1. Code
package com.xtm.java.test;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Comparator;
import java.util.List;
/**
* Group implementation
*/
public class Test{
//entity class
class Apple {
String color;
int weight;
Apple(String color, int weight) {
super();
this.color = color;
this.weight = weight;
}
@Override
public String toString() {
return "Apple{" +
"颜'" + color + '\'' +
", 重：" + weight +
'}';
}
}
public static void main(String args[]){
List<Apple> list = new ArrayList<>();
list.add(new Test().new Apple("红", 205));
list.add(new Test().new Apple("绿", 248));
list.add(new Test().new Apple("红", 131));
list.add(new Test().new Apple("黄", 119));
list.add(new Test().new Apple("绿", 153));
list.add(new Test().new Apple("黄", 224));
// group by color
List<List<Apple>> byColorList = byColorGroupApple(list, Comparator.comparing(t -> t.color));
printList(byColorList);
// group by weight
List<List<Apple>> byWeightList = byColorGroupApple(list, (t1, t2) -> t1.weight/100==t2.weight/100?0:1);
printList(byWeightList);
}
private static void printList(List<List<Apple>> byColorList) {
System.out.println("After grouping: "+byColorList);
}
/**
* Group the collection according to the incoming comparator
* @param data set of elements to be grouped
* @param c incoming comparator
* @param <T> element
* @return returns the grouped list of sets
*/
private static <T> List<List<T>> byColorGroupApple(Collection<T> data, Comparator<? super T> c) {//Use generics on the method, remember to add <T> before the return value
List<List<T>> result = new ArrayList<>();
for (T t : data) {//1. Loop out each element in the collection
boolean isSameGroup = false;//2. The flag is not the same group
for (List<T> aResult : result) {//4. Loop to find out whether the current element belongs to a created group
if (c.compare(t, aResult.get(0)) == 0) {//aResult.get(0) means: as long as the current element and the first element of a certain group are compared through the comparator, they belong to this Group
isSameGroup = true;//5. If it is found that the current element belongs to a certain group, set the flag bit to prevent it from creating a new group
aResult.add(t);//6. Add the current element to the current group
break;
}
}
if (!isSameGroup) {//3. If it does not belong to any group, create a new group and add elements to the group
// create
List<T> innerList = new ArrayList();
innerList.add(t);
result.add(innerList);
}
}
return result;
}
}
Code 2: java8
package com.xtm.java.test;
import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;
import static java.util.stream.Collectors.groupingBy;
public class Student {
private int id;
private String sex;
private String name;
public static void main(String[] args) {
List<Student> list = new ArrayList();
list.add(new Student(1,"男","zhangsan"));
list.add(new Student(2,"女","lisi"));
list.add(new Student(3,"男","wangsu"));
list.add(new Student(2,"女","zhaoliu"));
list.add(new Student(1,"男","zhuoqi"));
list.add(new Student(3,"女","cangkong"));
// group all students by gender
Map<String, List<Student>> collect = list.stream().collect(Collectors.groupingBy(Student::getSex));
// More readable with method references
Map<String, List<Student>> collect2 = list.stream().collect(groupingBy(Student::getSex));
// group by id
Map<Integer, List<Student>> collect3 = list.stream().collect(groupingBy(Student::getId));
//custom function grouping
Map<Integer, List<Student>> collect4 = list.stream().collect(groupingBy(stu -> stu.hashCode() % 5));
System.out.println(collect);
System.out.println(collect2);
System.out.println(collect3);
System.out.println(collect4);
}
public Student(int id, String sex, String name) {
this.id = id;
this.sex = sex;
this.name = name;
}
public int getId() {
return id;
}
public String getSex() {
return sex;
}
public String getName() {
return name;
}
@Override
public String toString() {
return "Student{" +
"id=" + id +
", sex='" + sex + '\'' +
", name='" + name + '\'' +
'}';
}
}
2. Running results
After grouping: [[Apple{color 'red', weight: 205}, Apple{color 'red', weight: 131}], [Apple{color 'green', weight: 248}, Apple{color 'green', weight: 153}], [Apple{yan'huang', weight: 119}, Apple{yan'huang', weight: 224}]]
After grouping: [[Apple{color 'red', weight: 205}, Apple{color 'green', weight: 248}, Apple{color 'yellow', weight: 224}], [Apple{color 'red', Weight: 131}, Apple{Yellow', weight: 119}, Apple{Yan'green', weight: 153}]]
{女=[Student{id=2, sex='女', name='lisi'}, Student{id=2, sex='女', name='zhaoliu'}], 男=[Student{id=1, sex='男', name='zhangsan'}, Student{id=3, sex='男', name='wangsu'}, Student{id=1, sex='男', name='zhuoqi'}, Student{id=3, sex='男', name='cangkong'}]}
{女=[Student{id=2, sex='女', name='lisi'}, Student{id=2, sex='女', name='zhaoliu'}], 男=[Student{id=1, sex='男', name='zhangsan'}, Student{id=3, sex='男', name='wangsu'}, Student{id=1, sex='男', name='zhuoqi'}, Student{id=3, sex='男', name='cangkong'}]}
{1=[Student{id=1, sex='男', name='zhangsan'}, Student{id=1, sex='男', name='zhuoqi'}], 2=[Student{id=2, sex='女', name='lisi'}, Student{id=2, sex='女', name='zhaoliu'}], 3=[Student{id=3, sex='男', name='wangsu'}, Student{id=3, sex='男', name='cangkong'}]}
{0=[Student{id=2, sex='女', name='lisi'}, Student{id=2, sex='女', name='zhaoliu'}], 1=[Student{id=3, sex='男', name='wangsu'}], 2=[Student{id=1, sex='男', name='zhangsan'}, Student{id=1, sex='男', name='zhuoqi'}], 4=[Student{id=3, sex='男', name='cangkong'}]}