Java 按属性对对象列表进行分组
我需要使用特定对象的属性（Java 按属性对对象列表进行分组,java,list,grouping,Java,List,Grouping,我需要使用特定对象的属性（Location）对对象列表（Student）进行分组。代码如下所示： public class Grouping { public static void main(String[] args) { List<Student> studlist = new ArrayList<Student>(); studlist.add(new Student("1726", "Joh
LocationStudentpublic class Grouping {
public static void main(String[] args) {
List<Student> studlist = new ArrayList<Student>();
studlist.add(new Student("1726", "John", "New York"));
studlist.add(new Student("4321", "Max", "California"));
studlist.add(new Student("2234", "Andrew", "Los Angeles"));
studlist.add(new Student("5223", "Michael", "New York"));
studlist.add(new Student("7765", "Sam", "California"));
studlist.add(new Student("3442", "Mark", "New York"));
}
}
class Student {
String stud_id;
String stud_name;
String stud_location;
Student(String sid, String sname, String slocation) {
this.stud_id = sid;
this.stud_name = sname;
this.stud_location = slocation;
}
}
公共类分组{
公共静态void main（字符串[]args）{
List studlist=new ArrayList（）；
新增（新学生（“1726”、“约翰”、“纽约”）；
新增（新学生（“4321”、“Max”、“California”）；
新增（新学生（“2234”、“安德鲁”、“洛杉矶”）；
新增（新学生（“5223”、“迈克尔”、“纽约”）；
新增（新生（“7765”、“山姆”、“加利福尼亚”）；
新增（新学生（“3442”，“马克”，“纽约”））；
}
}
班级学生{
串螺柱；
字符串名称；
管柱位置；
学生（字符串sid、字符串sname、字符串位置）{
this.stud_id=sid；
this.stud_name=sname；
此.stud_位置=插槽位置；
}
}
Map<String, List<Student>> map = new HashMap<String, List<Student>>();
List<Student> studlist = new ArrayList<Student>();
studlist.add(new Student("1726", "John", "New York"));
map.put("New York", studlist);
这将把students对象添加到带有
locationIDHashMap<Integer, List<Student>> hashMap = new HashMap<Integer, List<Student>>();
hashMap.get(locationId);
这将使您获得具有相同位置ID的所有学生。您可以按如下方式排序：
Collections.sort(studlist, new Comparator<Student>() {
@Override
public int compare(Student o1, Student o2) {
return o1.getStud_location().compareTo(o2.getStud_location());
}
});
Collections.sort（studlist，newcomparator（）{
@凌驾
公共整数比较（学生o1，学生o2）{
返回o1.getStud_location（）.compareTo（o2.getStud_location（））；
}
});
假设您在学生课堂上也有位置获取工具。您可以使用以下工具：
Map<String, List<Student>> groupedStudents = new HashMap<String, List<Student>>();
for (Student student: studlist) {
String key = student.stud_location;
if (groupedStudents.get(key) == null) {
groupedStudents.put(key, new ArrayList<Student>());
}
groupedStudents.get(key).add(student);
}
Map groupedStudents=newhashmap（）；
用于（学生：学生名单）{
字符串键=student.stud\u位置；
if（groupedStudents.get（key）==null）{
groupedStudents.put（key，newarraylist（））；
}
groupedStudents.get（key）.add（student）；
}
Set<String> groupedStudentsKeySet = groupedCustomer.keySet();
for (String location: groupedStudentsKeySet) {
List<Student> stdnts = groupedStudents.get(location);
for (Student student : stdnts) {
System.out.println("ID : "+student.stud_id+"\t"+"Name : "+student.stud_name+"\t"+"Location : "+student.stud_location);
}
}
Set-groupedStudentsKeySet=groupedCustomer.keySet（）；
for（字符串位置：groupedStudentsKeySet）{
List stdnts=groupedStudents.get（位置）；
适用于（学生：stdnts）{
System.out.println（“ID:+student.stud\u ID+”\t“+”名称：“+student.stud\u名称+”\t“+”位置：“+student.stud\u位置”）；
}
}
Map Map=newhashmap（）；
用于（学生：学生名单）{
字符串键=student.stud\u位置；
if（地图容器（图例））{
List=map.get（键）；
列表。添加（学生）；
}否则{
列表=新的ArrayList（）；
列表。添加（学生）；
地图。放置（键、列表）；
}
}
public class GroupByFeatureInJava {
public static void main(String[] args) {
ProductBean p1 = new ProductBean("P1", 20, new Date());
ProductBean p2 = new ProductBean("P1", 30, new Date());
ProductBean p3 = new ProductBean("P2", 20, new Date());
ProductBean p4 = new ProductBean("P1", 20, new Date());
ProductBean p5 = new ProductBean("P3", 60, new Date());
ProductBean p6 = new ProductBean("P1", 20, new Date());
List<ProductBean> list = new ArrayList<ProductBean>();
list.add(p1);
list.add(p2);
list.add(p3);
