我有对象数组person (int age;字符串名称;)。
我如何按名字的字母顺序排序这个数组，然后按年龄?
你会用哪种算法呢?
当前回答
对于那些能够使用Java 8流API的人来说，这里有一个更整洁的方法: lambda和排序
我正在寻找相当于c# LINQ:
.ThenBy(...)
我在Comparator上找到了Java 8的机制:
.thenComparing(...)
下面是演示算法的代码片段。
Comparator<Person> comparator = Comparator.comparing(person -> person.name);
comparator = comparator.thenComparing(Comparator.comparing(person -> person.age));
请查看上面的链接，以获得更简洁的方法，并解释Java的类型推断如何使其与LINQ相比定义起来更笨拙。
下面是完整的单元测试供参考:
@Test
public void testChainedSorting()
{
// Create the collection of people:
ArrayList<Person> people = new ArrayList<>();
people.add(new Person("Dan", 4));
people.add(new Person("Andi", 2));
people.add(new Person("Bob", 42));
people.add(new Person("Debby", 3));
people.add(new Person("Bob", 72));
people.add(new Person("Barry", 20));
people.add(new Person("Cathy", 40));
people.add(new Person("Bob", 40));
people.add(new Person("Barry", 50));
// Define chained comparators:
// Great article explaining this and how to make it even neater:
// http://blog.jooq.org/2014/01/31/java-8-friday-goodies-lambdas-and-sorting/
Comparator<Person> comparator = Comparator.comparing(person -> person.name);
comparator = comparator.thenComparing(Comparator.comparing(person -> person.age));
// Sort the stream:
Stream<Person> personStream = people.stream().sorted(comparator);
// Make sure that the output is as expected:
List<Person> sortedPeople = personStream.collect(Collectors.toList());
Assert.assertEquals("Andi", sortedPeople.get(0).name); Assert.assertEquals(2, sortedPeople.get(0).age);
Assert.assertEquals("Barry", sortedPeople.get(1).name); Assert.assertEquals(20, sortedPeople.get(1).age);
Assert.assertEquals("Barry", sortedPeople.get(2).name); Assert.assertEquals(50, sortedPeople.get(2).age);
Assert.assertEquals("Bob", sortedPeople.get(3).name); Assert.assertEquals(40, sortedPeople.get(3).age);
Assert.assertEquals("Bob", sortedPeople.get(4).name); Assert.assertEquals(42, sortedPeople.get(4).age);
Assert.assertEquals("Bob", sortedPeople.get(5).name); Assert.assertEquals(72, sortedPeople.get(5).age);
Assert.assertEquals("Cathy", sortedPeople.get(6).name); Assert.assertEquals(40, sortedPeople.get(6).age);
Assert.assertEquals("Dan", sortedPeople.get(7).name); Assert.assertEquals(4, sortedPeople.get(7).age);
Assert.assertEquals("Debby", sortedPeople.get(8).name); Assert.assertEquals(3, sortedPeople.get(8).age);
// Andi : 2
// Barry : 20
// Barry : 50
// Bob : 40
// Bob : 42
// Bob : 72
// Cathy : 40
// Dan : 4
// Debby : 3
}
/**
* A person in our system.
*/
public static class Person
{
/**
* Creates a new person.
* @param name The name of the person.
* @param age The age of the person.
*/
public Person(String name, int age)
{
this.age = age;
this.name = name;
}
/**
* The name of the person.
*/
public String name;
/**
* The age of the person.
*/
public int age;
@Override
public String toString()
{
if (name == null) return super.toString();
else return String.format("%s : %d", this.name, this.age);
}
}
其他回答
让你的person类实现Comparable< person >，然后实现compareTo方法，例如:
public int compareTo(Person o) {
int result = name.compareToIgnoreCase(o.name);
if(result==0) {
return Integer.valueOf(age).compareTo(o.age);
}
else {
return result;
}
}
它将首先按名称(大小写不敏感)排序，然后按年龄排序。然后可以在Person对象的集合或数组上运行Arrays.sort()或Collections.sort()。
使用Java 8 Streams方法，在getter上引用方法…
// Create a stream...
var sortedList = persons.stream()
// sort it (does not sort the original list)...
.sorted(Comparator.comparing(Person::getName)
.thenComparing(Person::getAge));
// and collect to a new list
.collect(Collectors.toList());
集合到数组ist也可以:
persons.stream()
.sorted(Comparator.comparing(Person::getName)
.thenComparing(Person::getAge));
.toArray(String[]::new);
Java 8 Lambda方法……
//Sorts the original list Lambda style
persons.sort((p1, p2) -> {
if (p1.getName().compareTo(p2.getName()) == 0) {
return p1.getAge().compareTo(p2.getAge());
} else {
return p1.getName().compareTo(p2.getName());
}
});
最后……
// This syntax is similar to the Streams example above, but sorts the original list!!!
