How do I write a compareTo method which compares objects

Question

I am learning about arrays  and basically I have an array that collects a last name  first name  and score   I need to write a compareTo method that will compare the last name and then the first name so the list could be sorted alphabetically starting with the last names  and then if two people have the same last name then it will sort the first name   I m confused  because all of the information in my book is comparing numbers  not objects and Strings   Here is what I have coded so far  I know it s wrong but it at least explains what I think I m doing   public int compare Object obj     creating a method to compare           Student s    Student  obj     creating a student object         I guess here I m telling it to compare the last names      int studentCompare   this lastName compareTo s getLastName           if  studentCompare    0          return studentCompare      else                if  this getLastName    lt  s getLastName                return - 1           if  this getLastName    gt  s getLastName                return 1            return 0      I know the  lt  and  gt  symbols are wrong  but like I said my book only shows you how to use the compareTo

User · Answer

Listen to  milkplusvellocet  I d recommend you to implement the Comparable interface to your class as well   Just contributing to the answers of others   String compareTo   will tell you how different a string is from another   e g  System out println   Test  compareTo  Tesu      will print -1 and  System out println   Test  compareTo  Tesa      will print 19  and nerdy and geeky one-line solution to this task would be   return this lastName equals s getLastName      this lastName compareTo s getLastName      this firstName compareTo s getFirstName      Explanation   this lastName equals s getLastName    checks whether lastnames are the same or not this lastName compareTo s getLastName    if yes  then returns comparison of last name  this firstName compareTo s getFirstName    if not  returns the comparison of first name

User · Answer

This is the right way to compare strings   int studentCompare   this lastName compareTo s getLastName        This won t even compile   if  this getLastName    lt  s getLastName      Use     if  this getLastName   compareTo s getLastName     lt  0  instead   So to compare fist last name order you need   int d   getFirstName   compareTo s getFirstName     if  d    0      d   getLastName   compareTo s getLastName     return d

User · Answer

I wouldn t have an Object type parameter  no point in casting it to Student if we know it will always be type Student   As for an explanation   result    0  will only occur when the last names are identical  at which point we compare the first names and return that value instead   public int Compare Object obj               Student student    Student  obj      int result   this getLastName   compareTo  student getLastName           if   result    0                 result   this getFirstName   compareTo  student getFirstName                 return result

User · Answer

If you using compare To method of the Comparable interface in any class  This can be used to arrange the string in Lexicographically   public class Student   implements Comparable lt Student gt    public int compareTo Object obj   if this  obj       return 0    if obj  null       String objName     Student obj  getName        return this name comapreTo  objName

User · Answer

A String is an object in Java   you could compare like so   if this lastName compareTo s getLastName      0   last names are the same

User · Answer

You re almost all the way there   Your first few lines  comparing the last name  are right on track   The compareTo   method on string will return a negative number for a string in alphabetical order before  and a positive number for one in alphabetical order after   Now  you just need to do the same thing for your first name and score     In other words  if Last Name 1    Last Name 2  go on a check your first name next   If the first name is the same  check your score next    Think about nesting your if then blocks

User · Answer

if  s compareTo t   gt  0  will compare string s to string t and return the int value you want       public int Compare Object obj     creating a method to compare              Student s    Student  obj    creating a student object             compare last names         return  this lastName compareTo s getLastName                 Now just test for a positive negative return from the method as you would have normally   Cheers

User · Answer

Consider using the Comparator interface described here which uses generics so you can avoid casting Object to Student    As Eugene Retunsky said  your first part is the correct way to compare Strings  Also if the lastNames are equal I think you meant to compare firstNames  in which case just use compareTo in the same way

User · Answer

The compareTo method is described as follows      Compares this object with the specified object for order  Returns a   negative integer  zero  or a positive integer as this object is less   than  equal to  or greater than the specified object    Let s say we would like to compare Jedis by their age   class Jedi implements Comparable lt Jedi gt         private final String name      private final int age                    Then if our Jedi is older than the provided one  you must return a positive  if they are the same age  you return 0  and if our Jedi is younger you return a negative   public int compareTo Jedi jedi       return this age  gt  jedi age   1   this age  lt  jedi age   -1   0      By implementing the compareTo method  coming from the Comparable interface  your are defining what is called a natural order  All sorting methods in JDK will use this ordering by default   There are ocassions in which you may want to base your comparision in other objects  and not on a primitive type  For instance  copare Jedis based on their names  In this case  if the objects being compared already implement Comparable then you can do the comparison using its compareTo method   public int compareTo Jedi jedi       return this name compareTo jedi getName         It would be simpler in this case   Now  if you inted to use both name and age as the comparison criteria then you have to decide your oder of comparison  what has precedence  For instance  if two Jedis are named the same  then you can use their age to decide which goes first and which goes second   public int compareTo Jedi jedi       int result   this name compareTo jedi getName         if result    0           result   this age  gt  jedi age   1   this age  lt  jedi age   -1   0            return result      If you had an array of Jedis  Jedi   jediAcademy    new Jedi  Obiwan  80   new Jedi  Anakin   30         All you have to do is to ask to the class java util Arrays to use its sort method   Arrays sort jediAcademy     This Arrays sort method will use your compareTo method to sort the objects one by one

[java] How do I write a compareTo method which compares objects?

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