list.add(p4);
list.add(p5);
list.add(p6);
for (Iterator iterator = list.iterator(); iterator.hasNext();) {
ProductBean bean = (ProductBean) iterator.next();
System.out.println(bean);
}
System.out.println("******** AFTER GROUP BY PRODUCT_ID ******");
Collections.sort(list, new ProductBean().new CompareByProductID());
for (Iterator iterator = list.iterator(); iterator.hasNext();) {
ProductBean bean = (ProductBean) iterator.next();
System.out.println(bean);
}
System.out.println("******** AFTER GROUP BY PRICE ******");
Collections.sort(list, new ProductBean().new CompareByProductPrice());
for (Iterator iterator = list.iterator(); iterator.hasNext();) {
ProductBean bean = (ProductBean) iterator.next();
System.out.println(bean);
}
}
}
class ProductBean {
String productId;
int price;
Date date;
@Override
public String toString() {
return "ProductBean [" + productId + " " + price + " " + date + "]";
}
ProductBean() {
}
ProductBean(String productId, int price, Date date) {
this.productId = productId;
this.price = price;
this.date = date;
}
class CompareByProductID implements Comparator<ProductBean> {
public int compare(ProductBean p1, ProductBean p2) {
if (p1.productId.compareTo(p2.productId) > 0) {
return 1;
}
if (p1.productId.compareTo(p2.productId) < 0) {
return -1;
}
// at this point all a.b,c,d are equal... so return "equal"
return 0;
}
@Override
public boolean equals(Object obj) {
// TODO Auto-generated method stub
return super.equals(obj);
}
}
class CompareByProductPrice implements Comparator<ProductBean> {
@Override
public int compare(ProductBean p1, ProductBean p2) {
// this mean the first column is tied in thee two rows
if (p1.price > p2.price) {
return 1;
}
if (p1.price < p2.price) {
return -1;
}
return 0;
}
public boolean equals(Object obj) {
// TODO Auto-generated method stub
return super.equals(obj);
}
}
class CompareByCreateDate implements Comparator<ProductBean> {
@Override
public int compare(ProductBean p1, ProductBean p2) {
if (p1.date.after(p2.date)) {
return 1;
}
if (p1.date.before(p2.date)) {
return -1;
}
return 0;
}
@Override
public boolean equals(Object obj) {
// TODO Auto-generated method stub
return super.equals(obj);
}
}
}
公共类GroupByFeatureInJava{
公共静态void main（字符串[]args）{
ProductBean p1=新ProductBean（“p1”，20，新日期（））；
ProductBean p2=新ProductBean（“P1”，30，新日期（））；
ProductBean p3=新ProductBean（“P2”，20，新日期（））；
ProductBean p4=新ProductBean（“P1”，20，新日期（））；
ProductBean p5=新ProductBean（“P3”，60，新日期（））；
ProductBean p6=新ProductBean（“P1”，20，新日期（））；
列表=新的ArrayList（）；
列表。添加（p1）；
列表。添加（p2）；
列表。添加（p3）；
列表。添加（p4）；
增加（p5）；
增加（第6页）；
for（Iterator Iterator=list.Iterator（）；Iterator.hasNext（）；）{
ProductBean=（ProductBean）迭代器；
System.out.println（bean）；
}
System.out.println（“*********在按产品划分的组后面**********”；
Collections.sort（list，new ProductBean（）.new CompareByProductID（））；
for（Iterator Iterator=list.Iterator（）；Iterator.hasNext（）；）{
ProductBean=（ProductBean）迭代器；
System.out.println（bean）；
}
System.out.println（“*********后按价格分组*******”；
Collections.sort（list，new ProductBean（）.new CompareByProductPrice（））；
for（Iterator Iterator=list.Iterator（）；Iterator.hasNext（）；）{
ProductBean=（ProductBean）迭代器；
System.out.println（bean）；
}
}
}
类ProductBean{
字符串productId；
国际价格；
日期；
@凌驾
公共字符串toString（）{
返回“ProductBean[”+productId+“”+price+“”+date+“”]”；
}
ProductBean（）{
}
ProductBean（字符串productId，int price，Date）{
this.productId=productId；
这个价格=价格；
this.date=日期；
}
类CompareByProductID实现Comparator{
公共int比较（ProductBean p1、ProductBean p2）{
如果（p1.productId.compareTo（p2.productId）>0）{
返回1；
}
如果（p1.productId.compareTo（p2.productId）<0）{
返回-1；
}
//此时所有a.b，c，d都相等…所以返回“相等”
返回0；
}
@凌驾
公共布尔等于（对象obj）{
//TODO自动生成的方法存根
返回super.equals（obj）；
}
}
类CompareByProductPrice实现Comparator{
@凌驾
公共int比较（ProductBean p1、ProductBean p2）{
