我读过关于使用比较器排序数组列表的内容，但在所有的例子中，人们都使用了compareTo，根据一些研究，它是字符串的一种方法。
我想根据自定义对象的一个属性(Date对象)对其数组列表进行排序 (getStartDay())。通常我通过item1.getStartDate().before(item2.getStartDate())比较它们，所以我想知道我是否可以写一些像这样的东西:
public class CustomComparator {
public boolean compare(Object object1, Object object2) {
return object1.getStartDate().before(object2.getStartDate());
}
}
public class RandomName {
...
Collections.sort(Database.arrayList, new CustomComparator);
...
}
当前回答
JAVA 8最简单的方法是英文字母排序
类的实现
public class NewspaperClass implements Comparable<NewspaperClass>{
public String name;
@Override
public int compareTo(NewspaperClass another) {
return name.compareTo(another.name);
}
}
Sort
Collections.sort(Your List);
如果你想为包含非英语字符的字母排序，你可以使用Locale…下面代码使用土耳其字符排序…
类的实现
public class NewspaperClass implements Comparator<NewspaperClass> {
public String name;
public Boolean isUserNewspaper=false;
private Collator trCollator = Collator.getInstance(new Locale("tr_TR"));
@Override
public int compare(NewspaperClass lhs, NewspaperClass rhs) {
trCollator.setStrength(Collator.PRIMARY);
return trCollator.compare(lhs.name,rhs.name);
}
}
Sort
Collections.sort(your array list,new NewspaperClass());
其他回答
我发现这些答案大部分(如果不是全部的话)依赖于底层类(Object)来实现类似的接口或具有类似的helper接口。
没有我的解决方案!下面的代码允许您通过知道对象的字符串名称来比较对象的字段。您可以很容易地修改它，不使用名称，但随后需要公开它或构造一个想要进行比较的对象。
Collections.sort(anArrayListOfSomeObjectPerhapsUsersOrSomething, new ReflectiveComparator(). new ListComparator("name"));
public class ReflectiveComparator {
public class FieldComparator implements Comparator<Object> {
private String fieldName;
public FieldComparator(String fieldName){
this.fieldName = fieldName;
}
@SuppressWarnings({ "unchecked", "rawtypes" })
@Override
public int compare(Object object1, Object object2) {
try {
Field field = object1.getClass().getDeclaredField(fieldName);
field.setAccessible(true);
Comparable object1FieldValue = (Comparable) field.get(object1);
Comparable object2FieldValue = (Comparable) field.get(object2);
return object1FieldValue.compareTo(object2FieldValue);
}catch (Exception e){}
return 0;
}
}
public class ListComparator implements Comparator<Object> {
private String fieldName;
public ListComparator(String fieldName) {
this.fieldName = fieldName;
}
@SuppressWarnings({ "unchecked", "rawtypes" })
@Override
public int compare(Object object1, Object object2) {
try {
Field field = object1.getClass().getDeclaredField(fieldName);
field.setAccessible(true);
Comparable o1FieldValue = (Comparable) field.get(object1);
Comparable o2FieldValue = (Comparable) field.get(object2);
if (o1FieldValue == null){ return -1;}
if (o2FieldValue == null){ return 1;}
return o1FieldValue.compareTo(o2FieldValue);
} catch (NoSuchFieldException e) {
throw new IllegalStateException("Field doesn't exist", e);
} catch (IllegalAccessException e) {
throw new IllegalStateException("Field inaccessible", e);
}
}
}
}
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.Date;
public class test {
public static class Person {
public String name;
public int id;
public Date hireDate;
public Person(String iname, int iid, Date ihireDate) {
name = iname;
id = iid;
hireDate = ihireDate;
}
public String toString() {
return name + " " + id + " " + hireDate.toString();
}
// Comparator
public static class CompId implements Comparator<Person> {
@Override
public int compare(Person arg0, Person arg1) {
return arg0.id - arg1.id;
}
}
public static class CompDate implements Comparator<Person> {
private int mod = 1;
public CompDate(boolean desc) {
if (desc) mod =-1;
}
@Override
public int compare(Person arg0, Person arg1) {
return mod*arg0.hireDate.compareTo(arg1.hireDate);
}
}
}
public static void main(String[] args) {
// TODO Auto-generated method stub
SimpleDateFormat df = new SimpleDateFormat("mm-dd-yyyy");
ArrayList<Person> people;
people = new ArrayList<Person>();
try {
people.add(new Person("Joe", 92422, df.parse("12-12-2010")));
people.add(new Person("Joef", 24122, df.parse("1-12-2010")));
people.add(new Person("Joee", 24922, df.parse("12-2-2010")));
} catch (ParseException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
Collections.sort(people, new Person.CompId());
System.out.println("BY ID");
for (Person p : people) {
System.out.println(p.toString());
}
Collections.sort(people, new Person.CompDate(false));
System.out.println("BY Date asc");
for (Person p : people) {
System.out.println(p.toString());
}
Collections.sort(people, new Person.CompDate(true));
System.out.println("BY Date desc");
for (Person p : people) {
System.out.println(p.toString());
}
}
}
你的customComparator类必须实现java.util.Comparator才能被使用。 它还必须覆盖compare() AND equals()
compare()必须回答这个问题:对象1是否小于、等于或大于对象2?
完整文档:http://java.sun.com/j2se/1.5.0/docs/api/java/util/Comparator.html
从Java 8开始，我们不需要直接使用Collections.sort()。List接口有一个默认的sort()方法:
List<User> users = Arrays.asList(user1,user2,user3);
users.sort( (u1, u2) -> {
return u1.getFirstName.compareTo(u2.getFirstName());});
见http://visvv.blogspot.in/2016/01/sorting -对象- - java 8.中的html。
函数和方法参考
的集合。sort方法可以使用传入的比较器对列表进行排序。该比较器可以使用Comparator. comparison方法实现，其中可以传递一个方法引用作为必要的函数。幸运的是，实际代码比这个描述简单得多。
对于Java 8:
Collections.sort(list, comparing(ClassName::getName));
or
Collections.sort(list, comparing(ClassName::getName).reversed());
另一种方法是
Collections.sort(list, comparing(ClassName::getName, Comparator.nullsLast(Comparator.naturalOrder())));
