【记录】【java】JDK8新特性Stream方式遍历集合

由于是以流方式，所以怎么操作也不改变原来的集合

1、普通遍历forEach

List<String> list = new ArrayList();
list.add("a");
list.add("b");
list.add("c");
list.add("d");
List<String> list2 = new ArrayList();
list.stream().forEach(str ->{
    list2.add(str);
});
System.out.println(list2);

2、过滤filter

List<String> list = new ArrayList();
list.add("a");
list.add("b");
list.add("c");
list.add("d");
List<String> list3 = new ArrayList();
// 过滤字符串为a的元素
list.stream().filter(str -> !"a".equals(str)).forEach(str -> {
    list3.add(str);
});
System.out.println(list3);

3、排序sorted

List<String> list = new ArrayList();
list.add("ab");
list.add("aa");
list.add("c");
list.add("d");
System.out.println("操作前list: " + list);
List<String> collect = list.stream().sorted((a, b) -> {
    return a.compareTo(b);
}).collect(Collectors.toList());
System.out.println("升序collect: " + collect);
List<String> collect2 = list.stream().sorted((a, b) -> {
    return b.compareTo(a);
}).collect(Collectors.toList());
System.out.println("降序collect2: " + collect2);
}

4、操作字符串集合，返回拼接字符串。collect的Collectors.joining

List<String> list = new ArrayList();
list.add("b");
list.add("a");
list.add("c");
list.add("d");
System.out.println("操作前list: " + list);
String collect = list.stream().sorted((a, b) -> {
    return b.compareTo(a);
}).collect(Collectors.joining(",", "前", "后"));
System.out.println("有前后collect: " + collect);
String collect2 = list.stream().sorted((a, b) -> {
    return b.compareTo(a);
}).collect(Collectors.joining(","));
System.out.println("没前后collect: " + collect2);

5、操作map集合

List<Map<String, Object>> list = new ArrayList();
Map<String, Object> map = new HashMap<>();
map.put("id", "wwww");
list.add(map);
Map<String, Object> map2 = new HashMap<>();
map2.put("id", "qqqq");
list.add(map2);
Map<String, Object> map3 = new HashMap<>();
map3.put("name", "qqqq");
list.add(map3);
System.out.println("list: " + list);
String collect = list.stream().filter(map1 -> null != map1.get("id"))
        .map(str -> str.get("id").toString())
        .collect(Collectors.joining(",", "前", "后"));
System.out.println("collect: " + collect);

6、操作对象

List<Student> list = new ArrayList();
Student student = new Student();
student.setAge("11");
student.setName("aaa");
list.add(student);
Student student2 = new Student();
student2.setAge("22");
student2.setName("bbb");
list.add(student2);
System.out.println("list: " + list);
String collect = list.stream().map(student3 -> student3.getName())
        .collect(Collectors.joining(",", "前", "后"));
System.out.println("collect: " + collect);

7、合并两个字符串数组flatMap

List<String[]> list = new ArrayList();
list.add(new String[]{"aa", "bb"});
list.add(new String[]{"ff", "ee"});
System.out.println("list: " + list);
String collect = list.stream().flatMap(Arrays::stream)
        .collect(Collectors.joining(",", "前", "后"));
System.out.println("collect: " + collect);

8、合并两个字符串集合flatMap

List<List<String>> list = new ArrayList();
list.add(Arrays.asList(new String[]{"aa", "bb"}));
list.add(Arrays.asList(new String[]{"ff", "ee"}));
System.out.println("list: " + list);
String collect = list.stream().flatMap(list2 -> list2.stream())
        .collect(Collectors.joining(",", "前", "后"));
System.out.println("collect: " + collect);

9、合并对象集合

List<List<Student>> list = new ArrayList();
List<Student> list1 = new ArrayList();
list1.add(new Student("23", "aaa"));
list1.add(new Student("43", "bbb"));
list.add(list1);
List<Student> list2 = new ArrayList();
list2.add(new Student("66", "qqq"));
list2.add(new Student("77", "www"));
list.add(list2);
System.out.println("list: " + list);
String collect = list.stream().flatMap(list4 -> list4.stream().map(student -> student.getName()))
        .collect(Collectors.joining(",", "前", "后"));
System.out.println("collect: " + collect);