Java String 的 equals() 方法可能的优化

JDK1.4, 1.5 的 String Class 代码如下

[code]

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;

[/code]

[code]

/**

* Initializes a newly created <code>String</code> 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</code> is needed, use of this

* constructor is unnecessary since Strings are immutable.

*

* @param original a <code>String</code>.

*/

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]

从这段构造函数中，我们可以看出，不同Reference的String之间有可能共享相同的 char[]。

[code]

/**

* Compares this string to the specified object.

* The result is <code>true</code> if and only if the argument is not

* <code>null</code> and is a <code>String</code> object that represents

* the same sequence of characters as this object.

*

* @param anObject the object to compare this <code>String</code>

* against.

* @return <code>true</code> if the <code>String </code>are equal;

* <code>false</code> 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 方法似乎忽略了这个可能。没有直接对两者的char[]的reference进行比较。

按照我的想法，应该加入这么一段。

[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]

这样就能够对应共享 char[] 的情况，能够加快比较速度。