An even faster and more compact version than using a HashMap is to use a good old integer array. A char can actually be typecasted to an int, which gives it's ASCII code value.

String str = "Hello World";
int[] counts = new int[256];

for (int i = 0; i < str.length(); i++) {
    char charAt = str.charAt(i);
    counts[(int) charAt]++;
}

System.out.println(Arrays.toString(counts));

As the above output is a bit big, by looping through the integer array you can output just the characters which actually occur:

for (int i = 0; i < counts.length; i++) {
    if (counts[i] > 0)
        System.out.println("Number of " + (char) i + ": " + counts[i]);
}

A possibly faster, and at least more compact version than using a HashMap is to use a good old integer array. A char can actually be typecasted to an int, which gives it's ASCII code value.

String str = "Hello World";
int[] counts = new int[(int) Character.MAX_VALUE];
// If you are certain you will only have ASCII characters, I would use `new int[256]` instead

for (int i = 0; i < str.length(); i++) {
    char charAt = str.charAt(i);
    counts[(int) charAt]++;
}

System.out.println(Arrays.toString(counts));

As the above output is a bit big, by looping through the integer array you can output just the characters which actually occur:

for (int i = 0; i < counts.length; i++) {
    if (counts[i] > 0)
        System.out.println("Number of " + (char) i + ": " + counts[i]);
}