java 实现可比较的接口
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Implementing the comparable interface
提问by John Curtsy
I just found this exam question and cannot figure it out :
我刚刚发现了这个考试问题，无法弄清楚：
The following depicts a contrived partial class which implements the Comparable interface. The only purpose of this contrived class is comparing its instances with a given string.
下面描述了一个实现 Comparable 接口的人为部分类。这个人为的类的唯一目的是将它的实例与给定的字符串进行比较。
There are two things we need to fill in in the class to finish it. Here is the class :
我们需要在类中填写两件事才能完成它。这是课程：
public class PrivateComparableClass // FILL IN PART 1 {
private String thing;
public PrivateComparableClass(String thing) {
this.thing=thing;
}
//FILL IN PART 2
}
I am assuming part 1 simply corresponds to :
我假设第 1 部分仅对应于：
public class PrivateComparableClass implements Comparable {
And part 2, I assume he is expecting an implementation of the compareTo method, but I don't really know how to properly go about implementing this:
第 2 部分，我假设他期待 compareTo 方法的实现，但我真的不知道如何正确地实现这个：
public static int compareTo() {
if this.thing.equals(thing){
return 1;
} else {
return -1;
}
}
How would I go about fixing this?
我将如何解决这个问题？
回答by NPE
First of all, part 1 should really be:
首先，第 1 部分应该是：
public class PrivateComparableClass implements Comparable<PrivateComparableClass> {
As to part 2, if thingis the only data member in the class, you can simply piggyback on String.compareTo:
至于第 2 部分，如果thing是类中唯一的数据成员，您可以简单地搭载String.compareTo：
public int compareTo(PrivateComparableClass rhs) {
return this.thing.compareTo(rhs.thing);
}
I recommend that you read up on how compareTois meant to work(there are three possible outcomes: less than, equal to and greater than).
我建议您阅读有关如何compareTo工作的内容（有三种可能的结果：小于、等于和大于）。
回答by digitalbath
To expand a bit:
稍微扩展一下：
Comparator functions typically take two arguments (let's call them A and B) and follow the convention of returning
比较器函数通常采用两个参数（让我们称它们为 A 和 B）并遵循返回约定
- -1 if A < B
- 0 if A == B
- 1 if A > B
- -1 如果 A < B
- 0 如果 A == B
- 1 如果 A > B
Also, compareTo should not be declared 'static' if you are using an instance variable.
此外，如果您使用的是实例变量，则不应将 compareTo 声明为“静态”。
回答by erickson
First, the Comparableinterface is generic; your declarations should specify a type parameter:
首先，Comparable接口是通用的；您的声明应指定一个类型参数：
public class PrivateComparableClass
implements Comparable<PrivateComparableClass> {
Then, you should compare the thingmembers of the class in a compareTo()method (which is an instance method, not a class member).
然后，您应该比较方法中thing类的成员compareTo()（这是一个实例方法，而不是类成员）。
@Override
public final int compareTo(PrivateComparableClass that) {
return this.thing.compareTo(that.thing);
}
A well-behaved Comparableshould implement an equals()method that is consistent with its compareTo()method:
一个行为端正的人Comparable应该实现一个equals()与其compareTo()方法一致的方法：
@Override
public final boolean equals(Object obj) {
if (obj == this)
return true;
if (!(obj instanceof PrivateComparableClass))
return false;
return compareTo((PrivateComparableClass) obj) == 0;
}
And, when you override equals(), you need to override hashCode()too:
而且，当您覆盖时equals()，您也需要覆盖hashCode()：
@Override
public final int hashCode() {
return thing.hashCode();
}
If thingis truly allowed to be null, suitable null-checking behavior should be added to each method.
如果thing确实允许null，则应为每个方法添加合适的空检查行为。
回答by james_bond
Well, this is more or less how the class should be declared and implemented
嗯，这或多或少是应该如何声明和实现类的
public class PrivateComparableClass implements Comparable<PrivateComparableClass>
{
private String thing;
//... other stuff
public int compareTo(PrivateComparableClass o)
{
return this.thing.compareTo(o.thing);
}
}
