小知识,大挑战!本文正在参与“程序员必备小知识”创作活动
查看源码可知,String底层是用final修饰的,而值得存储时用字符数组存储得
- charAt方法 返回指定索引处的字符值。索引范围从0到length()
public char charAt(int index) {
if ((index < 0) || (index >= value.length)) {
throw new StringIndexOutOfBoundsException(index);
}
return value[index];
}
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- getChars 将该字符串中的字符复制到目标字符数组中。
public void getChars(int srcBegin, int srcEnd, char dst[], int dstBegin) {
if (srcBegin < 0) {
throw new StringIndexOutOfBoundsException(srcBegin);
}
if (srcEnd > value.length) {
throw new StringIndexOutOfBoundsException(srcEnd);
}
if (srcBegin > srcEnd) {
throw new StringIndexOutOfBoundsException(srcEnd - srcBegin);
}
System.arraycopy(value, srcBegin, dst, dstBegin, srcEnd - srcBegin);
}
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- equals 另一篇文章有 如有需要可以点击
- compareTo 按字典顺序比较两个字符串 如果String对象按字典顺序排在参数String之前,则结果为负整数。如果String对象按字典顺序跟随参数String,则结果为正整数。如果两个字符串相等,则结果为零;当equals(Object)方法返回true时,compareTo恰好返回0
public int compareTo(String anotherString) {
int len1 = value.length;
int len2 = anotherString.value.length;
int lim = Math.min(len1, len2);
char v1[] = value;
char v2[] = anotherString.value;
int k = 0;
while (k < lim) {
char c1 = v1[k];
char c2 = v2[k];
if (c1 != c2) {
return c1 - c2;
}
k++;
}
return len1 - len2;
}
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- hashcode 返回此字符串的哈希码
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;
}
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- indexOf 返回该字符串中指定字符第一次出现的索引,并从指定索引处开始搜索。
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);
}
}
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- lastIndexOf 返回该字符串中指定字符最后一次出现的索引
public int lastIndexOf(int ch, int fromIndex) {
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;
int i = Math.min(fromIndex, value.length - 1);
for (; i >= 0; i--) {
if (value[i] == ch) {
return i;
}
}
return -1;
} else {
return lastIndexOfSupplementary(ch, fromIndex);
}
}
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- substring
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);
}
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);
}
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- concat 将指定的字符串连接到该字符串的末尾
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);
}
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- replace 返回一个字符串,将该字符串中出现的所有oldChar替换为newChar
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;
}
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- contains 当且仅当该字符串包含指定的字符值序列时返回true
public boolean contains(CharSequence s) {
return indexOf(s.toString()) > -1;
}
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- trim 返回值为此字符串的字符串,删除前导和尾随空格。
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;
}
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- toUpperCase(); toLowerCase() ;字符串大小写的转换
在读String源码得过程中,有许多重载方法(方法名相同,但参数列表不同),我上面分享了一些常用的方法以及部分的源码,其实看这些源码,发现和我们平常的代码差不多,只是他们的封装更好哇,我们多读读源码 对我们写代码也会有许多的启发和帮助 就例如上面每个方法,基本都会先对传入的参数进行验证,传入的数是否合法,若不合法直接抛出异常。(之前文章也分享过如何自己自定义异常如何抛出异常 若有兴趣可以去查看