Java源代码学习 -- java.lang.String
java.lang.String是使用频率非常高的类。要想更好的使用java.lang.String类,了解其源代码实现是非常有必要的。由java.lang.String,自然联想到java.lang.StringBuffer和java.lang.StringBuilder,下篇文章再来研究java.lang.StringBuffer和java.lang.StringBuilder。
重要属性
java.lang.String对象中字符串主要是以字符数组的形式存储。当调用对象方法获取字符串长度时,直接返回数组长度。判断字符串是否为空isEmpty()时,也直接检查数组长度是否为0。其部分发生代码如下所示:
1 /** The value is used for character storage. */
2 private final char value[];
3
4 /** Cache the hash code for the string */
5 private int hash; // Default to 0
value:存储字符串的字符数组。该数组为final变量,一旦赋值,将不会更改。
hash:该String对象的哈希值。
构造方法
java.lang.String对象构造方法比较多,列举如下:
1 public String()
2 public String(String original)
3 public String(char value[])
4 public String(char value[], int offset, int count)
5 public String(int[] codePoints, int offset, int count)
6 @Deprecated
7 public String(byte ascii[], int hibyte, int offset, int count)
8 @Deprecated
9 public String(byte ascii[], int hibyte)
10 public String(byte bytes[], int offset, int length, String charsetName) throws UnsupportedEncodingException
11 public String(byte bytes[], int offset, int length, Charset charset)
12 public String(byte bytes[], String charsetName) throws UnsupportedEncodingException
13 public String(byte bytes[], Charset charset)
14 public String(byte bytes[], int offset, int length)
15 public String(byte bytes[])
16 public String(StringBuffer buffer)
17 public String(StringBuilder builder)
在 public String(StringBuffer buffer) 中,传入形参为StringBuffer,StringBuffer为线程安全类。则在此构造方法内部进行了synchronized关键字锁同步。代码如下:
1 public String(StringBuffer buffer) {
2 synchronized(buffer) {
3 this.value = Arrays.copyOf(buffer.getValue(), buffer.length());
4 }
5 }
在 public String(StringBuilder builder) 中,传入形参为StringBuilder,StringBuilder为非线程安全类。则在此构造方法内部内部未做同步处理,对比 public String(StringBuffer buffer) 。代码如下:
1 public String(StringBuilder builder) {
2 this.value = Arrays.copyOf(builder.getValue(), builder.length());
3 }
常用方法
java.lang.String对象中封装方法非常多,仅针对常用方法源代码进行分析。如:equals(),replace(), indexOf(),startsWith(),compareTo(),regionMathes(),hashCode()。
public boolean equals(Object anObject)
用于比较两对象存储内容是否相同。采用比较巧妙的方式进行排除比较:
(1)先“==”比较两对象是否是同一对象,若是,直接返回true, 否则进一步判断;
(2)判断待比较对象类型是否是java.lang.String,若不是,直接返回false,否则进一步判断;
(3)判断两字符串长度是否相等,若不是直接返回false,否则进一步判断;
(4)从字符数组中第一个字符开始,依次进行比较,一旦发现不相同字符直接返回false,若所在字符均相同则返回true。对字符数组中字符依次进行比较是一件非常耗时的操作,将此操作放在最后执行,先利用其它条件进行对其进行判断。比较巧妙!
1 public boolean equals(Object anObject) {
2 if (this == anObject) {
3 return true;
4 }
5 if (anObject instanceof String) {
6 String anotherString = (String)anObject;
7 int n = value.length;
8 if (n == anotherString.value.length) {
9 char v1[] = value;
10 char v2[] = anotherString.value;
11 int i = 0;
12 while (n-- != 0) {
13 if (v1[i] != v2[i])
14 return false;
15 i++;
16 }
17 return true;
18 }
19 }
20 return false;
21 }
public String replace(char oldChar, char newChar)
将字符串中指定字符替换为新的字符。
(1)先判断待替换字符和新字符是否相同,若相同,则直接返回原字符串,若不同,则继续执行;
(2)找出第一次出现待替换字符位置i,创建新的等长字符数组,将该位置之前的字符依次放入新的字符数组中;
(3)从位置i处依次遍历比较原字符数组中字符是否是待替换字符,若是,则将新字符放入新字符数组对应位置,若不是,则将原字符数组中字符放入对应位置。巧妙做了一个小优化,直接找出第一次出现待替换字符的位置,再从此处开始遍历,提高效率。
1 public String replace(char oldChar, char newChar) {
2 if (oldChar != newChar) {
3 int len = value.length;
4 int i = -1;
5 char[] val = value; /* avoid getfield opcode */
6
7 while (++i < len) {
8 if (val[i] == oldChar) {
9 break;
10 }
11 }
12 if (i < len) {
13 char buf[] = new char[len];
14 for (int j = 0; j < i; j++) {
15 buf[j] = val[j];
16 }
17 while (i < len) {
18 char c = val[i];
19 buf[i] = (c == oldChar) ? newChar : c;
20 i++;
21 }
22 return new String(buf, true);
23 }
24 }
25 return this;
26 }
public String replace(CharSequence target, CharSequence replacement)
