假设你有一些对象,它们有几个字段可以比较:

public class Person {

    private String firstName;
    private String lastName;
    private String age;

    /* Constructors */

    /* Methods */

}

所以在这个例子中,当你问if:

a.compareTo(b) > 0

你可能会问a的姓是不是在b的姓之前,或者a的年龄是不是比b大,等等……

在不增加不必要的混乱或开销的情况下,在这些类型的对象之间进行多重比较的最干净的方法是什么?

comparable接口只允许通过一个字段进行比较 在我看来,添加大量的比较方法(如compareByFirstName(), compareByAge()等)是混乱的。

那么最好的解决办法是什么呢?


当前回答

import com.google.common.collect.ComparisonChain;

/**
 * @author radler
 * Class Description ...
 */
public class Attribute implements Comparable<Attribute> {

    private String type;
    private String value;

    public String getType() { return type; }
    public void setType(String type) { this.type = type; }

    public String getValue() { return value; }
    public void setValue(String value) { this.value = value; }

    @Override
    public String toString() {
        return "Attribute [type=" + type + ", value=" + value + "]";
    }

    @Override
    public int compareTo(Attribute that) {
        return ComparisonChain.start()
            .compare(this.type, that.type)
            .compare(this.value, that.value)
            .result();
    }

}

其他回答

您可以实现一个Comparator来比较两个Person对象,并且可以检查任意数量的字段。你可以在比较器中放入一个变量,告诉它与哪个字段进行比较,尽管只编写多个比较器可能会更简单。

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);
    }
}

你也可以看看实现Comparator的Enum。

http://tobega.blogspot.com/2008/05/beautiful-enums.html

e.g.

Collections.sort(myChildren, Child.Order.ByAge.descending());

要连续排序多个字段,请尝试ComparatorChain

A ComparatorChain is a Comparator that wraps one or more Comparators in sequence. The ComparatorChain calls each Comparator in sequence until either 1) any single Comparator returns a non-zero result (and that result is then returned), or 2) the ComparatorChain is exhausted (and zero is returned). This type of sorting is very similar to multi-column sorting in SQL, and this class allows Java classes to emulate that kind of behaviour when sorting a List. To further facilitate SQL-like sorting, the order of any single Comparator in the list can >be reversed. Calling a method that adds new Comparators or changes the ascend/descend sort after compare(Object, Object) has been called will result in an UnsupportedOperationException. However, take care to not alter the underlying List of Comparators or the BitSet that defines the sort order. Instances of ComparatorChain are not synchronized. The class is not thread-safe at construction time, but it is thread-safe to perform multiple comparisons after all the setup operations are complete.

我认为如果你的比较算法是“聪明的”,那就更令人困惑了。我会选择你建议的众多比较方法。

对我来说唯一的例外就是平等。对于单元测试,重写. equals(在.net中)对我来说很有用,以便确定两个对象之间的几个字段是否相等(而不是引用是否相等)。