Why should a Java class implement comparable

Question

Why is Java Comparable used   Why would someone implement Comparable in a class  What is a real life example where you need to implement comparable

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OK  but why not just  define a compareTo   method without implementing comparable interface  For example a class City defined by its name and temperature and   public int compareTo City theOther        if  this temperature  lt  theOther temperature          return -1      else if  this temperature  gt  theOther temperature          return 1      else         return 0

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The fact that a class implements Comparable means that you can take two objects from that class and compare them  Some classes  like certain collections  sort function in a collection  that keep objects in order rely on them being comparable  in order to sort you need to know which object is the  biggest  and so forth

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Quoted from the javadoc      This interface imposes a total   ordering on the objects of each class   that implements it  This ordering is   referred to as the class s natural   ordering  and the class s compareTo   method is referred to as its natural   comparison method       Lists  and arrays  of objects that   implement this interface can be sorted   automatically by Collections sort  and   Arrays sort   Objects that implement   this interface can be used as keys in   a sorted map or as elements in a   sorted set  without the need to   specify a comparator    Edit    and made the important bit bold

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Comparable is used to compare instances of your class  We can compare instances from many ways that is why we need to implement a method compareTo in order to know how  attributes  we want to compare instances   Dog class   package test  import java util Arrays   public class Main        public static void main String   args            Dog d1   new Dog  brutus            Dog d2   new Dog  medor            Dog d3   new Dog  ara            Dog   dogs   new Dog 3           dogs 0    d1          dogs 1    d2          dogs 2    d3           for  int i   0  i  lt  3  i                  System out println dogs i  getName                                      Output             brutus            medor            ara                      Arrays sort dogs  Dog NameComparator           for  int i   0  i  lt  3  i                  System out println dogs i  getName                                      Output             ara            medor            brutus                        Main class   package test   import java util Arrays   public class Main        public static void main String   args            Dog d1   new Dog  brutus            Dog d2   new Dog  medor            Dog d3   new Dog  ara            Dog   dogs   new Dog 3           dogs 0    d1          dogs 1    d2          dogs 2    d3           for  int i   0  i  lt  3  i                  System out println dogs i  getName                                      Output             brutus            medor            ara                      Arrays sort dogs  Dog NameComparator           for  int i   0  i  lt  3  i                  System out println dogs i  getName                                      Output             ara            medor            brutus                        Here is a good example how to use comparable in Java   http   www onjava com pub a onjava 2003 03 12 java comp html page 2

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Comparable defines a natural ordering  What this means is that you re defining it when one object should be considered  less than  or  greater than     Suppose you have a bunch of integers and you want to sort them  That s pretty easy  just put them in a sorted collection  right    TreeSet lt Integer gt  m   new TreeSet lt Integer gt      m add 1   m add 3   m add 2   for  Integer i   m         values will be sorted   But now suppose I have some custom object  where sorting makes sense to me  but is undefined  Let s say  I have data representing districts by zipcode with population density  and I want to sort them by density   public class District     String zipcode     Double populationDensity      Now the easiest way to sort them is to define them with a natural ordering by implementing Comparable  which means there s a standard way these objects are defined to be ordered    public class District implements Comparable lt District gt     String zipcode     Double populationDensity    public int compareTo District other          return populationDensity compareTo other populationDensity           Note that you can do the equivalent thing by defining a comparator  The difference is that the comparator defines the ordering logic outside the object  Maybe in a separate process I need to order the same objects by zipcode - in that case the ordering isn t necessarily a property of the object  or differs from the objects natural ordering  You could use an external comparator to define a custom ordering on integers  for example by sorting them by their alphabetical value   Basically the ordering logic has to exist somewhere  That can be -   in the object itself  if it s naturally comparable  extends Comparable -e g  integers  supplied in an external comparator  as in the example above

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An easy way to implement multiple field comparisons is with Guava s ComparisonChain - then you can say     public int compareTo Foo that         return ComparisonChain start             compare lastName  that lastName            compare firstName  that firstName            compare zipCode  that zipCode            result           instead of    public int compareTo Person other        int cmp   lastName compareTo other lastName       if  cmp    0          return cmp            cmp   firstName compareTo other firstName       if  cmp    0          return cmp            return Integer compare zipCode  other zipCode

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Most of the examples above show how to reuse an existing comparable object in the compareTo function  If you would like to implement your own compareTo when you want to compare two objects of the same class  say an AirlineTicket object that you would like to sort by price less is ranked first   followed by number of stopover  again  less is ranked first   you would do the following   class AirlineTicket implements Comparable lt Cost gt        public double cost      public int stopovers      public AirlineTicket double cost  int stopovers                this cost   cost  this stopovers   stopovers              public int compareTo Cost o                if this cost    o cost            return Double compare this cost  o cost     sorting in ascending order           if this stopovers    o stopovers            return this stopovers - o stopovers    again  ascending but swap the two if you want descending         return 0

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Here is a real life sample  Note that String also implements Comparable   class Author implements Comparable lt Author gt       String firstName      String lastName        Override     public int compareTo Author other              compareTo should return  lt  0 if this is supposed to be            less than other   gt  0 if this is supposed to be greater than             other and 0 if they are supposed to be equal         int last   this lastName compareTo other lastName           return last    0   this firstName compareTo other firstName    last            later           List the authors  Sort them by name so it will look good      public List lt Author gt  listAuthors        List lt Author gt  authors   readAuthorsFromFileOrSomething        Collections sort authors       return authors            List unique authors  Sort them by name so it will look good      public SortedSet lt Author gt  listUniqueAuthors        List lt Author gt  authors   readAuthorsFromFileOrSomething        return new TreeSet lt Author gt  authors

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For example when you want to have a sorted collection or map

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When you implement Comparable interface  you need to implement method compareTo    You need it to compare objects  in order to use  for example  sorting method of ArrayList class  You need a way to compare your objects to be able to sort them  So you need a custom compareTo   method in your class so you can use it with the ArrayList sort method  The compareTo   method returns -1 0 1    I have just read an according chapter in Java Head 2 0  I m still learning

[java] Why should a Java class implement comparable?

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