字符串廣泛應用 在 Java 編程中,在 Java 中字符串屬於對象,Java 提供了 String 類來創建和操作字符串。
字符串常量池
作爲最基礎的引用數據類型,Java 爲 String 提供了字符串常量池以提高其性能。字符串常量池存在於方法區中。
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字符串的分配,和其他的對象分配一樣,耗費高昂的時間與空間代價,作爲最基礎的數據類型,大量頻繁的創建字符串,極大程度地影響程序的性能
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JVM爲了提高性能和減少內存開銷,在實例化字符串常量的時候進行了一些優化
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爲字符串開闢一個字符串常量池,類似於緩存區
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創建字符串常量時,首先檢查字符串常量池是否存在該字符串
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存在該字符串,返回引用實例,不存在,實例化該字符串並放入池中
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實現的基礎
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實現該優化的基礎是因爲字符串是不可變的,可以不用擔心數據衝突進行共享
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運行時實例創建的全局字符串常量池中有一個表,總是爲池中每個唯一的字符串對象維護一個引用,這就意味着它們一直引用着字符串常量池中的對象,所以,在常量池中的這些字符串不會被垃圾收集器回收
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public final class String
implements java.io.Serializable, Comparable<String>, CharSequence
String 類爲 final類,不可被繼承(final類中的中的方法默認爲final)。String 實現了序列化的接口 Serializable,所以 String 是支持序列化和反序列化的。String 類還實現了 Comparable 接口,Comparable<T>接口只有一個方法 public int compareTo(T o),實現了這個接口就意味着該類支持排序,即可用 Collections.sort 或 Arrays.sort 等方法對該類的對象列表或數組進行排序。
/** 該值用於字符存儲。 */
private final char value[];
/** 緩存字符串的哈希 */
private int hash; // Default to 0
/** 使用JDK 1.0.2中的serialVersionUID實現互操作性 */
private static final long serialVersionUID = -6849794470754667710L;
/**
*用於指定哪些字段需要被默認序列化
*/
private static final ObjectStreamField[] serialPersistentFields =
new ObjectStreamField[0];
/**
*按{compareToIgnoreCase}排序的比較器。這個比較器是可序列化的
*/
public static final Comparator<String> CASE_INSENSITIVE_ORDER
= new CaseInsensitiveComparator();
+ 操作
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String 在執行 + 操作時,編譯時期會進行優化。
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如果是多個字面量相加,則會在編譯時期直接把組合好的字符串放入常量池。
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如果有字符串常量參與,以最左邊的字符串爲參數創建StringBuilder對象,然後依次對右邊進行append操作,最後將StringBuilder對象通過toString()方法轉換成String對象(注意:中間的多個字符串常量不會自動拼接)。
下面我們用字節碼看一下這2種情況。
//在常量池中創建 "ab",不會使用StringBuilder
String s1 = "a" + "b";
System.out.println(s1);
字節碼:
0 ldc #2 <ab>
2 astore_1
3 getstatic #3 <java/lang/System.out>
6 aload_1
7 invokevirtual #4 <java/io/PrintStream.println>
10 return
從字節碼可以看出常量相加的時候並沒有使用StringBuilder,而是在常量池中創建。
String s1 = "a";
String s2 = "b";
//使用StringBuilder實現 + 操作
String s3 = s1 + s2;
System.out.println(s3);
字節碼:
0 ldc #2 <a>
2 astore_1
3 ldc #2 <a>
5 astore_2
6 new #3 <java/lang/StringBuilder>
9 dup
10 invokespecial #4 <java/lang/StringBuilder.<init>>
13 aload_1
14 invokevirtual #5 <java/lang/StringBuilder.append>
17 aload_2
18 invokevirtual #5 <java/lang/StringBuilder.append>
21 invokevirtual #6 <java/lang/StringBuilder.toString>
