在寫代碼的時候，遇到Double類型轉換為Integer類型，代碼如下：

Double d = 3d;  
Integer i = Integer.valueOf(d.toString()); 

Exception in thread "main" java.lang.NumberFormatException: For input string: "3.0"

分析：

　　1）d.toString() = 3.0；

　　2）查看Integer valueOf源碼 ，調用paserInt方法

public static int parseInt(String s, int radix)
            throws NumberFormatException
{
    /*
     * WARNING: This method may be invoked early during VM initialization
     * before IntegerCache is initialized. Care must be taken to not use
     * the valueOf method.
     */

    if (s == null) {
        throw new NumberFormatException("null");
    }

    if (radix < Character.MIN_RADIX) {
        throw new NumberFormatException("radix " + radix +
                                        " less than Character.MIN_RADIX");
    }

    if (radix > Character.MAX_RADIX) {
        throw new NumberFormatException("radix " + radix +
                                        " greater than Character.MAX_RADIX");
    }

    int result = 0;
    boolean negative = false;
    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 "-"
            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;
        }
    } else {
        throw NumberFormatException.forInputString(s);
    }
    return negative ? result : -result;
}

斷點跟蹤，發現如下

，字符“.”在拋出NumverFormatException異常。該方法返回的是在指定的基數返回字符ch的數值，默認是10進制，沒有"."的字符，返回-1。

　　3）經過，分析，發現Integer中轉換為Integer類型方法：Integer(String s)，valuserOf(String s)都是調用上面的方法。那麽這條路行不通。Integer類，Double類繼承了Number類，Double類裏面有對實現了Numbe類r的intValue()的抽象方法。使用該方法轉換為int類型，然後轉換為Integer類型，但是精度會丟失。

類型轉換的坑