本文章学习参考的文章：
https://www.cnblogs.com/duanxz/p/3511695.html
https://blog.csdn.net/weixin_44365021/article/details/85775813
https://blog.csdn.net/windy8833/article/details/5292481
String中的属性
// 这里的这个value常量就是String不可变的原因，因为用了final修饰
private final char value[];// 这个是jdk1.8的底层，使用了char，而在1.9底层变成了byte
private int hash; // 默认值是0
private static final long serialVersionUID = -6849794470754667710L;
String中的常用方法
public String() {
this.value = "".value;
}
默认生成一个空的字符串
public String(String original) {
this.value = original.value;
this.hash = original.hash;
}
当构造函数中传递一个值的时候，value就是值，hash就是值的hash
public String(char value[]) {
this.value = Arrays.copyOf(value, value.length);
}
Arrays.copyOf的返回值是一个新的char[]，第一个参数是值，第二个参数是新数组的长度
public int length() {
return value.length;
}
返回int类型，获取字符串的长度
public boolean isEmpty() {
return value.length == 0;
}
判断字符串是否为空
public char charAt(int index) {
if ((index < 0) || (index >= value.length)) {
throw new StringIndexOutOfBoundsException(index);
}
return value[index];
}
获取指定下标处的char的具体值，下标从0开始
public int codePointAt(int index) {
if ((index < 0) || (index >= value.length)) {
throw new StringIndexOutOfBoundsException(index);
}
return Character.codePointAtImpl(value, index, value.length);
}
获取对应索引下的char 的code值
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);
}
应用：
String str = "123456";
char[] chars = {'a', 'b', 'c'};
str.getChars(1, 3, chars, 1);
for (char aChar : chars) {
System.out.print(aChar);
}
运行结果：
由此可以看出，这个方法的作用是从str的1索引开始到3，复制覆盖到chars的从第1个索引位置开始
索引从0开始
public byte[] getBytes() {
return StringCoding.encode(value, 0, value.length);
}
将字符串转换成byte数组，（“1” 的byte是49）
public boolean equals(Object anObject) {
if (this == anObject) {
return true;
}
if (anObject instanceof String) {
String anotherString = (String)anObject;
int n = value.length;
if (n == anotherString.value.length) {
char v1[] = value;
char v2[] = anotherString.value;
int i = 0;
while (n-- != 0) {
if (v1[i] != v2[i])
return false;
i++;
}
return true;
}
}
return false;
}
java在String的底层重写了equals方法，比较的是两个字符串的值value是否相同，而如果是自定义的类的话，没有重写equals会比较两个对象的地址。
这里我创建了两个不同的对象在堆内存中，通过 两个不同的引用来指向这两个字符串，两个的地址是不同的但是值相同，equals返回的是true，由此可以看出比较的是两个的值。
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;
}
继承了Comparable接口，重写了里面的compareTo方法，用来比较排序
返回为正数表示a1>a2, 返回为负数表示a1
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;
}
获取这个字符串对象的hashcode值
public int indexOf(int ch) {
return indexOf(ch, 0);
}
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);
}
}
默认从第0索引开始，判断字符串中有没有byte为ch的字符，没有就返回-1，有则返回这个字符的索引位置。这个方法也可以直接传入一个字符串或者字符，也可以查询出来。
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);
}
将两个字符串首尾连接起来，返回一个新的String对象
public String replace(char oldChar, char newChar) {
if (oldChar != newChar) {
int len = value.length;
int i = -1;
char[] val = value;
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;
}
将字符串的存在的子串进行替换，换成新的字符串，然后返回一个新的String对象。
public boolean contains(CharSequence s) {
return indexOf(s.toString()) > -1;
}
判断字符串中是否存在相应的子串，boolean
public String toLowerCase() {
return toLowerCase(Locale.getDefault());
}
public String toLowerCase(Locale locale) {
if (locale == null) {
throw new NullPointerException();
}
int firstUpper;
final int len = value.length;
scan: {
for (firstUpper = 0 ; firstUpper < len; ) {
char c = value[firstUpper];
if ((c >= Character.MIN_HIGH_SURROGATE)
&& (c <= Character.MAX_HIGH_SURROGATE)) {
int supplChar = codePointAt(firstUpper);
if (supplChar != Character.toLowerCase(supplChar)) {
break scan;
}
firstUpper += Character.charCount(supplChar);
} else {
if (c != Character.toLowerCase(c)) {
break scan;
}
firstUpper++;
}
}
return this;
}
char[] result = new char[len];
int resultOffset = 0;
System.arraycopy(value, 0, result, 0, firstUpper);
String lang = locale.getLanguage();
boolean localeDependent =
(lang == "tr" || lang == "az" || lang == "lt");
char[] lowerCharArray;
int lowerChar;
int srcChar;
int srcCount;
for (int i = firstUpper; i < len; i += srcCount) {
srcChar = (int)value[i];
if ((char)srcChar >= Character.MIN_HIGH_SURROGATE
&& (char)srcChar <= Character.MAX_HIGH_SURROGATE) {
srcChar = codePointAt(i);
srcCount = Character.charCount(srcChar);
} else {
srcCount = 1;
}
if (localeDependent ||
srcChar == 'u03A3' || // GREEK CAPITAL LETTER SIGMA
srcChar == 'u0130') { // LATIN CAPITAL LETTER I WITH DOT ABOVE
lowerChar = ConditionalSpecialCasing.toLowerCaseEx(this, i, locale);
} else {
lowerChar = Character.toLowerCase(srcChar);
}
if ((lowerChar == Character.ERROR)
|| (lowerChar >= Character.MIN_SUPPLEMENTARY_CODE_POINT)) {
if (lowerChar == Character.ERROR) {
lowerCharArray =
ConditionalSpecialCasing.toLowerCaseCharArray(this, i, locale);
} else if (srcCount == 2) {
resultOffset += Character.toChars(lowerChar, result, i + resultOffset) - srcCount;
continue;
} else {
lowerCharArray = Character.toChars(lowerChar);
}
int mapLen = lowerCharArray.length;
if (mapLen > srcCount) {
char[] result2 = new char[result.length + mapLen - srcCount];
System.arraycopy(result, 0, result2, 0, i + resultOffset);
result = result2;
}
for (int x = 0; x < mapLen; ++x) {
result[i + resultOffset + x] = lowerCharArray[x];
}
resultOffset += (mapLen - srcCount);
} else {
result[i + resultOffset] = (char)lowerChar;
}
}
return new String(result, 0, len + resultOffset);
}
将字符串中大写的字符变成小写
public String toUpperCase() {
return toUpperCase(Locale.getDefault());
}
将字符串中小写的字符变成大写
public String trim() {
int len = value.length;
int st = 0;
char[] val = value;
while ((st < len) && (val[st] <= ' ')) {
st++;
}
while ((st < len) && (val[len - 1] <= ' ')) {
len--;
}
return ((st > 0) || (len < value.length)) ? substring(st, len) : this;
}
除去String中的空格
public String toString() {
return this;
}
返回字符串的值，如果对象是null会抛出异常NullPointerException
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;
}
将字符串转化成char数组
public static String valueOf(Object obj) {
return (obj == null) ? "null" : obj.toString();
}
返回字符串的值，如果对象是null不会抛出异常，会返回null