24 astore_3
25 getstatic #7 <java/lang/System.out>
28 aload_3
29 invokevirtual #8 <java/io/PrintStream.println>
32 return
equals 和 ==
在Java中一些基本數據類型如int、short、long等,可以使用==比較2個值是否相等。對於引用類型則比較的是引用是否相等而不是值是否相等。equals方法是Object中的方法,在String中進行了重寫。當我們需要比較2個字符串的值是否相同的時候,則調用equals方法。
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;
}
intern()
public native String intern();
返回字符串對象的規範表示形式,字符串池最初爲空,由String類私下維護.調用intern方法時,如果池中已包含由equals方法確定的與此字符串對象相等的字符串,則返回池中的字符串。否則,將此字符串對象添加到池中,並返回對此字符串對象的引用。適當的使用可以減少內存的消耗,每次new String都會產生一個新的對象,如果適當的使用String s=new String("1").intern(); 能夠獲取常量池的常量。
public final class String
implements java.io.Serializable, Comparable<String>, CharSequence {
/** 該值用於字符存儲。 */
private final char value[];
/** 緩存字符串的哈希 */
private int hash; // Default to 0
/** 使用JDK 1.0.2中的serialVersionUID實現互操作性 */
private static final long serialVersionUID = -6849794470754667710L;
/**
*用於指定哪些字段需要被默認序列化
*/
private static final ObjectStreamField[] serialPersistentFields =
new ObjectStreamField[0];
/**
*按{compareToIgnoreCase}排序的比較器。這個比較器是可序列化的
*/
public static final Comparator<String> CASE_INSENSITIVE_ORDER
= new CaseInsensitiveComparator();
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;
}
/* 用於邊界檢查字節數組和String(byte[],..)構造函數使用的請求偏移量和長度值
*/
private static void checkBounds(byte[] bytes, int offset, int length) {
if (length < 0)
throw new StringIndexOutOfBoundsException(length);
if (offset < 0)
throw new StringIndexOutOfBoundsException(offset);
if (offset > bytes.length - length)
throw new StringIndexOutOfBoundsException(offset + length);
}
/**
* 通過使用指定的字符集解碼指定的字節數組,構造新的String
*/
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());
}
String(char[] value, boolean share) {
// assert share : "unshared not supported";
this.value = value;
}
/**
*返回字符串長度
*/
public int length() {
return value.length;
}
/**
* 字符串是否爲空
*/
public boolean isEmpty() {
return value.length == 0;
}
/**
* 返回指定索引處的char值
*/
public char charAt(int index) {
if ((index < 0) || (index >= value.length)) {
throw new StringIndexOutOfBoundsException(index);
}
return value[index];
}
/**
* 返回指定索引處的字符(Unicode)
*/
public int codePointAt(int index) {
if ((index < 0) || (index >= value.length)) {
throw new StringIndexOutOfBoundsException(index);
}
return Character.codePointAtImpl(value, index, value.length);
}
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);
}
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);
}
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);
}
/**
* 將此字符串中的字符複製到從dstBegin開始的dst中。
*/
void getChars(char dst[], int dstBegin) {
System.arraycopy(value, 0, dst, dstBegin, value.length);
}
/**
* @param srcBegin 要複製的字符串中第一個字符的索引.
* @param srcEnd 要複製的字符串中最後一個字符後的索引.
* @param dst 目標數組.
* @param dstBegin 目標數組中的起始偏移量.
