implementing the hash map

[sahana mithra]

I have a code where i count number of occurences of each letter. For performance sake i want to convet the same using hash map. Can any of you help me on how to implement hash map?
Here is my code

import java.io.*;

class Count_Char {
public static void main(String[] args) {
	try
	{
FileInputStream file = new FileInputStream("D:\\trial.txt");
DataInputStream dis = new DataInputStream(file);
BufferedReader br = new BufferedReader(new InputStreamReader(dis));
String Contents="";
String str="";
while ((Contents = br.readLine()) != null) {
str+=Contents;
}
char[]char_array =str.toCharArray();
for(int count =0;count<char_array.length;count++){
char ch= char_array[count];
int counter=0;
for ( int i=0; i<char_array.length; i++){
if (ch==char_array[i])
counter++;
}
boolean flag=false;
int j=count-1;
while(j>=0)
	{
	
	if(ch==char_array[j])
		flag=true;
		j--;
	}
if(!flag){
System.out.println(ch+" "+counter);
}
}
	}catch(IOException e1){
		System.out.println(e1);
	}
	}
}
 

[Rob Spoor]

sahana mithra wrote:

char[]char_array =str.toCharArray();
for(int count =0;count<char_array.length;count++){
char ch= char_array[count];

[/code]

First of all, a small tip. You now are copying the entire String into a new char[] (that's what toCharArray() does). You can instead write the same without this copying:

for (int count = 0; count < str.length(); count++) {
    char ch = str.charAt(count);

Now, back to the problem. You need a Map<Character,Integer>. The keys will be the characters, the values the number of occurrences. When you encounter a character you must:
1) retrieve its old value.
2) increase the old value. Keep in mind that for the first occurrence of the character there will be no old value so it will be null. I'll leave it up to you to find an appropriate new value.
3) put back the new value into the map, thereby overwriting the old value.

You can use auto-boxing to make life easier for you, as long as you remember that possible null value - unboxing null will lead to a NullPointerException.

As for the Map type to use, a LinkedHashMap will preserve the insertion order where a TreeMap will sort the keys.