I am practicing for interviews and I am given a question to create the String method indexOf without the use of string methods. My first thought was to process the String into a char[] but I am not sure how to do so without the use of .toCharArray()
If anyone has ever had this interview question asked to them I would love your input.
Without using any method provided by String the only option to "convert" a string to a character array would be the use of reflection:
char[] c = String.class.getDeclaredField( "value" ).get( "your string" );
Note that you'd have to catch exceptions etc.
And a big note: this is very unsafe, since you never know if the field is called value in any implementation. This is not an intended usage of the String class. Also note that the resulting array might be bigger than the actual string, i.e. the null termination character might be at any location.
char[]If your input is a CharSequence, you can do so like this:
CharSequence str = yourString;
char[] chars = new char[str.length()];
for (int i = chars.length; i-->0;) {
chars[i] = str.charAt(i);
}
//chars now contains the characters of the string
length() and chatAt() methods, so I assume not the best answer!
That's a hard one; honestly I don't know.
I wonder of these answers help
What is the easiest/best/most correct way to iterate through the characters of a string in Java?
There's one answer which uses StringCharacterIterator.
public static int customIndexOf(String string,char character) {
String c = String.valueOf(character);
Scanner sc = new Scanner(string).useDelimiter(Pattern.compile(""));
int i=0;
while(sc.hasNext()) {
if (sc.next().equals(c)) {
return i;
}
i++;
}
return -1;
}
You can define an utility method that gets access to the value instance variable of String and returns the first position of a charachter in that string if it exists or -1 if it does not:
public class ReflectedUtils {
public static final int indexOf(String s, char c)
{
int position = -1;
try {
Class clazz = Class.forName("java.lang.String");
Constructor constructor = clazz.getDeclaredConstructor(clazz);
String ztring = (String) constructor.newInstance(s);
Field field = ztring.getClass().getDeclaredField("value");
field.setAccessible(true);
char[] chars = (char[]) field.get(ztring);
for (int i=0; i<chars.length; i++)
{
if(chars[i] == c)
{
position = i;
break;
}
}
} catch (ClassNotFoundException e) {
e.printStackTrace();
} catch (InstantiationException e) {
e.printStackTrace();
} catch (IllegalAccessException e) {
e.printStackTrace();
} catch (NoSuchMethodException e) {
e.printStackTrace();
} catch (InvocationTargetException e) {
e.printStackTrace();
} catch (NoSuchFieldException e) {
e.printStackTrace();
}
finally {
return position;
}
}
public static void main(String... args)
{
System.out.print(String.valueOf(ReflectedUtils.indexOf("Hello", 'e')));
}
}
Stringat all - i.e.int indexOf(char[] str, char[] find)or similar. (Which is still not 'sensible', or course.)charAt()method.StringimplementsCharSequence. So you could write the method entirely against the abstractionCharSequence. Would that count as not 'using' methods fromString?