介绍
String 类是 final 修饰，说明不可以被继承，String 一旦被创建就不能改变，对 String 修改就会创建一个新的字符串，因此禁止使用循环修改字符串。
Sring 在内存中的位置
String str1 = new String("hello");
String str2 = new String("hello");
String str3 = "hello";
String str4 = "hello";
String str5 = "he"+"llo";
String str6 = "he";
String str7 = "llo";
System.out.println(str1==str2); //false
System.out.println(str1==str3); //false
System.out.println(str3==str4); //true
System.out.println(str4 == str5); //true
System.out.println(str3=="hello"); //true
System.out.println(str4==(str6+str7)); //false
使用 new 创建一个 String 对象，会在堆内分配内存，每个对象地址都不一样，而 str3 会先在字符串常量池查找，没有才会添加进去，如果存在直接引用地址。最后 str6+str7 是在 JVM 运行时才操作，会创建一个新的对象。
源码
继承
public final class String
implements java.io.Serializable, Comparable<String>, CharSequence {}
String 实现了 Serializable、Comparable 和 CharSequence 接口
成员变量
//存放final类型的char数组，不可修改
private final char value[];
//hash值
private int hash;
private static final ObjectStreamField[] serialPersistentFields =
new ObjectStreamField[0];
构造函数
//创建一个空的字符串
public String() {
this.value = "".value;
}
//新创建的String是参数String的副本
public String(String original) {
this.value = original.value;
this.hash = original.hash;
}
//传入char数组，直接拷贝数据到value
public String(char value[]) {
this.value = Arrays.copyOf(value, value.length);
}
//offset是第一个字符的索引，count是字节数，表示char数组从offset开始，获取长度为count的子数组
public String(char value[], int offset, int count) {
if (offset < 0) {
throw new StringIndexOutOfBoundsException(offset);
}
if (count <= 0) {
if (count < 0) {
throw new StringIndexOutOfBoundsException(count);
}
//长度为0
if (offset <= value.length) {
this.value = "".value;
return;
}
}
// 数组越界
if (offset > value.length - count) {
throw new StringIndexOutOfBoundsException(offset + count);
}
this.value = Arrays.copyOfRange(value, offset, offset+count);
}
//Unicode 代码点数组参数一个子数组的字符
public String(int[] codePoints, int offset, int count) {
if (offset < 0) {
throw new StringIndexOutOfBoundsException(offset);
}
if (count <= 0) {
if (count < 0) {
throw new StringIndexOutOfBoundsException(count);
}
if (offset <= codePoints.length) {
this.value = "".value;
return;
}
}
// Note: offset or count might be near -1>>>1.
if (offset > codePoints.length - count) {
throw new StringIndexOutOfBoundsException(offset + count);
}
final int end = offset + count;
// Pass 1: Compute precise size of char[]
int n = count;
for (int i = offset; i < end; i++) {
int c = codePoints[i];
if (Character.isBmpCodePoint(c))
continue;
else if (Character.isValidCodePoint(c))
n++;
else throw new IllegalArgumentException(Integer.toString(c));
}
// Pass 2: Allocate and fill in char[]
final char[] v = new char[n];
for (int i = offset, j = 0; i < end; i++, j++) {
int c = codePoints[i];
if (Character.isBmpCodePoint(c))
v[j] = (char)c;
else
Character.toSurrogates(c, v, j++);
}
this.value = v;
}
//不推荐使用
@Deprecated
public String(byte ascii[], int hibyte, int offset, int count) {
checkBounds(ascii, offset, count);
char value[] = new char[count];
if (hibyte == 0) {
for (int i = count; i-- > 0;) {
value[i] = (char)(ascii[i + offset] & 0xff);
}
} else {
hibyte <<= 8;
for (int i = count; i-- > 0;) {
value[i] = (char)(hibyte | (ascii[i + offset] & 0xff));
}
}
this.value = value;
}
//不推荐使用
@Deprecated
public String(byte ascii[], int hibyte) {
this(ascii, hibyte, 0, ascii.length);
}
//
public String(byte bytes[], int offset, int length, String charsetName)
throws UnsupportedEncodingException {
if (charsetName == null)
throw new NullPointerException("charsetName");
checkBounds(bytes, offset, length);
this.value = StringCoding.decode(charsetName, bytes, offset, length);
}
//传入字节数组，使用指定字符集
public String(byte bytes[], int offset, int length, Charset charset) {
if (charset == null)
throw new NullPointerException("charset");
checkBounds(bytes, offset, length);
this.value = StringCoding.decode(charset, bytes, offset, length);
}
public String(byte bytes[], String charsetName)
