Here you can find the source of collectionToString(Collection c)
static String collectionToString(Collection<?> c)
//package com.java2s; import java.util.Collection; public class Main { /**//from w w w . j a va 2 s . co m * An implementation of Collection.toString() suitable for classes * with locks. Instead of holding a lock for the entire duration of * toString(), or acquiring a lock for each call to Iterator.next(), * we hold the lock only during the call to toArray() (less * disruptive to other threads accessing the collection) and follows * the maxim "Never call foreign code while holding a lock". */ static String collectionToString(Collection<?> c) { final Object[] a = c.toArray(); final int size = a.length; if (size == 0) return "[]"; int charLength = 0; // Replace every array element with its string representation for (int i = 0; i < size; i++) { Object e = a[i]; // Extreme compatibility with AbstractCollection.toString() String s = (e == c) ? "(this Collection)" : objectToString(e); a[i] = s; charLength += s.length(); } return toString(a, size, charLength); } private static String objectToString(Object x) { // Extreme compatibility with StringBuilder.append(null) String s; return (x == null || (s = x.toString()) == null) ? "null" : s; } /** * Like Arrays.toString(), but caller guarantees that size > 0, * each element with index 0 <= i < size is a non-null String, * and charLength is the sum of the lengths of the input Strings. */ static String toString(Object[] a, int size, int charLength) { // assert a != null; // assert size > 0; // Copy each string into a perfectly sized char[] // Length of [ , , , ] == 2 * size final char[] chars = new char[charLength + 2 * size]; chars[0] = '['; int j = 1; for (int i = 0; i < size; i++) { if (i > 0) { chars[j++] = ','; chars[j++] = ' '; } String s = (String) a[i]; int len = s.length(); s.getChars(0, len, chars, j); j += len; } chars[j] = ']'; // assert j == chars.length - 1; return new String(chars); } }