What does implements do on a class

Question

If a class implements another class    what does that mean   I found this code sample  http   www java2s com Code Php Class extendsandimplement htm  But unfortunately it doesn t have any explanation with it

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In Java a class can implement an interface  See http   en wikipedia org wiki Interface  Java  for more details  Not sure about PHP   Hope this helps

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An interface defines a simple contract of methods all implementing classes must implement  When a class implements an interface  it must provide implementations for all its methods   I guess the poster assumes a certain level of knowledge about the language

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Interfaces are implemented through classes  They are purely abstract classes  if you will   In PHP when a class implements from an interface  the methods defined in that interface are to be strictly followed  When a class inherits from a parent class  method parameters may be altered  That is not the case for interfaces   interface ImplementMeStrictly      public function foo  a   b      class Obedient implements ImplementMeStrictly      public function foo  a   b   c                       will cause an error  because the interface wasn t implemented as defined  Whereas   class InheritMeLoosely      public function foo  a                     class IDoWhateverWithFoo extends InheritMeLoosely      public function foo                       Is allowed

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You should look into Java s interfaces  A quick Google search revealed this page  which looks pretty good   I like to think of an interface as a  promise  of sorts  Any class that implements it has certain behavior that can be expected of it  and therefore you can put an instance of an implementing class into an interface-type reference   A simple example is the java lang Comparable interface  By implementing all methods in this interface in your own class  you are claiming that your objects are  comparable  to one another  and can be partially ordered   Implementing an interface requires two steps    Declaring that the interface is implemented in the class declaration Providing definitions for ALL methods that are part of the interface    Interface java lang Comparable has just one method in it  public int compareTo Object other   So you need to provide that method   Here s an example  Given this class RationalNumber   public class RationalNumber       public int numerator      public int denominator       public RationalNumber int num  int den                this numerator   num          this denominator   den             Note  It s generally bad practice in Java to have public fields  but I am intending this to be a very simple plain-old-data type so I don t care about public fields    If I want to be able to compare two RationalNumber instances  for sorting purposes  maybe    I can do that by implementing the java lang Comparable interface  In order to do that  two things need to be done  provide a definition for compareTo and declare that the interface is implemented   Here s how the fleshed-out class might look   public class RationalNumber implements java lang Comparable       public int numerator      public int denominator       public RationalNumber int num  int den                this numerator   num          this denominator   den             public int compareTo Object other                if  other    null      other instanceof RationalNumber                         return -1     Put this object before non-RationalNumber objects                    RationalNumber r    RationalNumber other              Do the calculations by cross-multiplying  This isn t really important to            the answer  but the point is we re comparing the two rational numbers             And no  I don t care if it s mathematically inaccurate           int myTotal   this numerator   other denominator          int theirTotal   other numerator   this denominator           if  myTotal  lt  theirTotal  return -1          if  myTotal  gt  theirTotal  return 1          return 0            You re probably thinking  what was the point of all this  The answer is when you look at methods like this  sorting algorithms that just expect  some kind of comparable object    Note the requirement that all objects must implement java lang Comparable   That method can take lists of ANY kind of comparable objects  be they Strings or Integers or RationalNumbers   NOTE  I m using practices from Java 1 4 in this answer  java lang Comparable is now a generic interface  but I don t have time to explain generics

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It is called an interface  Many OO languages have this feature  You might want to read through the php explanation here  http   de2 php net interface

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An interface can be thought of as just a list of method definitions  without any body   If a class wants to implement and interface  it is entering into a contract  saying that it will provide an implementation for all of the methods listed in the interface  For more information  see http   download oracle com javase tutorial java concepts

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Implements means that it takes on the designated behavior that the interface specifies   Consider the following interface   public interface ISpeak      public String talk       public class Dog implements ISpeak      public String talk                return  bark            public class Cat implements ISpeak       public String talk                 return  meow              Both the Cat and Dog class implement the ISpeak interface   What s great about interfaces is that we can now refer to instances of this class through the ISpeak interface   Consider the following example     Dog dog   new Dog      Cat cat   new Cat       List lt ISpeak gt  animalsThatTalk   new ArrayList lt ISpeak gt        animalsThatTalk add dog     animalsThatTalk add cat      for  ISpeak ispeak   animalsThatTalk            System out println ispeak talk           The output for this loop would be      bark     meow    Interface provide a means to interact with classes in a generic way based upon the things they do  without exposing what the implementing classes are   One of the most common interfaces used in Java  for example  is Comparable   If your object implements this interface  you can write an implementation that consumers can use to sort your objects   For example   public class Person implements Comparable lt Person gt      private String firstName    private String lastName        Getters Setters    public int compareTo Person p          return this lastName compareTo p getLastName               Now consider this code       Some code in other class  List lt Person gt  people   getPeopleList     Collections sort people     What this code did was provide a natural ordering to the Person class   Because we implemented the Comparable interface  we were able to leverage the Collections sort   method to sort our List of Person objects by its natural ordering  in this case  by last name

[java] What does "implements" do on a class?

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