persons.sort(Comparator.comparing(Person::getName).thenComparing(Person::getAge));
根据需要创建尽可能多的比较器。之后，为每个订单类别调用“then comparison”方法。这是Streams的一种做法。看到的:
//Sort by first and last name
System.out.println("\n2.Sort list of person objects by firstName then "
+ "by lastName then by age");
Comparator<Person> sortByFirstName
= (p, o) -> p.firstName.compareToIgnoreCase(o.firstName);
Comparator<Person> sortByLastName
= (p, o) -> p.lastName.compareToIgnoreCase(o.lastName);
Comparator<Person> sortByAge
= (p, o) -> Integer.compare(p.age,o.age);
//Sort by first Name then Sort by last name then sort by age
personList.stream().sorted(
sortByFirstName
.thenComparing(sortByLastName)
.thenComparing(sortByAge)
).forEach(person->
System.out.println(person));
在多个字段上对用户定义的对象进行排序- Comparator (lambda stream)
对于那些能够使用Java 8流API的人来说，这里有一个更整洁的方法: lambda和排序
我正在寻找相当于c# LINQ:
.ThenBy(...)
我在Comparator上找到了Java 8的机制:
.thenComparing(...)
下面是演示算法的代码片段。
Comparator<Person> comparator = Comparator.comparing(person -> person.name);
comparator = comparator.thenComparing(Comparator.comparing(person -> person.age));
请查看上面的链接，以获得更简洁的方法，并解释Java的类型推断如何使其与LINQ相比定义起来更笨拙。
下面是完整的单元测试供参考:
@Test
public void testChainedSorting()
{
// Create the collection of people:
ArrayList<Person> people = new ArrayList<>();
people.add(new Person("Dan", 4));
people.add(new Person("Andi", 2));
people.add(new Person("Bob", 42));
people.add(new Person("Debby", 3));
people.add(new Person("Bob", 72));
people.add(new Person("Barry", 20));
people.add(new Person("Cathy", 40));
people.add(new Person("Bob", 40));
people.add(new Person("Barry", 50));
// Define chained comparators:
// Great article explaining this and how to make it even neater:
// http://blog.jooq.org/2014/01/31/java-8-friday-goodies-lambdas-and-sorting/
Comparator<Person> comparator = Comparator.comparing(person -> person.name);
comparator = comparator.thenComparing(Comparator.comparing(person -> person.age));
// Sort the stream:
Stream<Person> personStream = people.stream().sorted(comparator);
// Make sure that the output is as expected:
List<Person> sortedPeople = personStream.collect(Collectors.toList());
Assert.assertEquals("Andi", sortedPeople.get(0).name); Assert.assertEquals(2, sortedPeople.get(0).age);
Assert.assertEquals("Barry", sortedPeople.get(1).name); Assert.assertEquals(20, sortedPeople.get(1).age);
Assert.assertEquals("Barry", sortedPeople.get(2).name); Assert.assertEquals(50, sortedPeople.get(2).age);
Assert.assertEquals("Bob", sortedPeople.get(3).name); Assert.assertEquals(40, sortedPeople.get(3).age);
Assert.assertEquals("Bob", sortedPeople.get(4).name); Assert.assertEquals(42, sortedPeople.get(4).age);
Assert.assertEquals("Bob", sortedPeople.get(5).name); Assert.assertEquals(72, sortedPeople.get(5).age);
Assert.assertEquals("Cathy", sortedPeople.get(6).name); Assert.assertEquals(40, sortedPeople.get(6).age);
Assert.assertEquals("Dan", sortedPeople.get(7).name); Assert.assertEquals(4, sortedPeople.get(7).age);
Assert.assertEquals("Debby", sortedPeople.get(8).name); Assert.assertEquals(3, sortedPeople.get(8).age);
// Andi : 2
// Barry : 20
// Barry : 50
// Bob : 40
// Bob : 42
// Bob : 72
// Cathy : 40
// Dan : 4
// Debby : 3
}
/**
* A person in our system.
*/
public static class Person
{
/**
* Creates a new person.
* @param name The name of the person.
* @param age The age of the person.
*/
public Person(String name, int age)
{
this.age = age;
this.name = name;
}
/**
* The name of the person.
*/
public String name;
/**
* The age of the person.
*/
public int age;
@Override
public String toString()
{
if (name == null) return super.toString();
else return String.format("%s : %d", this.name, this.age);
}
}
Arrays.sort(persons, new PersonComparator());
import java.util.Comparator;
public class PersonComparator implements Comparator<? extends Person> {
@Override
public int compare(Person o1, Person o2) {
if(null == o1 || null == o2 || null == o1.getName() || null== o2.getName() ){
throw new NullPointerException();
}else{
int nameComparisonResult = o1.getName().compareTo(o2.getName());
if(0 == nameComparisonResult){
return o1.getAge()-o2.getAge();
}else{
return nameComparisonResult;
}
}
}
}
class Person{
int age; String name;
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
}
升级版:
public class PersonComparator implements Comparator<? extends Person> {
@Override
public int compare(Person o1, Person o2) {
int nameComparisonResult = o1.getName().compareToIgnoreCase(o2.getName());
return 0 == nameComparisonResult?o1.getAge()-o2.getAge():nameComparisonResult;
}
}
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