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分享看java String类里的substring()方法的实现产生的疑问，高手赐教。。。
substring()的实现代码：
其中的value是什么？在看到在String类里面倒是有声明一个char[] value数组，并且有一个构造函数：
但是想了很久，还是不明白这个char[] value和substring之间的关系
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);
}
其中的value是什么？在看到在String类里面倒是有声明一个char[] value数组，并且有一个构造函数：
public String(String original) {
this.value = original.value;
this.hash = original.hash;
}
但是想了很久，还是不明白这个char[] value和substring之间的关系
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姜小白- 2014-09-13
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我在家的源码看的也是这个，刚才我贴的应该是1.6的源码，你贴的是1.7的源码
jdk也在进步，一些不用的东西也会慢慢改进，删掉的
重点理解思想就可以了，不用太过纠结这个太过具体的东西
scott_129 2014-09-13
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public String(String original) {
int size = original.count;
char[] originalValue = original.value;
char[] v;
if (originalValue.length > size) {
// The array representing the String is bigger than the new
// String itself. Perhaps this constructor is being called
// in order to trim the baggage, so make a copy of the array.
int off = original.offset;
v = Arrays.copyOfRange(originalValue, off, off+size);
} else {
// The array representing the String is the same
// size as the String, so no point in making a copy.
v = originalValue;
}
this.offset = 0;
this.count = size;
this.value = v;
}
danceinstyle 2014-09-13
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你的这个构造方法我见到过，我家里的源码好像就是你的那个
把你的jdk种String 源码中上面的那一块贴出来，上面应该是有的
部分细微的差别可能是jdk版本不一致导致的，但大的应该是大差不差的。[/quote]
还是没找到啊！count的声明也找不到
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;
/**
* Class String is special cased within the Serialization Stream Protocol.
*
* A String instance is written into an ObjectOutputStream according to
* <a href="{@docRoot}/../platform/serialization/spec/output.html">
* Object Serialization Specification, Section 6.2, "Stream Elements"</a>
*/
private static final ObjectStreamField[] serialPersistentFields =
new ObjectStreamField[0];
/**
* Initializes a newly created {@code String} object so that it represents
* an empty character sequence. Note that use of this constructor is
* unnecessary since Strings are immutable.
*/
public String() {
this.value = new char[0];
}
/**
* Initializes a newly created {@code String} object so that it represents
* the same sequence of characters as the argument; in other words, the
* newly created string is a copy of the argument string. Unless an
* explicit copy of {@code original} is needed, use of this constructor is
* unnecessary since Strings are immutable.
*
* @param original
* A {@code String}
*/
public String(String original) {
this.value = original.value;
this.hash = original.hash;
}
/**
* Allocates a new {@code String} so that it represents the sequence of
* characters currently contained in the character array argument. The
* contents of the character array are copied; subsequent modification of
* the character array does not affect the newly created string.
*
* @param value
* The initial value of the string
*/
public String(char value[]) {
this.value = Arrays.copyOf(value, value.length);
}
/**
* Allocates a new {@code String} that contains characters from a subarray
* of the character array argument. The {@code offset} argument is the
* index of the first character of the subarray and the {@code count}
* argument specifies the length of the subarray. The contents of the
* subarray are copied; subsequent modification of the character array does
* not affect the newly created string.
*
* @param value
* Array that is the source of characters
*
* @param offset
* The initial offset
*
* @param count
* The length
*
* @throws IndexOutOfBoundsException
* If the {@code offset} and {@code count} arguments index
* characters outside the bounds of the {@code value} array
*/
public String(char value[], int offset, int count) {
if (offset < 0) {
throw new StringIndexOutOfBoundsException(offset);
}
if (count < 0) {
throw new StringIndexOutOfBoundsException(count);
}
// 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);
}
姜小白- 2014-09-13
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你的这个构造方法我见到过，我家里的源码好像就是你的那个
把你的jdk种String 源码中上面的那一块贴出来，上面应该是有的
部分细微的差别可能是jdk版本不一致导致的，但大的应该是大差不差的。
danceinstyle 2014-09-13
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谢谢你的回答！String在初始化的时候，JVM也自动地新建了一个char[] value，value的所有字符与String的分别一一对应，在执行String的substring操作的时候，就用这个char[] value来新建一个String再返回。这样解释合理！
现在我郁闷的是，我的JDK里的String的源代码跟你的是不同的。。。我的没有声明private final int count，
还有我的这个构造函数跟你的不一样。。。不知道是什么问题
public String(String original) {
this.value = original.value;
this.hash = original.hash;
}
姜小白- 2014-09-13
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public final class String
implements java.io.Serializable, Comparable<String>, CharSequence
{
/** The value is used for character storage. */
private final char value[];
/** The offset is the first index of the storage that is used. */
private final int offset;
/** The count is the number of characters in the String. */
private final int count;
/** Cache the hash code for the string */
private int hash; // Default to 0
public String(String original) {
int size = original.count;
char[] originalValue = original.value;
char[] v;
if (originalValue.length > size) {
// The array representing the String is bigger than the new
// String itself. Perhaps this constructor is being called
// in order to trim the baggage, so make a copy of the array.
int off = original.offset;
v = Arrays.copyOfRange(originalValue, off, off+size);
} else {
// The array representing the String is the same
// size as the String, so no point in making a copy.
v = originalValue;
}
this.offset = 0;
this.count = size;
this.value = v;
}
danceinstyle 2014-09-13
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我没有看到那个将String转化成char[]的代码，难道是自动进行的？substring函数一开始就value.length了。。。那个value代表的是什么？
danceinstyle 2014-09-13
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我没有看到那个将String转化成char[]的代码，难道是自动进行的？substring函数一开始就value.length了。。。那个value代表的是什么？
a12939026 2014-09-13
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value就是一个char的数组啊
string的本质其实就是一个char数组，substring就是从数组中取出几个char来组成新的一个char数组也就是新的string
danceinstyle 2014-09-13
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我在家的源码看的也是这个，刚才我贴的应该是1.6的源码，你贴的是1.7的源码
jdk也在进步，一些不用的东西也会慢慢改进，删掉的
重点理解思想就可以了，不用太过纠结这个太过具体的东西[/quote]
