流操作

排序

数据准备

List<Student> students = new ArrayList<>();
students.add(new Student(18, 88));
students.add(new Student(18, 90));
students.add(new Student(16, 78));
students.add(new Student(15, 98));
students.add(new Student(15, 58));

单字段升序

# 按年龄升序
students.stream()
        .sorted(Comparator.comparing(Student::getAge))
        .collect(Collectors.toList());

单字段降序

# 按年龄降序排列
students.stream()
        .sorted(Comparator.comparing(Student::getAge, Comparator.reverseOrder()))
        .collect(Collectors.toList());

# 按年龄降序排列
students.stream()
        .sorted(Comparator.comparing(Student::getAge).reversed())
        .collect(Collectors.toList());

多字段降序

# 按年龄、成绩降序排列
students.stream()
        .sorted(Comparator.comparing(Student::getAge, Comparator.reverseOrder())
                .thenComparing(Student::getScore, Comparator.reverseOrder()))
        .collect(Collectors.toList());

去重

单字段去重

List<Integer> numbers = Arrays.asList(1, 3, 1, 2, 3);

numbers = numbers.stream()
        .distinct()
        .collect(Collectors.toList());

对象去重

import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;
import java.util.function.Function;
import java.util.function.Predicate;

/**
 * @ClassName：StreamUtils
 * @Description：Stream 流工具类
 * @Author：MaRui
 * @Date：2020/6/20 14:46
 * @Version 1.0
 **/
public class StreamUtils {

    /**
     * 去重（去除List中某个字段重复的对象）
     * @param keyExtractor
     * @param <T>
     * @return
     */
    public static <T> Predicate<T> distinctByKey(Function<? super T, Object> keyExtractor) {
        Map<Object, Boolean> seen = new ConcurrentHashMap<>();
        return t -> seen.putIfAbsent(keyExtractor.apply(t), Boolean.TRUE) == null;
    }
}

List<Student> students = new ArrayList<>();
students.add(new Student(18, 88));
students.add(new Student(18, 90));
students.add(new Student(16, 78));

students = students.stream()
        .filter(StreamUtils.distinctByKey(Student::getAge))
        .collect(Collectors.toList());

随机数

左开右闭，也就是随机4到10的数

RandomUtils.nextInt(4, 11)

类型转换

字符串转换成 boolean 值：Boolean.getBoolean(String name);

// 源码
public final class Boolean implements java.io.Serializable,
                                      Comparable<Boolean>
{
	public static boolean getBoolean(String name) {
        boolean result = false;
        try {
            result = parseBoolean(System.getProperty(name));
        } catch (IllegalArgumentException | NullPointerException e) {
        }
        return result;
    }

    public static boolean parseBoolean(String s) {
        return ((s != null) && s.equalsIgnoreCase("true"));
    }
}