摘要：
1，List<EntityOld>转换为List<EntityNew>
正文：
1，List<EntityOld>转换为List<EntityNew>
List<EntityOld> list = oldList;
List<EntityNew> newList = list.stream().map(EntityNew::new).collect(Collectors.toList());
2，从一个实体类List中取出某个字段组成新的list
List<EntityOld> list = oldList;
List<String> idList = list.stream().map(EntityOld::getOldId).collect(Collectors.toList());
3，从list中筛选出符合条件的数据组成新的newList
普通for循环：
List<Student> list = new ArrayList<>(); //Student1: name:张三;age:18 //Student2: name:张三;age:20
List<StudentVo> newList = new ArrayList<>();
list.stream().forEach(item -> {
if (item.getAge() > 18) {
StudentVo info = new StudentVo();
info.setName(item.getName());
newList.add(info);
}
});
加filter：
//超过18岁
List<Student> newList = list.stream().filter(i -> this.checkAgeOver18(i.getAge())).collect(Collectors.toList()); private Boolean checkAgeOver18 (int age) {
return age > 18;
} //简化版，但是条件复杂的话最好就封装成一个方法了
List<Student> newList = list.stream().filter(i -> i.getAge() > 18).collect(Collectors.toList());
4，求list集合中某个字段的和
List<Integer> numbers = Arrays.asList(3, 2, 2, 3, 7, 3, 5);
IntSummaryStatistics stats = numbers.stream().mapToInt((x) -> x).summaryStatistics();
System.out.println("所有数之和 : " + stats.getSum());
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("平均数 : " + stats.getAverage());
List<Student> list = new ArrayList<>(); //Student1: name:张三;age:18 //Student2: name:张三;age:20
Integer result = list.stream().collect(Collectors.summingInt(Student::getAge));
System.out.println("所有学生年龄之和 : " + reuslt);
5，根据某字段对list进行分组
List<Fruit> fruitList = Lists.newArrayList(new Fruit("apple", 6),new Fruit("apple", 6),
new Fruit("banana", 7), new Fruit("banana", 7),
new Fruit("banana", 7), new Fruit("grape",8));
Map<String, List<Fruit>> groupMap = fruitList.stream().collect(Collectors.groupingBy(Fruit::getName));
6，根据某字段对list进行分组并计数
//输出结果是{banana=3, apple=2, grape=1}
Map<String, Long> map = fruitList.stream().collect(Collectors.groupingBy(Fruit::getName,Collectors.counting()));
7，根据字段排序
//根据name字段倒排
List<Fruit> list = fruitList.stream().sorted(Comparator.comparing(Fruit::getName).reversed()).collect(Collectors.toList());
8，多条数据取第一条
//equalsIgnoreCase：不考虑大小写;find.isPresent()可用来判断数据是否存在
Optional<Fruit> find = fruitList.stream().filter(i -> i.getName().equalsIgnoreCase("apple")).findFirst();
补充：
1，List<EntityOld>转换为List<EntityNew>
注意：实体类EntityNew中需要有一个EntityOld的转换方法
EntityNew.java
public class EntityNew {
private String newId;
private String avatar;
//转换方法*
public EntityNew (EntityOld item) {
this.newId = item.getOldId() == null ? "" : item.getOldId();
}
public String getNewId() {
return newId;
}
public void setNewId(String newId) {
this.newId = newId;
}
public String getAvatar() {
return avatar;
}
public void setAvatar(String avatar) {
this.avatar = avatar;
}
}
EntityOld.java
public class EntityOld {
private String oldId;
private String oldName;
public String getOldId() {
return oldId;
}
public void setOldId(String oldId) {
this.oldId = oldId;
}
public String getOldName() {
return oldName;
}
public void setOldName(String oldName) {
this.oldName = oldName;
}
}