1.public boolean startsWith(String prefix, int toffset) {
char ta[] = value;
int to = toffset;
char pa[] = prefix.value;
int po = 0;
int pc = prefix.value.length;
// Note: toffset might be near -1>>>1.
if ((toffset < 0) || (toffset > value.length - pc)) {
return false;
}
while (--pc >= 0) {
if (ta[to++] != pa[po++]) {
return false;
}
}
return true;
}
2.public boolean startsWith(String prefix) {
return startsWith(prefix, 0);
}
3.public boolean endsWith(String suffix) { return startsWith(suffix, value.length - suffix.value.length); }
以上三个方法,就是简单的字符串对比,以字符串开头,以字符串结尾(可能实际开发会以index of?)
4.public int hashCode() { int h = hash; if (h == 0 && value.length > 0) { char val[] = value; for (int i = 0; i < value.length; i++) { h = 31 * h + val[i]; } hash = h; } return h; }
划重点来了,String的hashCode竟然这么简单h = 31 * h + val[i],s[0]*31^(n-1) + s[1]*31^(n-2) + ... + s[n-1]
5.public int indexOf(int ch) { return indexOf(ch, 0); }
6.public int indexOf(int ch, int fromIndex) { final int max = value.length; if (fromIndex < 0) { fromIndex = 0; } else if (fromIndex >= max) { // Note: fromIndex might be near -1>>>1. return -1; } if (ch < Character.MIN_SUPPLEMENTARY_CODE_POINT) { // handle most cases here (ch is a BMP code point or a // negative value (invalid code point)) final char[] value = this.value; for (int i = fromIndex; i < max; i++) { if (value[i] == ch) { return i; } } return -1; } else { return indexOfSupplementary(ch, fromIndex); } }
各种indexof的方法好像没有什么比较值得看。
7.public String substring(int beginIndex) { if (beginIndex < 0) { throw new StringIndexOutOfBoundsException(beginIndex); } int subLen = value.length - beginIndex; if (subLen < 0) { throw new StringIndexOutOfBoundsException(subLen); } return (beginIndex == 0) ? this : new String(value, beginIndex, subLen); }
8.public String substring(int beginIndex, int endIndex) { if (beginIndex < 0) { throw new StringIndexOutOfBoundsException(beginIndex); } if (endIndex > value.length) { throw new StringIndexOutOfBoundsException(endIndex); } int subLen = endIndex - beginIndex; if (subLen < 0) { throw new StringIndexOutOfBoundsException(subLen); } return ((beginIndex == 0) && (endIndex == value.length)) ? this : new String(value, beginIndex, subLen); }
subString重点是这个方式,是用生成一个新的String的方式,如果indexOf是整个String的话,就用当前的String。
9.public String concat(String str) { int otherLen = str.length(); if (otherLen == 0) { return this; } int len = value.length; char buf[] = Arrays.copyOf(value, len + otherLen); str.getChars(buf, len); return new String(buf, true); }
拼接字符串,除非字符串不变不然返回的是新的字符串,所以大概可以发现在各种String的操作里面String不可变的特性很强
sum:一个小总结,编写方法的时候先考虑能直接返回结果的大情况,然后步步深入,比如,在对String的操作的时候,先判断操作前后String有没有变,没有变直接返回原来的,如果有变再继续操作,以及在equals的时候,先判断instanceof,类型不一样就直接fasle,长度不一样直接false,再进入遍历循环对比。
10.public String replace(char oldChar, char newChar) { if (oldChar != newChar) { int len = value.length; int i = -1; char[] val = value; /* avoid getfield opcode */ while (++i < len) { if (val[i] == oldChar) { break; } } if (i < len) { char buf[] = new char[len]; for (int j = 0; j < i; j++) { buf[j] = val[j]; } while (i < len) { char c = val[i]; buf[i] = (c == oldChar) ? newChar : c; i++; } return new String(buf, true); } } return this; }
出现新旧字符串不一致的时候,进行替换,但是有一个疑问,替换为空怎么做?找不到平常用的替换空格的方法,在第【12】
11.public boolean matches(String regex) { return Pattern.matches(regex, this); } 12.public String replace(CharSequence target, CharSequence replacement) { return Pattern.compile(target.toString(), Pattern.LITERAL).matcher( this).replaceAll(Matcher.quoteReplacement(replacement.toString())); }
替换空格的方式在这。
13.public String[] split(String regex, int limit) { /* fastpath if the regex is a (1)one-char String and this character is not one of the RegEx's meta characters ".$|()[{^?*+\\", or (2)two-char String and the first char is the backslash and the second is not the ascii digit or ascii letter. */ char ch = 0; if (((regex.value.length == 1 && ".$|()[{^?*+\\".indexOf(ch = regex.charAt(0)) == -1) || (regex.length() == 2 && regex.charAt(0) == '\\' && (((ch = regex.charAt(1))-'0')|('9'-ch)) < 0 && ((ch-'a')|('z'-ch)) < 0 && ((ch-'A')|('Z'-ch)) < 0)) && (ch < Character.MIN_HIGH_SURROGATE || ch > Character.MAX_LOW_SURROGATE)) { int off = 0; int next = 0; boolean limited = limit > 0; ArrayList<String> list = new ArrayList<>(); while ((next = indexOf(ch, off)) != -1) { if (!limited || list.size() < limit - 1) { list.add(substring(off, next)); off = next + 1; } else { // last one //assert (list.size() == limit - 1); list.add(substring(off, value.length)); off = value.length; break; } } // If no match was found, return this if (off == 0) return new String[]{this}; // Add remaining segment if (!limited || list.size() < limit) list.add(substring(off, value.length)); // Construct result int resultSize = list.size(); if (limit == 0) { while (resultSize > 0 && list.get(resultSize - 1).length() == 0) { resultSize--; } } String[] result = new String[resultSize]; return list.subList(0, resultSize).toArray(result); } return Pattern.compile(regex).split(this, limit); }
断开字符串
14.public String trim() { int len = value.length; int st = 0; char[] val = value; /* avoid getfield opcode */ while ((st < len) && (val[st] <= ' ')) { st++; } while ((st < len) && (val[len - 1] <= ' ')) { len--; } return ((st > 0) || (len < value.length)) ? substring(st, len) : this; }
去掉头尾空格
15.public char[] toCharArray() { // Cannot use Arrays.copyOf because of class initialization order issues char result[] = new char[value.length]; System.arraycopy(value, 0, result, 0, value.length); return result; }
注意返回的并不是String本身的value属性值,而是重新赋值一个char数组来返回(不可变性)
16.public native String intern();
在 JAVA 语言中有8中基本类型和一种比较特殊的类型String
。这些类型为了使他们在运行过程中速度更快,更节省内存,都提供了一种常量池的概念。常量池就类似一个JAVA系统级别提供的缓存。8种基本类型的常量池都是系统协调的,String
类型的常量池比较特殊。它的主要使用方法有两种:
直接使用双引号声明出来的String
对象会直接存储在常量池中。
如果不是用双引号声明的String
对象,可以使用String
提供的intern
方法。intern 方法会从字符串常量池中查询当前字符串是否存在,若不存在就会将当前字符串放入常量池中。
String.intern
方法中看到,这个方法是一个 native 的方法,但注释写的非常明了。“如果常量池中存在当前字符串, 就会直接返回当前字符串. 如果常量池中没有此字符串, 会将此字符串放入常量池中后, 再返回”。