*/
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);
}
/**
* 使用指定字符集將這個字符串編碼成一個字節序列,並將結果存儲到一個新的字節數組中。
*/
public byte[] getBytes(String charsetName)
throws UnsupportedEncodingException {
if (charsetName == null) throw new NullPointerException();
return StringCoding.encode(charsetName, value, 0, value.length);
}
public byte[] getBytes(Charset charset) {
if (charset == null) throw new NullPointerException();
return StringCoding.encode(charset, value, 0, value.length);
}
public byte[] getBytes() {
return StringCoding.encode(value, 0, value.length);
}
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;
}
public boolean contentEquals(StringBuffer sb) {
return contentEquals((CharSequence)sb);
}
private boolean nonSyncContentEquals(AbstractStringBuilder sb) {
char v1[] = value;
char v2[] = sb.getValue();
int n = v1.length;
if (n != sb.length()) {
return false;
}
for (int i = 0; i < n; i++) {
if (v1[i] != v2[i]) {
return false;
}
}
return true;
}
public boolean contentEquals(CharSequence cs) {
// Argument is a StringBuffer, StringBuilder
if (cs instanceof AbstractStringBuilder) {
if (cs instanceof StringBuffer) {
synchronized(cs) {
return nonSyncContentEquals((AbstractStringBuilder)cs);
}
} else {
return nonSyncContentEquals((AbstractStringBuilder)cs);
}
}
// Argument is a String
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;
}
public boolean equalsIgnoreCase(String anotherString) {
return (this == anotherString) ? true
: (anotherString != null)
&& (anotherString.value.length == value.length)
&& regionMatches(true, 0, anotherString, 0, value.length);
}
/**
* 比較兩個字符串詞典。比較基於字符串中每個字符的Unicode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;
}
private static class CaseInsensitiveComparator
implements Comparator<String>, java.io.Serializable {
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;
}
/** Replaces the de-serialized object. */
private Object readResolve() { return CASE_INSENSITIVE_ORDER; }
}
public int compareToIgnoreCase(String str) {
return CASE_INSENSITIVE_ORDER.compare(this, str);
}
/**
* 測試兩個字符串區域是否相等
*/
public boolean regionMatches(int toffset, String other, int ooffset,
int len) {
char ta[] = value;
int to = toffset;
char pa[] = other.value;
int po = ooffset;
// Note: toffset, ooffset, or len might be near -1>>>1.
if ((ooffset < 0) || (toffset < 0)
|| (toffset > (long)value.length - len)
|| (ooffset > (long)other.value.length - len)) {
return false;
}
while (len-- > 0) {
if (ta[to++] != pa[po++]) {
return false;
}
}
return true;
}
public boolean regionMatches(boolean ignoreCase, int toffset,
String other, int ooffset, int len) {
char ta[] = value;
int to = toffset;
char pa[] = other.value;
int po = ooffset;
// Note: toffset, ooffset, or len might be near -1>>>1.
if ((ooffset < 0) || (toffset < 0)
|| (toffset > (long)value.length - len)
|| (ooffset > (long)other.value.length - len)) {
return false;
}
while (len-- > 0) {
char c1 = ta[to++];
char c2 = pa[po++];
if (c1 == c2) {
continue;
}
if (ignoreCase) {
char u1 = Character.toUpperCase(c1);
char u2 = Character.toUpperCase(c2);
if (u1 == u2) {
continue;
}
if (Character.toLowerCase(u1) == Character.toLowerCase(u2)) {
continue;
}
}
return false;
}
return true;
}
/**
* 測試此字符串中以指定索引開頭的子字符串是否以指定前綴開頭
*/
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;
}
public boolean startsWith(String prefix) {
return startsWith(prefix, 0);
}
/**
* 測試此字符串是否以指定的後綴結尾.
*/
public boolean endsWith(String suffix) {
return startsWith(suffix, value.length - suffix.value.length);
}
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;
}
/**
* 返回指定字符第一次出現的字符串中的索引。
*/
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);
}
}
/**
* Handles (rare) calls of indexOf with a supplementary character.
*/
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;
}
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);
}
}
/**
* Handles (rare) calls of lastIndexOf with a supplementary character.
