JDK1.4, 1.5 的 String Class 代碼如下
[code]
public final class String
implements Java.io.Serializable, Comparable
{
/** 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;
[/code]
[code]
/**
* Initializes a newly created 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 original is needed, use of this
* constructor is unnecessary since Strings are immutable.
*
* @param original a String.
*/
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.
v = new char[size];
System.arraycopy(originalValue, original.offset, v, 0, 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;
}
[/code]
從這段構造函數中，我們可以看出，不同Reference的String之間有可能共享相同的 char[]。
[code]
/**
* Compares this string to the specifIEd object.
* The result is true if and only if the argument is not
* null and is a String object that represents
* the same sequence of characters as this object.
*
* @param anObject the object to compare this String
* against.
* @return true if the String are equal;
* false otherwise.
* @see java.lang.String#compareTo(Java.lang.String)
* @see java.lang.String#equalsIgnoreCase(Java.lang.String)
*/
public boolean equals(Object anObject) {
if (this == anObject) {
return true;
}
if (anObject instanceof String) {
String anotherString = (String)anObject;
int n = count;
if (n == anotherString.count) {
char v1[] = value;
char v2[] = anotherString.value;
int i = offset;
int j = anotherString.offset;
while (n-- != 0) {
if (v1[i++] != v2[j++])
return false;
}
return true;
}
}
return false;
}
[/code]
但是，equals 方法似乎忽略了這個可能。沒有直接對兩者的char[]的reference進行比較。
按照我的想法，應該加入這麼一段。
[code]
if (anObject instanceof String) {
String anotherString = (String)anObject;
int n = count;
if (n == anotherString.count) {
char v1[] = value;
char v2[] = anotherString.value;
int i = offset;
int j = anotherString.offset;
////{{
if(i == j && v1 == v2) return true; // NOTE: this line is added by me
////}}
while (n-- != 0) {
if (v1[i++] != v2[j++])
return false;
}
[/code]
這樣就能夠對應共享 char[] 的情況，能夠加快比較速度。
