Java – find friends of all users: how to implement Hadoop MapReduce?

Say I entered the following:

(1,2)(2,1)(1,3)(3,4)(4,1)

The output is expected to be as follows:

(1,(2,3,4)) -> (1,3) //second index is total friend #
(2,(1,4)) -> (2,3)
(3,2))   -> (3,2)
(4,2))   -> (4,2)

I know how to do this in Java using HashSet But I don't know how to use MapReduce model Can anyone throw out any ideas or sample code on this issue? I would appreciate it

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This is my naive solution: 1 mapper, 2 reducers Mapper will organize input (1,2), 1), 3);

Organize output as

*(1, HashSet's & lt 2 –), HashSet's & lt 1 is halogen), HashSet's & 3; 1 +), (3, the & lt 1 of HashSet is halogen;) *

Reducer1：

Take mapper's output as input and output:

*(1, HashSet < 2,3 >), HashSet < 1 >) and (2, HashSet < 1 >)*

Reducer2：

Take the output of reducer1 as input and output:

*(1,1) and (2,1)*

This is just my naive solution I'm not sure if this can be done with Hadoop code

Solution

I think there should be a simple way to solve this problem

Mapper Input: (1,1)

Only two records need to be issued for each pair, as follows:

Mapper Output/ Reducer Input:

Key => Value
1 => 2
2 => 1
2 => 1
1 => 2
1 => 3
3 => 1
3 => 2
2 => 3
2 => 4
4 => 2
4 => 1
1 => 1

In terms of reducer, you will get four different groups, as shown below:

Reducer Output:

Key => Values
1 => [2,4]
2 => [1,4]
3 => [1,2]
4 => [1,2]

You can now format the results as needed

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