throws UnsupportedEncodingException {
this(bytes, 0, bytes.length, charsetName);
}
public String(byte bytes[], Charset charset) {
this(bytes, 0, bytes.length, charset);
}
public String(byte bytes[], int offset, int length) {
checkBounds(bytes, offset, length);
this.value = StringCoding.decode(bytes, offset, length);
}
public String(byte bytes[]) {
this(bytes, 0, bytes.length);
}
public String(StringBuffer buffer) {
synchronized(buffer) {
this.value = Arrays.copyOf(buffer.getValue(), buffer.length());
}
}
public String(StringBuilder builder) {
this.value = Arrays.copyOf(builder.getValue(), builder.length());
}
String(char[] value, boolean share) {
// assert share : "unshared not supported";
this.value = value;
}
其他方法
//获取长度
public int length() {
return value.length;
}
//判断是否为空
public boolean isEmpty() {
return value.length == 0;
}
//获取char指定索引的值
public char charAt(int index) {
if ((index < 0) || (index >= value.length)) {
throw new StringIndexOutOfBoundsException(index);
}
return value[index];
}
//获取指定索引处的字符（Unicode代码点）
public int codePointAt(int index) {
if ((index < 0) || (index >= value.length)) {
throw new StringIndexOutOfBoundsException(index);
}
return Character.codePointAtImpl(value, index, value.length);
}
//获取指定索引之前的字符
public int codePointBefore(int index) {
int i = index - 1;
if ((i < 0) || (i >= value.length)) {
throw new StringIndexOutOfBoundsException(index);
}
return Character.codePointBeforeImpl(value, index, 0);
}
//获取String文本范围内Unicode代码点数
public int codePointCount(int beginIndex, int endIndex) {
if (beginIndex < 0 || endIndex > value.length || beginIndex > endIndex) {
throw new IndexOutOfBoundsException();
}
return Character.codePointCountImpl(value, beginIndex, endIndex - beginIndex);
}
//返回此 String 中从给定的 index 处偏移 codePointOffset 个代码点的索引。
public int offsetByCodePoints(int index, int codePointOffset) {
if (index < 0 || index > value.length) {
throw new IndexOutOfBoundsException();
}
return Character.offsetByCodePointsImpl(value, 0, value.length,
index, codePointOffset);
}
//将字符串中的字符复制到目标数组中，dstBegin是目标数组中的起始偏移量
void getChars(char dst[], int dstBegin) {
System.arraycopy(value, 0, dst, dstBegin, value.length);
}
//同上，要复制字符串中，从srcBegin开始，到srcEnd-1结束
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);
}
//获取命名的字符集字节数组
public byte[] getBytes(String charsetName)
throws UnsupportedEncodingException {
if (charsetName == null) throw new NullPointerException();
return StringCoding.encode(charsetName, value, 0, value.length);
}
//获取指定字符集的字节数组
public byte[] getBytes(Charset charset) {
if (charset == null) throw new NullPointerException();
return StringCoding.encode(charset, value, 0, value.length);
}
//获取默认字符集的字节数组
public byte[] getBytes() {
return StringCoding.encode(value, 0, value.length);
}
//重写equals方法
public boolean equals(Object anObject) {
//如果对象地址相同，就是相同的对象
if (this == anObject) {
return true;
}
//首先判断是否是String对象
if (anObject instanceof String) {
String anotherString = (String)anObject;
int n = value.length;
//判断长度是否相同，不相同不是相同的字符串
if (n == anotherString.value.length) {
char v1[] = value;
char v2[] = anotherString.value;
int i = 0;
//判断每一个字符是否相同
while (n-- != 0) {
if (v1[i] != v2[i])
return false;
i++;
}
return true;
}
}
return false;
}
//equals对比的是String对象，contentEquals对比CharSequence或其子类的对象
public boolean contentEquals(StringBuffer sb) {
return contentEquals((CharSequence)sb);
}
public boolean contentEquals(CharSequence cs) {
// Argument is a StringBuffer, StringBuilder
if (cs instanceof AbstractStringBuilder) {
if (cs instanceof StringBuffer) {
//StringBuffer 是线程安全的，用synchronized块
synchronized(cs) {
return nonSyncContentEquals((AbstractStringBuilder)cs);
}
} else {
return nonSyncContentEquals((AbstractStringBuilder)cs);
}
}
// 如果是String，就用equals方法
if (cs instanceof String) {
return equals(cs);
}
// Argument is a generic CharSequence
char v1[] = value;
int n = v1.length;
if (n != cs.length()) {
