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今天就跟大家聊聊有关 Java8中怎么操作Stream 对集合,可能很多人都不太了解,为了让大家更加了解,小编给大家总结了以下内容,希望大家根据这篇文章可以有所收获。
首先为了说明Stream对对象集合的操作,新建一个Student类(学生类),覆写了equals()和hashCode()方法
public class Student { private Long id; private String name; private int age; private String address; public Student() {} public Student(Long id, String name, int age, String address) { this.id = id; this.name = name; this.age = age; this.address = address; } @Override public String toString() { return "Student{" + "id=" + id + ", name='" + name + '\'' + ", age=" + age + ", address='" + address + '\'' + '}'; } @Override public boolean equals(Object o) { if (this == o) return true; if (o == null || getClass() != o.getClass()) return false; Student student = (Student) o; return age == student.age && Objects.equals(id, student.id) && Objects.equals(name, student.name) && Objects.equals(address, student.address); } @Override public int hashCode() { return Objects.hash(id, name, age, address); } public Long getId() { return id; } public void setId(Long id) { this.id = id; } public String getName() { return name; } public void setName(String name) { this.name = name; } public int getAge() { return age; } public void setAge(int age) { this.age = age; } public String getAddress() { return address; } public void setAddress(String address) { this.address = address; } }
public static void main(String [] args) { Student s1 = new Student(1L, "肖战", 15, "浙江"); Student s2 = new Student(2L, "王一博", 15, "湖北"); Student s3 = new Student(3L, "杨紫", 17, "北京"); Student s4 = new Student(4L, "李现", 17, "浙江"); List<Student> students = new ArrayList<>(); students.add(s1); students.add(s2); students.add(s3); students.add(s4); List<Student> streamStudents = testFilter(students); streamStudents.forEach(System.out::println); } /** * 集合的筛选 * @param students * @return */ private static List<Student> testFilter(List<Student> students) { //筛选年龄大于15岁的学生 // return students.stream().filter(s -> s.getAge()>15).collect(Collectors.toList()); //筛选住在浙江省的学生 return students.stream().filter(s ->"浙江".equals(s.getAddress())).collect(Collectors.toList()); }
运行结果:
这里我们创建了四个学生,经过filter的筛选,筛选出地址是浙江的学生集合。
public static void main(String [] args) { Student s1 = new Student(1L, "肖战", 15, "浙江"); Student s2 = new Student(2L, "王一博", 15, "湖北"); Student s3 = new Student(3L, "杨紫", 17, "北京"); Student s4 = new Student(4L, "李现", 17, "浙江"); List<Student> students = new ArrayList<>(); students.add(s1); students.add(s2); students.add(s3); students.add(s4); testMap(students); } /** * 集合转换 * @param students * @return */ private static void testMap(List<Student> students) { //在地址前面加上部分信息,只获取地址输出 List<String> addresses = students.stream().map(s ->"住址:"+s.getAddress()).collect(Collectors.toList()); addresses.forEach(a ->System.out.println(a)); }
运行结果
map就是将对应的元素按照给定的方法进行转换。
public static void main(String [] args) { testDistinct1(); } /** * 集合去重(基本类型) */ private static void testDistinct1() { //简单字符串的去重 List<String> list = Arrays.asList("111","222","333","111","222"); list.stream().distinct().forEach(System.out::println); }
运行结果:
