How to implement Three Level Sort ?

Question

I am performing a complex search and get a List of Object. Now, the problem is I have to sort this list in three levels. That is,
If I have Object like this
public class A {
private String id ; // unique for every record
private String attr1;
private String attr2;
private String attr3;
private String attr4;
then I have to Sort the List of above type of objects
First level -attr1 as Asc
Second level -attr2 as Asc
third level - attr3 as desc
Could u suggest some way to implement this.

Answer 1

I would suggest using the built in Collections framework for sorting.
have class A implement the Comparable interface then implement the compareTo method as follows:
public int compareTo(Object o) {
    A a = (A)o;  // <-- may cause ClassCastException if (o instanceof A) != true, but that should not be a problem.
    int value = attr1.compareTo(a.attr1);
    if (value != 0) return value;
    value = attr2.compareTo(a.attr2);
    if (value != 0) return value;
    value = a.attr3.compareTo(attr3);  //Notice I switched a.attr3 and attr3 for desc
    return value;
}That should do it for you.
Now put everything into an instance of java.util.List and pass that to java.util.Collections.sort(java.util.List);
If you didn't create class A, and therefore can't add a method to it, then implement a Comparator. The idea is the same, just look at the java.util.Comparator API and use java.util.Collection.sort(java.util.List, java.util.Comparator) method to sort;