*/
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);
}
static int indexOf(char[] source, int sourceOffset, int sourceCount,
String target, int fromIndex) {
return indexOf(source, sourceOffset, sourceCount,
target.value, 0, target.value.length,
fromIndex);
}
static int indexOf(char[] source, int sourceOffset, int sourceCount,
char[] target, int targetOffset, int targetCount,
int fromIndex) {
if (fromIndex >= sourceCount) {
return (targetCount == 0 ? sourceCount : -1);
}
if (fromIndex < 0) {
fromIndex = 0;
}
if (targetCount == 0) {
return fromIndex;
}
char first = target[targetOffset];
int max = sourceOffset + (sourceCount - targetCount);
for (int i = sourceOffset + fromIndex; i <= max; i++) {
/* Look for first character. */
if (source[i] != first) {
while (++i <= max && source[i] != first);
}
/* Found first character, now look at the rest of v2 */
if (i <= max) {
int j = i + 1;
int end = j + targetCount - 1;
for (int k = targetOffset + 1; j < end && source[j]
== target[k]; j++, k++);
if (j == end) {
/* Found whole string. */
return i - sourceOffset;
}
}
}
return -1;
}
public int lastIndexOf(String str) {
return lastIndexOf(str, value.length);
}
public int lastIndexOf(String str, int fromIndex) {
return lastIndexOf(value, 0, value.length,
str.value, 0, str.value.length, fromIndex);
}
static int lastIndexOf(char[] source, int sourceOffset, int sourceCount,
String target, int fromIndex) {
return lastIndexOf(source, sourceOffset, sourceCount,
target.value, 0, target.value.length,
fromIndex);
}
static int lastIndexOf(char[] source, int sourceOffset, int sourceCount,
char[] target, int targetOffset, int targetCount,
int fromIndex) {
/*
* Check arguments; return immediately where possible. For
* consistency, don't check for null str.
*/
int rightIndex = sourceCount - targetCount;
if (fromIndex < 0) {
return -1;
}
if (fromIndex > rightIndex) {
fromIndex = rightIndex;
}
/* Empty string always matches. */
if (targetCount == 0) {
return fromIndex;
}
int strLastIndex = targetOffset + targetCount - 1;
char strLastChar = target[strLastIndex];
int min = sourceOffset + targetCount - 1;
int i = min + fromIndex;
startSearchForLastChar:
while (true) {
while (i >= min && source[i] != strLastChar) {
i--;
}
if (i < min) {
return -1;
}
int j = i - 1;
int start = j - (targetCount - 1);
int k = strLastIndex - 1;
while (j > start) {
if (source[j--] != target[k--]) {
i--;
continue startSearchForLastChar;
}
}
return start - sourceOffset + 1;
}
}
/**
*返回此字符串的子字符串.
*/
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 CharSequence subSequence(int beginIndex, int endIndex) {
return this.substring(beginIndex, endIndex);
}
/**
* 將指定的字符串連接到此字符串的結尾.
*/
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);
}
/**
* 返回一個字符串,該字符串中所有出現的oldChar替換爲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;
}
/**
* 此字符串是否與給定的正則表達式匹配
*/
public boolean matches(String regex) {
return Pattern.matches(regex, this);
}
/**
* 僅當此字符串包含指定的字符值序列時返回true。
*/
public boolean contains(CharSequence s) {
return indexOf(s.toString()) > -1;
}
/**
*用給定的replacement替換此字符串中與給定正則表達式匹配的第一個子字符串.
*
*/
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()));
}
/**
* 給定正則表達式的匹配項拆分此字符串.limit爲拆分的數量
*/
public String[] split(String regex, int limit) {
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);
}
/**
* 返回一個新字符串,該字符串由{CharSequence elements}的副本和指定的{delimiter}的副本組成.
*
*/
public static String join(CharSequence delimiter, CharSequence... elements) {
Objects.requireNonNull(delimiter);
Objects.requireNonNull(elements);
// Number of elements not likely worth Arrays.stream overhead.
