Java String 的 equals() 方法可能的最佳化

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最佳化




JDK1.4, 1.5 的 String Class 代碼如下











以下內容為程式碼











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;
























 


 



 



以下內容為程式碼











    /**











     * 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;











    }























從這段建構函式中,我們可以看出,不同Reference的String之間有可能共用相同的 char[]。












 


 



 



以下內容為程式碼











    /**











     * 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;











    }























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。












 


 



 



以下內容為程式碼











            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;











                        }
























 


 



 



這樣就能夠對應共用 char[] 的情況,能夠加快比較速度。














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