how to find occurrence in List/ArrayList

Question

I have a question on how to find the number of occurrences in a list. In my case, for part of my program, I am trying to find the number of occurrences of 1, 2, 3, and 4's of a hurricane category and displaying them. I tried using a for loop and an if statement but when I ran it, I got zero for the output. How can I fix this? Any help will be greatly appreciated. Below is a snippet of my program:

Hurricanes2.java:

// category 1 occurrence
     int i = 0;
     int ii = 0;
     int category1 = category.get(i);
     for(int j = 0; j < category.size(); j++){
         if(category1 == 1){
             ii++;
         }
     }
System.out.printf("%1s%10d%n", "Category 1 occurrence ~", ii);

When ran I get this:

run:
                        Hurricanes 1980 - 2006

Year           Hurricane            Category        Pressure(mb)    Wind Speed (mph)
____________________________________________________________________________________
1980               Allen                   2                 100                 945
1983              Alicia                   2                 100                 962
1984               Diana                   2                 100                 949
1985                 Bob                   1                  65                1002
1985               Danny                   1                  80                 987
1985               Elena                   2                 100                 959
1985              Gloria                   1                  90                 942
1985                Juan                   1                  75                 971
1985                Kate                   1                  85                 967
1986              Bonnie                   1                  75                 990
1986             Charley                   1                  65                 990
1987               Floyd                   1                  65                 993
1988            Florence                   1                  70                 984
1989             Chantal                   1                  70                 986
1989                Hugo                   3                 120                 934
1989               Jerry                   1                  75                 983
1991                 Bob                   1                  90                 962
1992              Andrew                   4                 145                 922
1993               Emily                   2                 100                 960
1995                Erin                   1                  85                 973
1995                Opal                   2                 100                 942
1996              Bertha                   1                  90                 974
1996                Fran                   2                 100                 954
1997               Danny                   1                  70                 984
1998              Bonnie                   1                  95                 964
1998                Earl                   1                  70                 987
1998             Georges                   1                  90                 964
1999                Bret                   2                 100                 951
1999               Floyd                   1                  90                 956
1999               Irene                   1                  70                 987
2002                Lili                   1                  80                 963
2003           Claudette                   1                  80                 979
2003              Isabel                   1                  90                 957
2004                Alex                   1                  70                 972
2004             Charley                   4                 130                 941
2004              Gaston                   1                  65                 985
2004             Frances                   1                  90                 960
2004                Ivan                   2                 105                 946
2004              Jeanne                   2                 105                 950
2005               Cindy                   1                  65                 992
2005              Dennis                   4                 130                 930
2005               Emily                   4                 135                 929
2005               Irene                   1                  85                 975
2005             Katrina                   4                 150                 902
2005               Maria                   2                 100                 960
2005                Nate                   1                  80                 979
2005             Ophelia                   1                  80                 976
2005            Phillipe                   1                  70                 985
2005                Rita                   4                 150                 897
2005                Stan                   1                  70                 979
2005               Vince                   1                  65                 987
2005               Wilma                   4                 150                 882
2005                Beta                   2                 100                 960
2005             Epsilon                   1                  75                 979
2006             Ernesto                   1                  65                 995
2006            Florence                   1                  80                 972
2006              Gordon                   2                 105                 955
2006              Helene                   2                 110                 954
2006               Isaac                   1                  75                 985
____________________________________________________________________________________
Average ~                                  1                  91                 963
Maximum ~                                  4                 150                1002
Minimum ~                                  1                  65                 882
Category 1 occurrence ~         0
BUILD SUCCESSFUL (total time: 0 seconds)

Answer 1

Use Collections.frequency().

Example : int frequency = Collections.frequency(category, category1)

Answer 2

Use an int[] as a counter, like this:

// we won't use the 0 position
int[] counter = new int[5];

Iterate over all the categories. Every time you find a new category, add one to the counter:

for (int i = 0; i < category.size(); i++) {
    int cat = category.get(i);
    // assuming that cat is 1, 2, 3 or 4
    counter[cat]++;
}

When the iteration ends, counter[1] will hold the number of occurrences of the first category, counter[2] will hold the number of occurrences of the second category, and so on.

Answer 3

you need to update what you're checking within the loop. As it stands each iteration of the loop is checking against the same value.

You probably want a

category.get(j) == 1 

or

if (category.get(j) <= 4)

Answer 4

Move the line

int category1 = category.get(i);

down a line so that it's inside the loop, and change the i to a j. That way, you'll be checking a different storm's category each time, rather than just looking at the first storm over and over.