Here is the function where your get the occurrence of each character in any string with O(n) times.

public static void main(String args[]){
String str="mam is going";
HashMap<String, Integer> freqOfChar=calculateCharFrequency(str!=null?str.trim():"");
printFreeOfChar(freqOfChar);
}

static HashMap<String, Integer> calculateCharFrequency(String str){
	HashMap<String, Integer> freqOfChar = new HashMap<String, Integer>();
	String ch;
	if(str!=null){
		for(int i=0;i<str.length();i++){
			ch=Character.toString(str.charAt(i)).trim();
			if(ch.length()>0){
				if(freqOfChar.get(ch)!=null){
					freqOfChar.put(ch, (freqOfChar.get(ch)+1));
				}else{
					freqOfChar.put(ch, 1);
				}
			}
		}
	}
	return freqOfChar;
}

static void printFreeOfChar(HashMap<String, Integer>  freqOfChar){
	 for (String key : freqOfChar.keySet()) {
		 System.out.println("Char: "+key+ "; Occurance: "+freqOfChar.get(key));
	 }
	
}

output:

Char: o; Occurance: 1
Char: i; Occurance: 2
Char: m; Occurance: 2
Char: a; Occurance: 1
Char: n; Occurance: 1
Char: g; Occurance: 2
Char: s; Occurance: 1