how to input in a nested arraylist

You clear the list after adding it to the list of lists. You need to create a new list instead:

import java.util.*;

public class MyClass {
    public static void main(String args[]) {
        List<List<Integer>> l = new ArrayList<List<Integer>>();
        List<Integer> a = new ArrayList<Integer>();
        a.add(1);
        a.add(2);
        l.add(a);
        List<Integer> b = new ArrayList<>();  // new list!
        b.add(4);
        b.add(5);
        l.add(b);
        System.out.println(l+" | ");
    }
}

Background: When you create the list a, that is an object. You then add elements to that list. Then you add list a to l. l now has a reference to a, it does not make a copy of a. Then you clear a, which means l now has a reference to the empty list a. Then you add new items to a, and add a to l again. That means that l now has two references to the same list a. Hence when you print l, it prints a twice.

As @Robert has explained, a in the original code is a single reference added several times to the list l. However, it is possible to simply create a copy of a list when adding it to the outer list using its constructor:

List<List<Integer>> l = new ArrayList<>();
List<Integer> a = new ArrayList<>();
a.add(1);
a.add(2);
l.add(new ArrayList<>(a)); // copy added to l

a.clear();
a.add(4);
a.add(5);
l.add(new ArrayList<>(a)); // another copy added to l
System.out.println(l+" | ");

Another approach would be to avoid creating a temporary list a at all and populate list l using Arrays.asList which creates a fixed-size list:

List<List<Integer>> l = new ArrayList<>(); // modifiable

l.add(Arrays.asList(1, 2));
l.add(Arrays.asList(4, 5));

System.out.println(l+" | ");

you are cleaning the list before printing, you can type the below code:

import java.util.*;

public class Main {
    public static void main(String[] args) throws Exception {
        List<List<Integer>> l = new ArrayList<List<Integer>>();
        List<Integer> y = new ArrayList<Integer>();
        y.add(1);
        y.add(2);
        l.add(y);
        List<Integer> x = new ArrayList<>();  
        x.add(4);
        x.add(5);
        l.add(x);
        System.out.println(l+" | ");
    }
}