你认为类字符串实现了Java中可比的接口

Question

I'm doing this for my own understanding and I don't understand this question.

Assume there is an interface named comparable with the following definition:

public interface Comparable { 
        int compareTo(Object obj);
     }

Do you think class String implements interface Comparable? Provide a reason for your answer.

I would say no because there is no string anywhere in the class, but I believe I am misunderstanding the question. Can Someone explain this one better for me?

Answer 1

You should look into the source code or the Java doc of the class String . It is explicitly implementing the interface Comparable .

http://docs.oracle.com/javase/7/docs/api/java/lang/String.html

java.lang

Class String

java.lang.Object

java.lang.String

All Implemented Interfaces:

Serializable, CharSequence, Comparable< String >

Answer 2

I'm going to give you other point of view for the question and I'm goint to say No, it doesn't.

Since Java 6, java.lang.String can't implement that interface because that is not compatible with Comparable<String> and Comparable is not the same as Comparable<String> . The diference is that you can't compare an String with an Object

Answer 3

String implements Comparable.

Why it does so?

So that sorting can be done on strings using compareTo method. For example you want to sort employees based on their name and name is string so you use this as:-

 import java.util.ArrayList;
import java.util.Collections;
import java.util.Iterator;
import java.util.List;

public class Employee implements Comparable {

    String name;

    public Employee(String name) {
        this.name = name;
    }

    public int compareTo(Object o) {
        Employee employeeAnother = (Employee) o;
        // natural alphabetical ordering by type
        // if equal returns 0, if greater returns +ve int,
        // if less returns -ve int
        return this.name.compareTo(employeeAnother.name);// String compareTo is
                                                            // called here
    }

    public static void main(String[] args) {
        List list = new ArrayList();
        list.add(new Employee("Ramesh"));
        list.add(new Employee("Deepak"));
        list.add(new Employee("Vivek"));
        Collections.sort(list); // sorts using compareTo method
        for (Iterator iter = list.iterator(); iter.hasNext();) {
            Employee employee = (Employee) iter.next();
            System.out.println(employee);
        }
    }

    public String toString() {
        return name;
    }
} 

I have not used Generics to keep things simple. You will get few warnings but you can ignore these.

Answer 4

If you want to check if the given class implements some interfaces or other class you could use instanceof :

System.out.println("string" instanceof Comparable);

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You could also try to execute method from the given interface:

"one".compareTo("two")

In this case your code will compile or appropriate error message will be shown.

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However the best way is to read the source code of the given class. In this case you don't have to write any additional code. Also, a good IDE (like Eclipse, Intellij Idea) could help to find the answer.

Answer 5

yes!! In Java 8; open Java doc and you will find below:

public final class String
    implements java.io.Serializable, Comparable<String>, CharSequence