Why not just String substr = word.substring(word.length() - 3)?

Update

Please make sure you check that the String is at least 3 characters long before calling substring():

if (word.length() == 3) {
  return word;
} else if (word.length() > 3) {
  return word.substring(word.length() - 3);
} else {
  // whatever is appropriate in this case
  throw new IllegalArgumentException("word has fewer than 3 characters!");
}

So there are two problems here:

Firstly you're calling fname.Length(3), which doesn't make sense as String doesn't have a Length(int n) method on it. What it does have is a substring(int) method and a length() method, which you can use as follows:

String middlePart = fname.substring(fname.length() - 3);

As outlined in the linked JavaDocs, String.substring() "Returns a new string that is a substring of this string. The substring begins with the character at the specified index and extends to the end of this string.". So if we can provide it with the index (or position) within the String fname where we want to start copying from.

If I've got a String such as "Chicken", and I want the last 3 characters, I'd call "Chicken".substring(4), and the result would be "ken" (Strings are zero-indexed, so the character 'k' has index 4).

Instead of hard coding the index where I want to start the substring from, I use the String.length() method which tells me how long a String is, and subtract 3. In the above example, "Chicken".length() is 7, and so "Chicken".length() - 3 is the index where you should start substring-ing if you want the last 3 characters.

Why has nobody given the obvious answer?

sed 's/.*\(...\)/\1/'

… or the slightly less obvious

grep -o '...$'

Admittedly, the second one has the drawback that lines with fewer than three characters vanish; but the question didn’t explicitly define the behavior for this case.

Assuming that the url will end with .XXX (continuous extension)

String[] splitted = url.split("\\.");
return (splitted[splitted.length-1]); //will return the last string after the last "."

Where url is your url (www.google.com in this case).

Note that if the url is http://www.google.com/ then the code will return .com/. So you need to perform a check if the result contains / character. (Hint: look for contains in the String API).

I think you should create a new list to storage the sub string. Do you know when your for-cycle break? I debugged it and get that it will break when your alist.size()=0. The exception causes when the aList.get(i).length()<3. So you just do like this:

`
        List<String> aList = new ArrayList<String>();
        aList.add("4:78:34");
        aList.add("5:8:34");
        aList.add("8:18:90");
        aList.add("2:8:40");
        List<String> subList = new ArrayList<String>();
        for (int i = 0; i < aList.size(); i++)
        {

            String str = aList.get(i).substring(0, aList.get(i).length() - 3);
            subList.add(str);

        }
        System.out.println(subList);`

Good try. The only problem is you choose the wrong remainder of the division since elements start from 0.

Try this condition:

if (i % 3 == 2)

That piece of code does exactly the opposite of what you want. Now let's see why and how we can modify it.

Quick solution

You can modify the code as follows to do what you want:

 String lastNchars = s.substring( Math.max(0, s.length()-n));

Explanation

According to the official documentation, Java String class has a special method called substring(). The signature of the method is the following (with overload):

 public String substring(int beginIndex, int endIndex))
 public String substring(int beginIndex)

The first method accepts 2 parameters as input:

1. beginIndex: the begin index of the substring, inclusive.
2. endIndex: the end index of the substring, exclusive.

The second overload will automatically consider as endIndex the length of the string, thus returning "the last part"

Both methods return a new String Object instance according to the input parameters just described.

How do you pick up the right sub-string from a string? The hint is to think at the strings as they are: an array of chars. So, if you have the string Hello world you can logically think of it as:

[H][e][l][l][o][ ][w][o][r][l][d]
[0]...............[6]......[9][10]

If you choose to extract only the string world you can thus call the substring method giving the right "array" indexes (remember the endIndex is exclusive!):

 String s = "Hello world";
 s.substring(6,11);

In the code snippet you provided, you give a special endIndex:

 Math.min(s.length(), n);

That is exactly up to the n th char index taking into account the length of the string (to avoid out of bound conditions).

What we did at the very beginning of this answer was just calling the method and providing it with the beginning index of the substring, taking into account the possible overflow condition if you choose a wrong index.

Please note that any String Object instance can take advantage of this method, take a look at this example, for instance:

 System.out.println("abc");
 String cde = "cde";
 System.out.println("abc" + cde);
 String c = "abc".substring(2,3);
 String d = cde.substring(1, 2);

As you see even "abc", of course, has the substring method!