java 源碼閱讀（一） String

java 源碼閱讀（一） String

String的屬性：

char[] value;

int hash;

String的重要方法

intern()

/**

* Returns a canonical representation for the string object.

* <p>

* A pool of strings, initially empty, is maintained privately by the

* class {@code String}.

* <p>

* When the intern method is invoked, if the pool already contains a

* string equal to this {@code String} object as determined by

* the {@link #equals(Object)} method, then the string from the pool is

* returned. Otherwise, this {@code String} object is added to the

* pool and a reference to this {@code String} object is returned.

* <p>

* It follows that for any two strings {@code s} and {@code t},

* {@code s.intern() == t.intern()} is {@code true}

* if and only if {@code s.equals(t)} is {@code true}.

* <p>

* All literal strings and string-valued constant expressions are

* interned. String literals are defined in section 3.10.5 of the

* <cite>The Java&trade; Language Specification</cite>.

*

* @return  a string that has the same contents as this string, but is

*          guaranteed to be from a pool of unique strings.

*/

public native String intern();

這是一個native方法，如果常量池中存在當前字符串，就會直接返回該字符串，如果常量池中沒有此字符串，會將此字符串放入常量池後，再返回。

hashcode()

/**
 * Returns a hash code for this string. The hash code for a
 * {@code String} object is computed as
 * <blockquote><pre>
 * s[0]*31^(n-1) + s[1]*31^(n-2) + ... + s[n-1]
 * </pre></blockquote>
 * using {@code int} arithmetic, where {@code s[i]} is the
 * <i>i</i>th character of the string, {@code n} is the length of
 * the string, and {@code ^} indicates exponentiation.
 * (The hash value of the empty string is zero.)
 *
 * @return  a hash code value for this object.
 */
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;
}

這裏他給了一個hashcode的計算公式：s[0]31^(n-1) + s[1]31^(n-2) + ... + s[n-1]。

這裏有一個問題：爲什麼hashcode乘子是31？

31 可以被JVM優化，31*i = (i << 5) -i

爲什麼不是2？

因爲2^5 = 31,hashcode的區間太小了，這樣hashcode的衝突概率比較大。

例子一：

public static void main(String[] args) {
    String a = new String("ab");
    String b = "ab";
    String c = "a" + "b";
    System.out.println(a.equals(b));
    System.out.println(a.equals(c));
    System.out.println(b.equals(c));
    System.out.println(a == b);
    System.out.println(a == c);
    System.out.println(b == c);
}

 

true
true
true
false
false
true

解釋

new String()無法在編譯器確定。

String b = "ab" 可以在編譯器確定，直接放入字符串常量池

String c = "a" + "b" 可以在編譯器優化爲 String c = "ab"

例子二：

public static void main(String[] args) {
    String a = new String("ab");
    String b = "ab";
    String c = "a" + new String("b");
    System.out.println(a.equals(b));
    System.out.println(a.equals(c));
    System.out.println(b.equals(c));
    System.out.println(a == b);
    System.out.println(a == c);
    System.out.println(b == c);
}

true
true
true
false
false
false

例子三：

public static void main(String[] args) {
    String a = "ab";
    String b = new String("ab");
    System.out.println(a == b);
    String c = b.intern();
    System.out.println(a == c);
}

false
true