Difference Between Comparable and Comparator

Both Comparable and Comparator interfaces are used to compare sizes. First, let's take a look at the definition of Comparable:

 package java.lang;
import java.util.*;
public interface Comparable<T> {
    public int compareTo(T o);
}

Comparator is defined as follows:

package java.util;
public interface Comparator<T> {
    int compare(T o1, T o2);
    boolean equals(Object obj);
}

Comparable sorts the objects of each class that implements it as a whole. This interface needs to be implemented by the class itself. If a class implements the Comparable interface, a List (or array) of objects of the class implementing the Comparable interface can be sorted by Collections.sort (or Arrays.sort). In addition, objects of classes that implement the Comparable interface can be used as keys in "ordered maps (such as TreeMap)" or elements in "ordered sets (TreeSet)" without specifying a comparator.
Example (class Person1 implements Comparable interface)

package collections;

public class Person1 implements Comparable<Person1>
{
    private int age;
    private String name;

    public Person1(String name, int age)
    {
        this.name = name;
        this.age = age;
    }
    @Override
    public int compareTo(Person1 o)
    {
        return this.age-o.age;
    }
    @Override 
    public String toString()
    {
        return name+":"+age;
    }
}

You can see that Person1 implements the compareTo method in the Comparable interface. Implementing the Comparable interface must modify its own class, that is, implement the corresponding method in the interface in its own class.
Test code:

        Person1 person1 = new Person1("zzh",18);
        Person1 person2 = new Person1("jj",17);
        Person1 person3 = new Person1("qq",19);

        List<Person1> list = new ArrayList<>();
        list.add(person1);
        list.add(person2);
        list.add(person3);

        System.out.println(list);
        Collections.sort(list);
        System.out.println(list);

Output result:

[zzh:18, jj:17, qq:19]
[jj:17, zzh:18, qq:19]

If our class cannot be modified, such as String, we have to sort it again. Of course, the Comparable interface has been implemented in String. If we simply use String as an example, it will not be very vivid. The class itself cannot be modified, which uses the Comparator interface (strategy pattern).

public final class Person2
{
    private int age;
    private String name;

    public Person2(String name, int age)
    {
        this.name = name;
        this.age = age;
    }

    @Override 
    public String toString()
    {
        return name+":"+age;
    }

    //getter and setter方法省略....
}

For example, the class Person2, this class has been fixed, and the modification of the class itself cannot be carried out, and the modifier is final. You don't want to inherit and implements Comparable, so what should you do at this time? Use the Comparator's interface outside the class. The following test code:

        Person2 p1 = new Person2("zzh",18);
        Person2 p2 = new Person2("jj",17);
        Person2 p3 = new Person2("qq",19);
        List<Person2> list2 = new ArrayList<Person2>();
        list2.add(p1);
        list2.add(p2);
        list2.add(p3);
        System.out.println(list2);
        Collections.sort(list2,new Comparator<Person2>(){

            @Override
            public int compare(Person2 o1, Person2 o2)
            {
                if(o1 == null || o2 == null)
                    return 0;
                return o1.getAge()-o2.getAge();
            }

        });
        System.out.println(list2);

Output result:

[zzh:18, jj:17, qq:19]
[jj:17, zzh:18, qq:19]

这里（public static <T> void sort(List<T> list, Comparator<? super T> c) ）采用了内部类的实现方式，实现compare方法，对类Person2的list进行排序。
再譬如博主遇到的真实案例中，需要对String进行排序，且不区分大小写，我们知道String中的排序是字典排序，譬如：A a D排序之后为A D a，这样显然不对，那么该怎么办呢？同上（下面代码中的list是一个String的List集合）：

        Collections.sort(list, new Comparator<String>()
        {
            @Override
            public int compare(String o1, String o2)
            {
                if(o1 == null || o2 == null)
                    return 0;
                return o1.toUpperCase().compareTo(o2.toUpperCase());
            }
        });

这样就可以实现不区分大小进行排序String的集合了，是不是很方便~

细心的同学可能会有疑问，明明在Comparator接口中定义了两个方法，为什么继承的时候只实现了一个方法，难道要颠覆我对Java接口常识的理解了嚒？
实际上，我们知道当一个类没有显式继承父类的时候，会有一个默认的父类，即java.lang.Object，在Object类中有一个方法即为equals方法，所以这里并不强制要求实现Comparator接口的类要实现equals方法，直接调用父类的即可，虽然你显式的实现了equals()方法 will be a better choice~

在《Effective Java》一书中，作者Joshua Bloch推荐大家在编写自定义类的时候尽可能的考虑实现一下Comparable接口，一旦实现了Comparable接口，它就可以跟许多泛型算法以及依赖于改接口的集合实现进行协作。你付出很小的努力就可以获得非常强大的功能。
事实上，Java平台类库中的所有值类都实现了Comparable接口。如果你正在编写一个值类，它具有非常明显的内在排序关系，比如按字母顺序、按数值顺序或者按年代顺序，那你就应该坚决考虑实现这个接口。
compareTo方法不但允许进行简单的等同性进行比较，而且语序执行顺序比较，除此之外，它与Object的equals方法具有相似的特征，它还是一个泛型。类实现了Comparable接口，就表明它的实例具有内在的排序关系，为实现Comparable接口的对象数组进行排序就这么简单： Arrays.sort(a);
对存储在集合中的Comparable对象进行搜索、计算极限值以及自动维护也同样简单。列如，下面的程序依赖于String实现了Comparable接口，它去掉了命令行参数列表中的重复参数，并按字母顺序打印出来：

public class WordList{
    public static void main(String args[]){
        Set<String> s = new TreeSet<String>();
        Collections.addAll(s,args);
        System.out.println(s);
    }
}

Comparable 是排序接口；若一个类实现了 Comparable 接口，就意味着 “该类支持排序”。而 Comparator 是比较器；我们若需要控制某个类的次序，可以建立一个 “该类的比较器” 来进行排序。
前者应该比较固定，和一个具体类相绑定，而后者比较灵活，它可以被用于各个需要比较功能的类使用。可以说前者属于 “静态绑定”，而后者可以 “动态绑定”。
我们不难发现：Comparable 相当于 “内部比较器”，而 Comparator 相当于 “外部比较器”。

Difference Between Comparable and Comparator

Guess you like