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
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());
}
}
}
其他回答
因为Date实现了Comparable，所以它有一个compareTo方法，就像String一样。
所以你的自定义比较器可以是这样的:
public class CustomComparator implements Comparator<MyObject> {
@Override
public int compare(MyObject o1, MyObject o2) {
return o1.getStartDate().compareTo(o2.getStartDate());
}
}
compare()方法必须返回一个int值，因此不能像计划的那样直接返回一个布尔值。
你的排序代码就像你写的那样:
Collections.sort(Database.arrayList, new CustomComparator());
如果你不需要重用你的比较器，一种稍短的方法是把它写成一个内联匿名类:
Collections.sort(Database.arrayList, new Comparator<MyObject>() {
@Override
public int compare(MyObject o1, MyObject o2) {
return o1.getStartDate().compareTo(o2.getStartDate());
}
});
因为java 8
现在你可以用比较器的lambda表达式来写最后一个例子:
Collections.sort(Database.arrayList,
(o1, o2) -> o1.getStartDate().compareTo(o2.getStartDate()));
List有一个sort(Comparator)方法，所以你可以进一步缩短:
Database.arrayList.sort((o1, o2) -> o1.getStartDate().compareTo(o2.getStartDate()));
这是一个非常常见的习惯用法，因此有一个内置方法可以为具有Comparable键的类生成Comparator:
Database.arrayList.sort(Comparator.comparing(MyObject::getStartDate));
所有这些都是等价的形式。
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());
具有自然排序顺序的类(例如类Number)应该实现Comparable接口，而没有自然排序顺序的类(例如类Chair)应该提供Comparator(或匿名Comparator类)。
两个例子:
public class Number implements Comparable<Number> {
private int value;
public Number(int value) { this.value = value; }
public int compareTo(Number anotherInstance) {
return this.value - anotherInstance.value;
}
}
public class Chair {
private int weight;
private int height;
public Chair(int weight, int height) {
this.weight = weight;
this.height = height;
}
/* Omitting getters and setters */
}
class ChairWeightComparator implements Comparator<Chair> {
public int compare(Chair chair1, Chair chair2) {
return chair1.getWeight() - chair2.getWeight();
}
}
class ChairHeightComparator implements Comparator<Chair> {
public int compare(Chair chair1, Chair chair2) {
return chair1.getHeight() - chair2.getHeight();
}
}
用法:
List<Number> numbers = new ArrayList<Number>();
...
Collections.sort(numbers);
List<Chair> chairs = new ArrayList<Chair>();
// Sort by weight:
Collections.sort(chairs, new ChairWeightComparator());
// Sort by height:
Collections.sort(chairs, new ChairHeightComparator());
// You can also create anonymous comparators;
// Sort by color:
Collections.sort(chairs, new Comparator<Chair>() {
public int compare(Chair chair1, Chair chair2) {
...
}
});
您可以使用Bean Comparator对自定义类中的任何属性进行排序。
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。
