Java - Create a new String instance with specified length and filled with specific character. Best solution? [duplicate]

Simply use the StringUtils class from apache commons lang project. You have a leftPad method:

StringUtils.leftPad("foobar", 10, '*'); // Returns "****foobar"

No need to do the loop, and using just standard Java library classes:

protected String getStringWithLengthAndFilledWithCharacter(int length, char charToFill) {
  if (length > 0) {
    char[] array = new char[length];
    Arrays.fill(array, charToFill);
    return new String(array);
  }
  return "";
}

As you can see, I also added suitable code for the length == 0 case.

In Java 11, you have repeat:

String s = " ";
s = s.repeat(1);

(Although at the time of writing still subject to change)

Some possible solutions.

This creates a String with length-times '0' filled and replaces then the '0' with the charToFill (old school).

String s = String.format("%0" + length + "d", 0).replace('0', charToFill);

This creates a List containing length-times Strings with charToFill and then joining the List into a String.

String s = String.join("", Collections.nCopies(length, String.valueOf(charToFill)));

This creates a unlimited java8 Stream with Strings with charToFill, limits the output to length and collects the results with a String joiner (new school).

String s = Stream.generate(() -> String.valueOf(charToFill)).limit(length).collect(Collectors.joining());

char[] chars = new char[10];
Arrays.fill(chars, '*');
String text = new String(chars);

To improve performance you could have a single predefined sting if you know the max length like:

String template = "####################################";

And then simply perform a substring once you know the length.

Solution using Google Guava

String filled = Strings.repeat("*", 10);

public static String fillString(int count,char c) {
    StringBuilder sb = new StringBuilder( count );
    for( int i=0; i<count; i++ ) {
        sb.append( c ); 
    }
    return sb.toString();
}

What is wrong?

using Dollar is simple:

String filled = $("=").repeat(10).toString(); // produces "=========="

Solution using Google Guava, since I prefer it to Apache Commons-Lang:

/**
 * Returns a String with exactly the given length composed entirely of
 * the given character.
 * @param length the length of the returned string
 * @param c the character to fill the String with
 */
public static String stringOfLength(final int length, final char c)
{
    return Strings.padEnd("", length, c);
}

The above is fine. Do you mind if I ask you a question - Is this causing you a problem? It seams to me you are optimizing before you know if you need to.

Now for my over engineered solution. In many (thou not all) cases you can use CharSequence instead of a String.

public class OneCharSequence implements CharSequence {
  private final char value;
  private final int length;
  public OneCharSequence(final char value, final int length) {
    this.value = value;
    this.length = length;
  }
  public char   charAt(int index)  {
     if(index < length) return value;
     throw new IndexOutOfBoundsException();
  }
  public int length() {
    return length;
  }
  public CharSequence subSequence(int start, int end) {
     return new OneCharSequence(value, (end-start));
  }
  public String toString() {
    char[] array = new char[length];
    Arrays.fill(array, value);
    return new String(array);
  }
}

One extra note: it seems that all public ways of creating a new String instance involves necessarily the copy of whatever buffer you are working with, be it a char[], a StringBuffer or a StringBuilder. From the String javadoc (and is repeated in the respective toString methods from the other classes):

The contents of the character array are copied; subsequent modification of the character array does not affect the newly created string.

So you'll end up having a possibly big memory copy operation after the "fast filling" of the array. The only solution that may avoid this issue is the one from @mlk, if you can manage working directly with the proposed CharSequence implementation (what may be the case).

PS: I would post this as a comment but I don't have enough reputation to do that yet.

Try this Using the substring(int start, int end); method

String myLongString = "abcdefghij";
if (myLongString .length() >= 10)
String shortStr = myLongString.substring(0, 5)+ "...";

this will return abcde.

Mi solution :

  pw = "1321";
    if (pw.length() < 16){
      for(int x = pw.length() ; x < 16 ; x++){
        pw  += "*";
      }
    }

The output :

1321************

Try this jobber

String stringy =null;
 byte[] buffer =  new byte[100000];
            for (int i = 0; i < buffer.length; i++) {
            buffer[i] =0;

        }
            stringy =StringUtils.toAsciiString(buffer);