Map in java - getting wrong map size

Question

I am trying to understand how Map interface works in Java. What I am trying to do is: run through the array of strings and for each name in the array name[] put a random gradebetween 0 to 5. Then map the gradeto name[i]. However, the map size gets weird, although I have 10 elements in array, the map.size()is 5 after mapping. Why does the program count the same size several times (see output)? Here is the code and output below:

    import java.util.*;  

    class MapInterfaceExample{  

    public static void main(String[] args){  

    int grade = 0; 
    String[] name = {"Lisa", "Dan", "John", "Adam", "George", "Amanda", "Sarah", "James", "Derek", "Sam"}; 

    Map<Integer,String> map=new HashMap<Integer,String>();  

    for(int i=0; i<name.length; i++){
    grade = (int)(Math.random()*5+1); 
    map.put(grade, name[i]);  
    //System.out.println(grade + "\t"+ name[i]);  
    System.out.println("Size of map "+ map.size());}
    } 
    }

Output:

Size of map 1

Size of map 2

Size of map 3

Size of map 4

Size of map 5

Are you sure the grades are the keys? If yes, do you want to collect al names with identical grades? — , Mar 08 '18 at 12:04
Check what kind of magic will happen if you map.put(name[i],grade); — Antoniossss, Mar 08 '18 at 12:11

Kayaman · Accepted Answer · 2018-03-08T12:06:34.390

5

Assigning a new value to the same key will override the old value, so if you have a maximum of 5 different keys (1-5) your map has a maximum size of 5.

You should use the name as the key, since they're unique. Then several students can have the same grade.

edited Mar 08 '18 at 12:06

answered Mar 08 '18 at 12:02

Kayaman

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@Kayaman, ah thanks, I understand what is going on now. The size is triggered by the value I get from `grade`. – sumu00 Mar 08 '18 at 12:05
@Pshemo, how do I get 0-5? Thanks. – sumu00 Mar 08 '18 at 12:06
@Pshemo err yeah, of course. – Kayaman Mar 08 '18 at 12:06
1

@sumu00 Lets start with what you have now: `(int)(Math.random()*5+1);`. `Math.random()` can only generate value from range `[0;1)` (or lets write it as `[0; 0.9999999...]`). When we multiply it `5*[0;1)` it gives us new range `[0; 4.9999999...]`. After adding `+1` range becomes `[1; 5.99999999]`. By casting to `(int)` we remove `.999...` part so range is `[1; 5]`. If you want to get `[0; 5]` you can create range `[0; 5.9999....]` before casting. To generate such range you need `6*[0; 0.99999...]`. So value from such range can be generated using `int value = (int)(6*Math.random())`. – Pshemo Mar 08 '18 at 12:16
@Pshemo, such a good answer! Thanks a lot! – sumu00 Mar 08 '18 at 13:22

score 3 · Answer 2 · edited Mar 08 '18 at 12:17

3

You use the grades as keys

map.put(grade, name[i]);

Since the range of grades is [1, 5], the map will not become larger than 5.

edited Mar 08 '18 at 12:17

Pshemo

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answered Mar 08 '18 at 12:02

thanks, now I figured out. – sumu00 Mar 08 '18 at 12:07

Map in java - getting wrong map size

2 Answers2