java.lang.*中Integer類 源代碼詳解

java.lang.*中Integer類 源代碼詳解

核心方法：

int parseInt(String s) 字符串轉爲int
Integer valueOf(String s) 字符串轉爲Integer對象

parseInt方法

public static int parseInt(String s) throws NumberFormatException {
        return parseInt(s,10); //默認轉化爲10進制數
}
/*
* @param      s   目標字符串
* @param      radix   轉化的進制
* @exception  NumberFormatException if the {@code String}
*             does not contain a parsable {@code int}.
*/
public static int parseInt(String s, int radix)
                throws NumberFormatException
{
        if (s == null) {
            throw new NumberFormatException("null");
        }
		//最小轉化爲2進制
        if (radix < Character.MIN_RADIX) {
            throw new NumberFormatException("radix " + radix +
                                            " less than Character.MIN_RADIX");
        }
		//最大轉化爲36進制
        if (radix > Character.MAX_RADIX) {
            throw new NumberFormatException("radix " + radix +
                                            " greater than Character.MAX_RADIX");
        }

        int result = 0;//接收計算結果
        boolean negative = false;//標記符號
        int i = 0, len = s.length();
        int limit = -Integer.MAX_VALUE;
        int multmin;
        int digit;

        if (len > 0) {
            char firstChar = s.charAt(0);
            if (firstChar < '0') { // Possible leading "+" or "-"			//判斷符號
                if (firstChar == '-') {
                    negative = true;
                    limit = Integer.MIN_VALUE;
                } else if (firstChar != '+')
                    throw NumberFormatException.forInputString(s);

                if (len == 1) // Cannot have lone "+" or "-"
                    throw NumberFormatException.forInputString(s);
                i++;
            }
            multmin = limit / radix;
            while (i < len) {
                // Accumulating negatively avoids surprises near MAX_VALUE
                digit = Character.digit(s.charAt(i++),radix);
                if (digit < 0) {
                    throw NumberFormatException.forInputString(s);
                }
                if (result < multmin) {
                    throw NumberFormatException.forInputString(s);
                }
                result *= radix;
                if (result < limit + digit) {
                    throw NumberFormatException.forInputString(s);
                }
                result -= digit;
                //result = result * radix - digit; 這樣更好理解
            }
        } else {
            throw NumberFormatException.forInputString(s);
        }
        return negative ? result : -result;
}

valueOf方法

1.它本身底層調用的還是parseInt方法
2.區別於parseInt，它返回的是Integer對象
3.對於-128-127的值，它默認緩存了，執行速度更快

public static Integer valueOf(String s) throws NumberFormatException {
        return Integer.valueOf(parseInt(s, 10));
}

public static Integer valueOf(int i) {
        if (i >= IntegerCache.low && i <= IntegerCache.high)
            return IntegerCache.cache[i + (-IntegerCache.low)];
        return new Integer(i);
    }

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