数 : 1
    所有数之和 : 85
    平均数 : 9.444444444444445
    随机数:
    -1947809395
    -1892983897
    -447017742
    -309736456
    -167605453
    -98838548
    182724635
    213186029
    665727226
    2125424645
    空字符串的数量为: 2
    
    
    Process finished with exit code 0

[https_www.runoob.com_java_java8-streams.html]: https://www.runoob.com/java/java8-streams.html