该方法是我们通常意义所用到的 public String replace(String target, String replacement) ,java.lang.String实现了java.lang.CharSequence接口。方法内部调用正则表达式匹配替换来实现。
1 public String replace(CharSequence target, CharSequence replacement) {
2 return Pattern.compile(target.toString(), Pattern.LITERAL).matcher(
3 this).replaceAll(Matcher.quoteReplacement(replacement.toString()));
4 }
public int indexOf(String str)
该方法是找出目标字符串是第一次出现指定子字符串的位置,若不存在,则返回-1,若存在,则返回位置坐标。具体实现是调用 static int indexOf(char[] source, int sourceOffset, int sourceCount, char[] target, int targetOffset, int targetCount, int fromIndex) 方法。先对目标字符串中出现子字符串的位置可能范围,然后在此范围中遍历找出与子字符串第一个字符相同的位置,并对后面字符进行比较分析。
1 /**
2 * Returns the index within this string of the first occurrence of the
3 * specified substring.
4 */
5 public int indexOf(String str) {
6 return indexOf(str, 0);
7 }
8
9 /**
10 * Returns the index within this string of the first occurrence of the
11 * specified substring, starting at the specified index.
12 */
13 public int indexOf(String str, int fromIndex) {
14 return indexOf(value, 0, value.length,
15 str.value, 0, str.value.length, fromIndex);
16 }
17
18 /**
19 * Code shared by String and StringBuffer to do searches. The
20 * source is the character array being searched, and the target
21 * is the string being searched for.
22 *
23 * @param source the characters being searched.
24 * @param sourceOffset offset of the source string.
25 * @param sourceCount count of the source string.
26 * @param target the characters being searched for.
27 * @param targetOffset offset of the target string.
28 * @param targetCount count of the target string.
29 * @param fromIndex the index to begin searching from.
30 */
31 static int indexOf(char[] source, int sourceOffset, int sourceCount,
32 char[] target, int targetOffset, int targetCount,
33 int fromIndex) {
34 if (fromIndex >= sourceCount) {
35 return (targetCount == 0 ? sourceCount : -1);
36 }
37 if (fromIndex < 0) {
38 fromIndex = 0;
39 }
40 if (targetCount == 0) {
41 return fromIndex;
42 }
43
44 char first = target[targetOffset];
45 int max = sourceOffset + (sourceCount - targetCount);
46
47 for (int i = sourceOffset + fromIndex; i
48 /* Look for first character. */
49 if (source[i] != first) {
50 while (++i ]
51 }
52
53 /* Found first character, now look at the rest of v2 */
54 if (i
55 int j = i + 1;
56 int end = j + targetCount - 1;
57 for (int k = targetOffset + 1; j < end && source[j]
58 == target[k]; j++, k++);
59
60 if (j == end) {
61 /* Found whole string. */
62 return i - sourceOffset;
63 }
64 }
65 }
66 return -1;
67 }
public int compareTo(String anotherString)
该方法是对字符串集合进行排序的基础,通过此方法可比较两字符串大小,原理很简单,源代码如下:
1 public int compareTo(String anotherString) {
2 int len1 = value.length;
3 int len2 = anotherString.value.length;
4 int lim = Math.min(len1, len2);
5 char v1[] = value;
6 char v2[] = anotherString.value;
7
8 int k = 0;
9 while (k < lim) {
10 char c1 = v1[k];
11 char c2 = v2[k];
12 if (c1 != c2) {
13 return c1 - c2;
14 }
15 k++;
16 }
17 return len1 - len2;
18 }
public boolean startsWith(String prefix)
判断目标字符串是否以指定字符子串开关,该方法内部是调用 public boolean startsWith(String prefix, int toffset) 方法实现,原理很简单,代码如下:
1 /**
2 * Tests if this string starts with the specified prefix.
3 *
4 * @param prefix the prefix.
5 */
6 public boolean startsWith(String prefix) {
7 return startsWith(prefix, 0);
8 }
public int hashCode()
其hashCode()代码如下:
1 public int hashCode() {
2 int h = hash;
3 if (h == 0 && value.length > 0) {
4 char val[] = value;
5
6 for (int i = 0; i < value.length; i++) {
7 h = 31 * h + val[i];
8 }
9 hash = h;
10 }
11 return h;
12 }
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