java - alphabetically sort elements in a List -

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• sorting arraylist of objects object attribute 2 answers

i have shape class , in class there method called getrepr() can char representation of shape. example,

shapea.getrepr() ->'a'  shapeb.getrepr() ->'b' shapec.getrepr() ->'c' 

now have arraylist stores several shapes including shapee, shapea, shaped, shapec, , shapeb.

the question how can use collections.sort() alphabetically rearrange these shapes in arraylist according char representations?

the expected result in arraylist after sorting should shapea, shapeb, shapec, shaped, shapee.

or there way reach purpose without collections.sort()?

you need implement comparable interface in superclass

public class myclasssuperclass implements comparable<myclasssuperclass>{      @override     public int compareto(myclasssuperclass o) {         return this.getrepr().compareto(o.getrepr());     }   } 

then can call .sort method on collection

1. if obj a>b compareto shall return value > 0
2. if obj
3. if obj == b compareto shall return 0

for mathematically inclined, relation defines natural ordering on given class c is:

   {(x, y) such x.compareto(y) <= 0}.   quotient total order is:    {(x, y) such x.compareto(y) == 0}.   follows contract compareto quotient 

equivalence relation on c, , natural ordering total order on c. when class's natural ordering consistent equals, mean quotient natural ordering equivalence relation defined class's equals(object) method: {(x, y) such x.equals(y)}.

for more information please check link:

https://docs.oracle.com/javase/7/docs/api/java/lang/comparable.html