StringJoiner joiner = new StringJoiner(delimiter);
for (CharSequence cs: elements) {
joiner.add(cs);
}
return joiner.toString();
}
public static String join(CharSequence delimiter,
Iterable<? extends CharSequence> elements) {
Objects.requireNonNull(delimiter);
Objects.requireNonNull(elements);
StringJoiner joiner = new StringJoiner(delimiter);
for (CharSequence cs: elements) {
joiner.add(cs);
}
return joiner.toString();
}
/**
* 轉換成小寫
*/
public String toLowerCase(Locale locale) {
if (locale == null) {
throw new NullPointerException();
}
int firstUpper;
final int len = value.length;
/* Now check if there are any characters that need to be changed. */
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; /* result may grow, so i+resultOffset
* is the write location in result */
/* Just copy the first few lowerCase characters. */
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);
}
/* Grow result if needed */
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);
}
public String toLowerCase() {
return toLowerCase(Locale.getDefault());
}
/**
*轉換成大寫
*/
public String toUpperCase(Locale locale) {
if (locale == null) {
throw new NullPointerException();
}
int firstLower;
final int len = value.length;
/* Now check if there are any characters that need to be changed. */
scan: {
for (firstLower = 0 ; firstLower < len; ) {
int c = (int)value[firstLower];
int srcCount;
if ((c >= Character.MIN_HIGH_SURROGATE)
&& (c <= Character.MAX_HIGH_SURROGATE)) {
c = codePointAt(firstLower);
srcCount = Character.charCount(c);
} else {
srcCount = 1;
}
int upperCaseChar = Character.toUpperCaseEx(c);
if ((upperCaseChar == Character.ERROR)
|| (c != upperCaseChar)) {
break scan;
}
firstLower += srcCount;
}
return this;
}
/* result may grow, so i+resultOffset is the write location in result */
int resultOffset = 0;
char[] result = new char[len]; /* may grow */
/* Just copy the first few upperCase characters. */
System.arraycopy(value, 0, result, 0, firstLower);
String lang = locale.getLanguage();
boolean localeDependent =
(lang == "tr" || lang == "az" || lang == "lt");
char[] upperCharArray;
int upperChar;
int srcChar;
int srcCount;
for (int i = firstLower; 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) {
upperChar = ConditionalSpecialCasing.toUpperCaseEx(this, i, locale);
} else {
upperChar = Character.toUpperCaseEx(srcChar);
}
if ((upperChar == Character.ERROR)
|| (upperChar >= Character.MIN_SUPPLEMENTARY_CODE_POINT)) {
if (upperChar == Character.ERROR) {
if (localeDependent) {
upperCharArray =
ConditionalSpecialCasing.toUpperCaseCharArray(this, i, locale);
} else {
upperCharArray = Character.toUpperCaseCharArray(srcChar);
}
} else if (srcCount == 2) {
resultOffset += Character.toChars(upperChar, result, i + resultOffset) - srcCount;
continue;
} else {
upperCharArray = Character.toChars(upperChar);
}
/* Grow result if needed */
int mapLen = upperCharArray.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] = upperCharArray[x];
}
resultOffset += (mapLen - srcCount);
} else {
result[i + resultOffset] = (char)upperChar;
}
}
return new String(result, 0, len + resultOffset);
}
public String toUpperCase() {
return toUpperCase(Locale.getDefault());
}
/**
* 去掉前後空格
*/
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;
}
public String toString() {
return this;
}
/**
*將此字符串轉換爲新的字符數組.
*/
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;
}
/**
* 使用指定的格式字符串和參數返回格式化字符串。
*/
public static String format(String format, Object... args) {
return new Formatter().format(format, args).toString();
}
public static String format(Locale l, String format, Object... args) {
return new Formatter(l).format(format, args).toString();
}
public static String valueOf(Object obj) {
return (obj == null) ? "null" : obj.toString();
}
public static String valueOf(char data[]) {
return new String(data);
}
public static String valueOf(char data[], int offset, int count) {
return new String(data, offset, count);
}
public static String copyValueOf(char data[], int offset, int count) {
return new String(data, offset, count);
}
public static String copyValueOf(char data[]) {
return new String(data);
}
public static String valueOf(boolean b) {
return b ? "true" : "false";
}
public static String valueOf(char c) {
char data[] = {c};
return new String(data, true);
}
public static String valueOf(int i) {
return Integer.toString(i);
}
public static String valueOf(long l) {
return Long.toString(l);
}
public static String valueOf(float f) {
return Float.toString(f);
}
public static String valueOf(double d) {
return Double.toString(d);
}
/**
*返回字符串對象的規範表示形式
*字符串池最初爲空,由類{String}私下維護.
*調用intern方法時,
*如果池中已包含由{equals(object)}方法確定的與此{string}對象相等的字符串
*則返回池中的字符串。否則,將此{@code String}對象添加到池中,
*並返回對此{@code String}對象的引用。
*/
public native String intern();
}
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