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Java 8- Sorting Collections using lambda expression


Java 7 (old way)-
1. Create a comparator
2. Use Collections.sort or pass the comparator to constructor if you are using treeset or treemap
3. Use the comparator
a)     Option 1- Pass comparator as argument- Collections.sort(collection, comparator);
b)     Option 2- Keep collection sorted always
TreeSet<Integer> treeset = new TreeSet<>(comparator);
TreeMap<Integer, String> treemap = new TreeMap<>(comparator);
Below are few examples- note this can be done only for collections which maintain the sorting order and at the time of initialization you don’t default sorting or order but some custom sorting order. Below is an example of one of the constructors which directly accepts a comparator

Java 8- Using lambda expressions

As comparator is a functional interface (consists of single abstract method), we can use lambda expression to create the sorting expression. Below is Lamda expression for sorting a list in ascending order
(i1, i2) -> (i1-i2)
// for integers
Ascending sort- Collection.sort (col, (i1, i2) -> (i1-i2)); // follow regular sequence 1st & then 2nd
Descending sort- Collection.sort (col, (i1, i2) -> (i2-i1)); //Reverse the sequence, 2nd & then 1st
// for strings
Ascending sort- Collection.sort (col, (s1, s2) -> (s1.compareTo(s2)); // follow regular sequence 1st & then 2nd
Descending sort- Collection.sort (col, (s1, s2) -> (s2.compareTo(s1)); //Reverse the sequence, 2nd & then 1st

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