Please help me to uderstand the use of "->" in Arrays.sort()

Issue

I was solving problems in geeksforgeeks and suddenly got this. I understand the use of "Integer.compare" but could not get the use of "->" in the sortBySetBitCount method.

static void sortBySetBitCount(Integer arr[], int n)
{ 
    Arrays.sort(arr, (a, b) -> Integer.compare(countSetBit(b), countSetBit(a)));
         //I mean here the "->" operator
} 

static int countSetBit(int n) {
    int count = 0;
    while (n>0) {
        n = n & (n - 1);
        count += 1;
    }
    return count;
}

Solution

Lambda expressions are added in Java 8 and provide below functionalities.

Enable to treat functionality as a method argument, or code as data.

A function that can be created without belonging to any class.

A lambda expression can be passed around as if it was an object and executed on demand.

Syntax:

lambda operator -> body

where lambda operator can be:

Zero parameter:

() -> System.out.println("Zero parameter lambda");

One parameter:–

(p) -> System.out.println("One parameter: " + p);

It is not mandatory to use parentheses, if the type of that variable can be inferred from the context

Multiple parameters :

(p1, p2) -> System.out.println("Multiple parameters: " + p1 + ", " + p2);

Source for above examples GeeksForGeeks.

import java.util.*;

class MainClass{
    public static void main(String ... $){
        var out = System.out;
        Integer arr[] = {10, 32, 31};
        sortBySetBitCount(arr);
        //output is 31, 10, 32
        //31 has 5 set bits
        //10 has 3 set bits
        //32 has 1 set bit
        out.println(Arrays.asList(arr));

    }
    static void sortBySetBitCount(Integer arr[]){ 
        //first argument is array they we want to sort
        //second argument is a lambda method
        Arrays.sort(arr, (a, b) -> Integer.compare(countSetBit(b), countSetBit(a)));
    } 
    //Returns numbers of bit set
    static int countSetBit(int n) {
        int count = 0;
        while (n != 0) {
            n = n & (n - 1);
            count += 1;
        }
        return count;
    }
}

output:

$ javac MainClass.java && java MainClass 
[31, 10, 32]

More about lambda methods:

Lambda method in java

Answered By - Dev Parzival