//这意味着第一列并列在两行中
如果（p1.价格>p2.价格）{
返回1；
}
如果（p1.价格Set<String> groupedStudentsKeySet = groupedCustomer.keySet();
for (String location: groupedStudentsKeySet) {
List<Student> stdnts = groupedStudents.get(location);
for (Student student : stdnts) {
System.out.println("ID : "+student.stud_id+"\t"+"Name : "+student.stud_name+"\t"+"Location : "+student.stud_location);
}
}
Map<String, List<Student>> map = new HashMap<String, List<Student>>();
for (Student student : studlist) {
String key = student.stud_location;
if(map.containsKey(key)){
List<Student> list = map.get(key);
list.add(student);
}else{
List<Student> list = new ArrayList<Student>();
list.add(student);
map.put(key, list);
}
}
public class GroupByFeatureInJava {
public static void main(String[] args) {
ProductBean p1 = new ProductBean("P1", 20, new Date());
ProductBean p2 = new ProductBean("P1", 30, new Date());
ProductBean p3 = new ProductBean("P2", 20, new Date());
ProductBean p4 = new ProductBean("P1", 20, new Date());
ProductBean p5 = new ProductBean("P3", 60, new Date());
ProductBean p6 = new ProductBean("P1", 20, new Date());
List<ProductBean> list = new ArrayList<ProductBean>();
list.add(p1);
list.add(p2);
list.add(p3);
list.add(p4);
list.add(p5);
list.add(p6);
for (Iterator iterator = list.iterator(); iterator.hasNext();) {
ProductBean bean = (ProductBean) iterator.next();
System.out.println(bean);
}
System.out.println("******** AFTER GROUP BY PRODUCT_ID ******");
Collections.sort(list, new ProductBean().new CompareByProductID());
for (Iterator iterator = list.iterator(); iterator.hasNext();) {
ProductBean bean = (ProductBean) iterator.next();
System.out.println(bean);
}
System.out.println("******** AFTER GROUP BY PRICE ******");
Collections.sort(list, new ProductBean().new CompareByProductPrice());
for (Iterator iterator = list.iterator(); iterator.hasNext();) {
ProductBean bean = (ProductBean) iterator.next();
System.out.println(bean);
}
}
}
class ProductBean {
String productId;
int price;
Date date;
@Override
public String toString() {
return "ProductBean [" + productId + " " + price + " " + date + "]";
}
ProductBean() {
}
ProductBean(String productId, int price, Date date) {
this.productId = productId;
this.price = price;
this.date = date;
}
class CompareByProductID implements Comparator<ProductBean> {
public int compare(ProductBean p1, ProductBean p2) {
if (p1.productId.compareTo(p2.productId) > 0) {
return 1;
}
if (p1.productId.compareTo(p2.productId) < 0) {
return -1;
}
// at this point all a.b,c,d are equal... so return "equal"
return 0;
}
@Override
public boolean equals(Object obj) {
// TODO Auto-generated method stub
return super.equals(obj);
}
}
class CompareByProductPrice implements Comparator<ProductBean> {
@Override
public int compare(ProductBean p1, ProductBean p2) {
// this mean the first column is tied in thee two rows
if (p1.price > p2.price) {
return 1;
}
if (p1.price < p2.price) {
return -1;
}
return 0;
}
public boolean equals(Object obj) {
// TODO Auto-generated method stub
return super.equals(obj);
}
}
class CompareByCreateDate implements Comparator<ProductBean> {
@Override
public int compare(ProductBean p1, ProductBean p2) {
if (p1.date.after(p2.date)) {
return 1;
}
if (p1.date.before(p2.date)) {
return -1;
}
return 0;
}
@Override
public boolean equals(Object obj) {
// TODO Auto-generated method stub
return super.equals(obj);
}
}
}
Map<String, List<Student>> studlistGrouped =
studlist.stream().collect(Collectors.groupingBy(w -> w.stud_location));
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;
import java.util.stream.Stream;
class Student {
String stud_id;
String stud_name;
String stud_location;
public String getStud_id() {
return stud_id;
}
public String getStud_name() {
return stud_name;
