[imp] how can we sort the objects in Collections

[shyam sunder prasad]

class student{
int id;

}

List list=new ArrayList();
list.add(student);

i have added 10 student objects to list.

so ,i want to sort them in ascending order by student id;

can you help me.

[Rajasekar Krishnan]

Hi shyam,

You can use
1) <T> void java.util.Collections.sort(List<T> list) to sort the ArrayList, if your the student objects implements Comparable. or
2) <T> void java.util.Collections.sort(List<T> list, Comparator<? super T> c), if you have Comparator to compare Student Objects.

Thanks,
Rajasekar.

[shyam sunder prasad]

thanks rajashekar,

my code is like this:

class Student{
int id;

}
Student std1=new Student();
Student std2=new Student();
Student std3=new Student();

List ls=new ArrayList();
ls.add(std1);
ls.add(std1);
ls.add(std1);




so can you help me with code how to sort in ascending order.

[Rajasekar Krishnan]

Hi Shyam,

class Student implements Comparable{ 
	int id;

	Student(int i) {
		id = i;
	}
	public int compareTo(Object o) {
		return ((Student)o).id-this.id;
	} 

	public String toString(){
		return ""+id;
	}
}

this is one way of doing the sorting in java, We can try out Comparator if number of sorting method required by you.

Thanks,
Rajasekar.

[shyam sunder prasad]

thankyou Rajashekar.

[Campbell Ritchie]

Rajasekar Krishnan wrote:Hi Shyam,

class Student implements Comparable{ 
	int id;

	Student(int i) {
		id = i;
	}
	public int compareTo(Object o) {
		return ((Student)o).id-this.id;
	} 

	public String toString(){
		return ""+id;
	}
}

. ..

Not a good way to do it. You should be using generics

class Student implements Comparable<Student>{ 
	int id;

	Student(int i) {
		id = i;
	}
	public int compareTo(Student s) {
		return s.id-this.id;
	} 

	public String toString(){
		return String.valueOf(id);
	}
}

And of course, if there is a great difference between the ids you will have problems

Student s = new Student(2000000000);
Student ss = new Student(-200000000);
System.err.printf("s.compareTo(ss) returns %d%n", s.compareTo(ss));

and shyam sunder prasad, welcome to the Ranch

[Rajasekar Krishnan]

Hi Campbell,

Thanks for your comment.

Form my next post onwards I'll post the code with Generics (when Required).

thanks,
Rajasekar.

[shyam sunder prasad]

thanks for your reply.
your telling me that comparing two objects but i want to put them in ascending order in list as well.
tell me how.

[Joshua Barrett]

Write a class that implements Comparator.
This class should have a single method called compare, that returns an int, and takes 2 Object objects as parameters.
Inside this method Cast the Objects to your classes within your list. And write some code that compares the fields you want to sort by.
Basically if Object 1 should come before Object 2 you should return -1.

Check the API of the Interface if you are unsure what methods are required and what they should return.

Then use this static method to sort your list using the given comparator object

Collections.sort(yourList, new YourComparator());

Perhaps not the most efficent way, but it works, and its simple to understand.


This was the best way I could describe what you have to do without just giving you the code

[Campbell Ritchie]

Joshua Barrett wrote: . . . Cast the Objects to your classes within your list. . . .

Surely you write a class which implements the Comparator<Student> interface, then you can dispense with those casts.

[Joshua Barrett]

Campbell Ritchie wrote:

Joshua Barrett wrote: . . . Cast the Objects to your classes within your list. . . .

Surely you write a class which implements the Comparator<Student> interface, then you can dispense with those casts.

Blimeh, that would do away with the extra class! Never knew that existed. Ya learn something new every day

[Campbell Ritchie]

Joshua Barrett wrote: . . . Ya learn something new every day

That's why I came here in the first place. I have learned lots and lots here.