public static void main(String [] args) { testDistinct2(); } /** * 集合去重(引用对象) */ private static void testDistinct2() { //引用对象的去重,引用对象要实现hashCode和equal方法,否则去重无效 Student s1 = new Student(1L, "肖战", 15, "浙江"); Student s2 = new Student(2L, "王一博", 15, "湖北"); Student s3 = new Student(3L, "杨紫", 17, "北京"); Student s4 = new Student(4L, "李现", 17, "浙江"); Student s5 = new Student(1L, "肖战", 15, "浙江"); List<Student> students = new ArrayList<>(); students.add(s1); students.add(s2); students.add(s3); students.add(s4); students.add(s5); students.stream().distinct().forEach(System.out::println); }
运行结果:
可以看出,两个重复的“肖战”同学进行了去重,这不仅因为使用了distinct()方法,而且因为Student对象重写了equals和hashCode()方法,否则去重是无效的。
public static void main(String [] args) { testSort1(); } /** * 集合排序(默认排序) */ private static void testSort1() { List<String> list = Arrays.asList("333","222","111"); list.stream().sorted().forEach(System.out::println); }
运行结果:
public static void main(String [] args) { testSort2(); } /** * 集合排序(指定排序规则) */ private static void testSort2() { Student s1 = new Student(1L, "肖战", 15, "浙江"); Student s2 = new Student(2L, "王一博", 15, "湖北"); Student s3 = new Student(3L, "杨紫", 17, "北京"); Student s4 = new Student(4L, "李现", 17, "浙江"); List<Student> students = new ArrayList<>(); students.add(s1); students.add(s2); students.add(s3); students.add(s4); students.stream() .sorted((stu1,stu2) ->Long.compare(stu2.getId(), stu1.getId())) .sorted((stu1,stu2) -> Integer.compare(stu2.getAge(),stu1.getAge())) .forEach(System.out::println); }
运行结果:
上面指定排序规则,先按照学生的id进行降序排序,再按照年龄进行降序排序
public static void main(String [] args) { testLimit(); } /** * 集合limit,返回前几个元素 */ private static void testLimit() { List<String> list = Arrays.asList("333","222","111"); list.stream().limit(2).forEach(System.out::println); }
运行结果:
public static void main(String [] args) { testSkip(); } /** * 集合skip,删除前n个元素 */ private static void testSkip() { List<String> list = Arrays.asList("333","222","111"); list.stream().skip(2).forEach(System.out::println); }
运行结果:
public static void main(String [] args) { testReduce(); } /** * 集合reduce,将集合中每个元素聚合成一条数据 */ private static void testReduce() { List<String> list = Arrays.asList("欢","迎","你"); String appendStr = list.stream().reduce("北京",(a,b) -> a+b); System.out.println(appendStr); }
运行结果:
public static void main(String [] args) { testMin(); } /** * 求集合中元素的最小值 */ private static void testMin() { Student s1 = new Student(1L, "肖战", 14, "浙江"); Student s2 = new Student(2L, "王一博", 15, "湖北"); Student s3 = new Student(3L, "杨紫", 17, "北京"); Student s4 = new Student(4L, "李现", 17, "浙江"); List<Student> students = new ArrayList<>(); students.add(s1); students.add(s2); students.add(s3); students.add(s4); Student minS = students.stream().min((stu1,stu2) ->Integer.compare(stu1.getAge(),stu2.getAge())).get(); System.out.println(minS.toString()); }
运行结果:
上面是求所有学生中年龄最小的一个,max同理,求最大值。
public static void main(String [] args) { testMatch(); } private static void testMatch() { Student s1 = new Student(1L, "肖战", 15, "浙江"); Student s2 = new Student(2L, "王一博", 15, "湖北"); Student s3 = new Student(3L, "杨紫", 17, "北京"); Student s4 = new Student(4L, "李现", 17, "浙江"); List<Student> students = new ArrayList<>(); students.add(s1); students.add(s2); students.add(s3); students.add(s4); Boolean anyMatch = students.stream().anyMatch(s ->"湖北".equals(s.getAddress())); if (anyMatch) { System.out.println("有湖北人"); } Boolean allMatch = students.stream().allMatch(s -> s.getAge()>=15); if (allMatch) { System.out.println("所有学生都满15周岁"); } Boolean noneMatch = students.stream().noneMatch(s -> "杨洋".equals(s.getName())); if (noneMatch) { System.out.println("没有叫杨洋的同学"); } }
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