}
public String getStud_location() {
return stud_location;
}
Student(String sid, String sname, String slocation) {
this.stud_id = sid;
this.stud_name = sname;
this.stud_location = slocation;
}
}
class Temp
{
public static void main(String args[])
{
Stream<Student> studs =
Stream.of(new Student("1726", "John", "New York"),
new Student("4321", "Max", "California"),
new Student("2234", "Max", "Los Angeles"),
new Student("7765", "Sam", "California"));
Map<String, Map<Object, List<Student>>> map= studs.collect(Collectors.groupingBy(Student::getStud_name,Collectors.groupingBy(Student::getStud_location)));
System.out.println(map);//print by name and then location
}
}
{
Max={
Los Angeles=[Student@214c265e],
California=[Student@448139f0]
},
John={
New York=[Student@7cca494b]
},
Sam={
California=[Student@7ba4f24f]
}
}
@Canonical
class Persion {
String name
Integer age
}
List<Persion> list = [
new Persion("qianzi", 100),
new Persion("qianzi", 99),
new Persion("zhijia", 99)
]
println Multimaps.index(list, { Persion p -> return p.name })
Function<Student, List<Object>> compositKey = std ->
Arrays.asList(std.stud_location());
studentList.stream().collect(Collectors.groupingBy(compositKey, Collectors.toList()));
Function<Student, List<Object>> compositKey = std ->
Arrays.asList(std.stud_location(),std.stud_name());
studentList.stream().collect(Collectors.groupingBy(compositKey, Collectors.toList()));
public class Test9 {
static class Student {
String stud_id;
String stud_name;
String stud_location;
public Student(String stud_id, String stud_name, String stud_location) {
super();
this.stud_id = stud_id;
this.stud_name = stud_name;
this.stud_location = stud_location;
}
public String getStud_id() {
return stud_id;
}
public void setStud_id(String stud_id) {
this.stud_id = stud_id;
}
public String getStud_name() {
return stud_name;
}
public void setStud_name(String stud_name) {
this.stud_name = stud_name;
}
public String getStud_location() {
return stud_location;
}
public void setStud_location(String stud_location) {
this.stud_location = stud_location;
}
@Override
public String toString() {
return " [stud_id=" + stud_id + ", stud_name=" + stud_name + "]";
}
}
public static void main(String[] args) {
List<Student> list = new ArrayList<Student>();
list.add(new Student("1726", "John Easton", "Lancaster"));
list.add(new Student("4321", "Max Carrados", "London"));
list.add(new Student("2234", "Andrew Lewis", "Lancaster"));
list.add(new Student("5223", "Michael Benson", "Leeds"));
list.add(new Student("5225", "Sanath Jayasuriya", "Leeds"));
list.add(new Student("7765", "Samuael Vatican", "California"));
list.add(new Student("3442", "Mark Farley", "Ladykirk"));
list.add(new Student("3443", "Alex Stuart", "Ladykirk"));
list.add(new Student("4321", "Michael Stuart", "California"));
Map<String, List<Student>> map1 =
list
.stream()
.sorted(Comparator.comparing(Student::getStud_id)
.thenComparing(Student::getStud_name)
.thenComparing(Student::getStud_location)
)
.collect(Collectors.groupingBy(
ch -> ch.stud_location
));
System.out.println(map1);
/*
Output :
{Ladykirk=[ [stud_id=3442, stud_name=Mark Farley],
[stud_id=3443, stud_name=Alex Stuart]],
Leeds=[ [stud_id=5223, stud_name=Michael Benson],
[stud_id=5225, stud_name=Sanath Jayasuriya]],
London=[ [stud_id=4321, stud_name=Max Carrados]],
Lancaster=[ [stud_id=1726, stud_name=John Easton],
[stud_id=2234, stud_name=Andrew Lewis]],
California=[ [stud_id=4321, stud_name=Michael Stuart],
[stud_id=7765, stud_name=Samuael Vatican]]}
*/
}// main
}
public static <E, K> Map<K, List<E>> groupBy(List<E> list, Function<E, K> keyFunction) {
return Optional.ofNullable(list)
.orElseGet(ArrayList::new)
.stream()
.collect(Collectors.groupingBy(keyFunction));
}