return false;
}
for (int i = 0; i < n; i++) {
if (v1[i] != cs.charAt(i)) {
return false;
}
}
return true;
}
private boolean nonSyncContentEquals(AbstractStringBuilder sb) {
char v1[] = value;
char v2[] = sb.getValue();
int n = v1.length;
//判断长度是否相同
if (n != sb.length()) {
return false;
}
//对比每一个字符
for (int i = 0; i < n; i++) {
if (v1[i] != v2[i]) {
return false;
}
}
return true;
}
//对比String，忽略大小写
public boolean equalsIgnoreCase(String anotherString) {
return (this == anotherString) ? true
: (anotherString != null)
&& (anotherString.value.length == value.length)
&& regionMatches(true, 0, anotherString, 0, value.length);
}
//如果字符串等于参数字符串anotherString，返回0；如果字符串小于参数字符串anotherString，返回小于0的值；如果字符串大于参数字符串anotherString，返回大于0的值
public int compareTo(String anotherString) {
int len1 = value.length;
int len2 = anotherString.value.length;
int lim = Math.min(len1, len2);//取长度最小的
char v1[] = value;
char v2[] = anotherString.value;
int k = 0;
while (k < lim) {
char c1 = v1[k];
char c2 = v2[k];
//如果不相同，就比较不相同的字符
if (c1 != c2) {
return c1 - c2;
}
k++;
}
//上面遍历结束还未返回，如果两个长度相同那说明两个字符串完全相同，返回0
return len1 - len2;
}
//忽略大小写的对比
public static final Comparator<String> CASE_INSENSITIVE_ORDER
= new CaseInsensitiveComparator();
private static class CaseInsensitiveComparator
implements Comparator<String>, java.io.Serializable {
// use serialVersionUID from JDK 1.2.2 for interoperability
private static final long serialVersionUID = 8575799808933029326L;
public int compare(String s1, String s2) {
int n1 = s1.length();
int n2 = s2.length();
int min = Math.min(n1, n2);
for (int i = 0; i < min; i++) {
char c1 = s1.charAt(i);
char c2 = s2.charAt(i);
if (c1 != c2) {
c1 = Character.toUpperCase(c1);
c2 = Character.toUpperCase(c2);
if (c1 != c2) {
c1 = Character.toLowerCase(c1);
c2 = Character.toLowerCase(c2);
if (c1 != c2) {
// No overflow because of numeric promotion
return c1 - c2;
}
}
}
}
return n1 - n2;
}
private Object readResolve() { return CASE_INSENSITIVE_ORDER; }
}
//忽略大小写
public int compareToIgnoreCase(String str) {
return CASE_INSENSITIVE_ORDER.compare(this, str);
}
public int indexOf(int ch) {
return indexOf(ch, 0);
}
//返回指定字符第一次出现的字符串内的索引，从fromIndex开始查找，ch是字符，Unicode代码点
public int indexOf(int ch, int fromIndex) {
final int max = value.length;
//如果偏移量小于0，则从0开始查
if (fromIndex < 0) {
fromIndex = 0;
} else if (fromIndex >= max) {
// 如果偏移量大于字符数量，那就返回-1，表示没找到
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;
//循环查找，只返回第一次出现的位置
for (int i = fromIndex; i < max; i++) {
if (value[i] == ch) {
return i;
}
}
return -1;
} else {
return indexOfSupplementary(ch, fromIndex);
}
}
//增补字符验证
private int indexOfSupplementary(int ch, int fromIndex) {
if (Character.isValidCodePoint(ch)) {
final char[] value = this.value;
final char hi = Character.highSurrogate(ch);
final char lo = Character.lowSurrogate(ch);
final int max = value.length - 1;
for (int i = fromIndex; i < max; i++) {
if (value[i] == hi && value[i + 1] == lo) {
return i;
}
}
}
return -1;
}
//返回指定字符最后一次出现的位置
public int lastIndexOf(int ch) {
return lastIndexOf(ch, value.length - 1);
}
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);
}
}
private int lastIndexOfSupplementary(int ch, int fromIndex) {
if (Character.isValidCodePoint(ch)) {
final char[] value = this.value;
char hi = Character.highSurrogate(ch);
char lo = Character.lowSurrogate(ch);
int i = Math.min(fromIndex, value.length - 2);
for (; i >= 0; i--) {
if (value[i] == hi && value[i + 1] == lo) {
return i;
}
}
}
return -1;
}
public int indexOf(String str) {
return indexOf(str, 0);
}
public int indexOf(String str, int fromIndex) {
return indexOf(value, 0, value.length,
str.value, 0, str.value.length, fromIndex);
}
//去除字符串两端空格
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--;
}
//判断前后空格，然后取子集
return ((st > 0) || (len < value.length)) ? substring(st, len) : this;
}
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