Java.String
String 类的两种赋值方式
String 可以表示出一个字符串,根据String的源码我们会发现String类实际上是使用字符数组char[]存储的.
如下为
String的源码实现
public final class String
implements java.io.Serializable, Comparable<String>, CharSequence {
/** The value is used for character storage. */
private final char value[];
/** Cache the hash code for the string */
private int hash; // Default to 0
/** use serialVersionUID from JDK 1.0.2 for interoperability */
private static final long serialVersionUID = -6849794470754667710L;
public String() {
this.value = "".value;
}
public String(String original) {
this.value = original.value;
this.hash = original.hash;
}
public String(char value[]) {
this.value = Arrays.copyOf(value, value.length);
}
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);
}
if (offset <= value.length) {
this.value = "".value;
return;
}
}
// Note: offset or count might be near -1>>>1.
if (offset > value.length - count) {
throw new StringIndexOutOfBoundsException(offset + count);
}
this.value = Arrays.copyOfRange(value, offset, offset+count);
}
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;
}
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);
}
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());
}
-
常见的两种String赋值方式
- String str1 = new String("name");
- String str2 = "name";
两种赋值方式分析
//会创建两个字符串对象，一个在常量池中创建便于下一次使用，一个在堆内存中创建
String str1 = new String("name");
//最多创建一个字符串对象，有可能不用创建对象（如果常量池中已经存在相应的常量）
String str2 = "name";//推荐使用
内存分析
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String类编译期与运行期分析
- 首先明确对象之间 "==" 是比较两个对象的地址。
public class StringDemo {
public static void main(String[] args) {
// 情况一 结果为true
String aString = "a1";
String a1String = "a" + 1;
System.out.println(aString == a1String);
// 情况二 结果为false
String bString = "b1";
int bb = 1;
String b1String = "b" + bb;
System.out.println(bString == b1String);
// 情况三 结果为true
String cString = "c1";
final int cc = 1;
String c1String = "c" + cc;
System.out.println(cString == c1String);
// 情况四 结果为false
String dString = "dd";
final int dd = getDD();
String d1String = dString + dd;
System.out.println(dString == d1String);
}
public static int getDD(){
return 1;
}
}
- 情况二为false，因为 b1String 在编译期时是还没有完全确定下来的，因为 bb 是一个变量，只有在执行期才能加载到。
- 情况四为false，注意与情况三进行对比，虽然使用了final关键字，但是根据函数返回值确定下来的，而函数要执行仍然是在运行期的时候确定的，编译期仍然无法确定。
String 类字符与字符串操作方法
- 字符与字符串操作
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- 字节与字符串操作
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- 是否以指定内容开头或结尾
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- String 类替换操作
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- String 类字符串截取操作
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- 字符串拆分操作
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- String 类字符串查找操作
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- String 类其他操作方法
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更多请见相关api文档
常见用例
System.out.println("abcde".charAt(3));//d
System.out.println("abcde".toCharArray()[2]);//c
byte[] bytes = "abcde".getBytes();
for(int i = 0; i < bytes.length; i++){
System.out.println(bytes[i]);
}
System.out.println(new String(bytes).toString());
System.out.println(new String(bytes,0,2).toString());//ab
System.out.println(new String(bytes,"utf-8").toString());
System.out.println("abcde".startsWith("ab"));//true
System.out.println("abcde".startsWith("cd",2));//true
System.out.println("abcde".endsWith("de"));//true
System.out.println("abc,de".replace(",", ":").toString());//abc:de
System.out.println("abcde".replace("ab", "AB"));//ABcde
//匹配正则表达式中26个字母替换为相应的字符串
System.out.println("abcde".replaceAll("[a-z]", "*"));//*****
System.out.println("abcde".replaceFirst("[a-z]", "*"));//*bcde
//包括起始位置
System.out.println("abcde".substring(2));//cde
//包括起始位置，不包括结束位置
System.out.println("abcde".substring(1, 3));//bc
String[] value = "ab-cde".split("-");//传入的参数为正则表达式
String[] value2 = "ab-c-de".split("-",3);//拆分成三个字符串数组
for(String s:value){
System.out.print(s);
}
System.out.println();
for(String s:value2){
System.out.print(s);
}
System.out.println("abcde".contains("abc"));//true
System.out.println("abcde".indexOf("a"));//0 参数虽然为整形，但会转为unicode字符
System.out.println("abcde".indexOf("bc"));//0 返回首地址
System.out.println("abcded".lastIndexOf("d"));//5
System.out.println("abcdede".lastIndexOf("de"));//5
System.out.println("abcde".isEmpty());//false
System.out.println("abcde".length());//5
System.out.println("abcde".toUpperCase());//ABCDE
System.out.println("ABCDE".toLowerCase());//abcde
System.out.println(" abcde ".trim());//abcde
System.out.println("abc".concat("de"));//abcde
- 源码
public char charAt(int index) {
if ((index < 0) || (index >= value.length)) {
throw new StringIndexOutOfBoundsException(index);
}
return value[index];
}
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;
}
// 使用平台默认的编码转换成字节数组，默认gbk，根据虚拟机
public byte[] getBytes(String charsetName)
throws UnsupportedEncodingException {
if (charsetName == null) throw new NullPointerException();
return StringCoding.encode(charsetName, value, 0, value.length);
}
public String(byte bytes[]) {
this(bytes, 0, bytes.length);
}
public String(byte bytes[], int offset, int length) {
checkBounds(bytes, offset, length);
this.value = StringCoding.decode(bytes, offset, length);
}
public String(byte bytes[], String charsetName)
throws UnsupportedEncodingException {
this(bytes, 0, bytes.length, charsetName);
}
public boolean startsWith(String prefix) {
return startsWith(prefix, 0);
}
public boolean startsWith(String prefix, int toffset) {
char ta[] = value;//调用该方法的对象的value
int to = toffset;//偏移量
char pa[] = prefix.value;//前缀的value
int po = 0;//前缀的偏移量
int pc = prefix.value.length;//前缀的总数
// 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;
}
public boolean endsWith(String suffix) {
return startsWith(suffix, value.length - suffix.value.length);
}
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;
}
public String replaceFirst(String regex, String replacement) {
return Pattern.compile(regex).matcher(this).replaceFirst(replacement);
}
public String replaceAll(String regex, String replacement) {
return Pattern.compile(regex).matcher(this).replaceAll(replacement);
}
public String replace(CharSequence target, CharSequence replacement) {
return Pattern.compile(target.toString(), Pattern.LITERAL).matcher(
this).replaceAll(Matcher.quoteReplacement(replacement.toString()));
}
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);
}
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);
}
public String[] split(String regex) {
return split(regex, 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);
}
}
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);
}
public int indexOf(int ch) {
return indexOf(ch, 0);
}
public boolean contains(CharSequence s) {
return indexOf(s.toString()) > -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);
}
}
public int lastIndexOf(int ch) {
return lastIndexOf(ch, value.length - 1);
}
public boolean isEmpty() {
return value.length == 0;
}
public int length() {
return value.length;
}
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;
}
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);
}
