Integer.parseInt() 源码

Integer.parseInt() 是 Java 中用于将字符串转换为 int 类型的静态方法，它的源码如下：

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Java
public static int parseInt(String s)
        throws NumberFormatException {
    return parseInt(s, 10);
}

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;
}
Java

1. 方法定义

Integer.parseInt() 方法是一个静态方法，它接收一个字符串参数，并返回一个 int 类型的值。

2. 参数检查

该方法首先检查参数是否为空，如果为空则抛出 NumberFormatException 异常。然后检查 radix 参数的范围，radix 必须在 Character.MIN_RADIX 到 Character.MAX_RADIX 之间，否则也会抛出 NumberFormatException 异常。

3. 字符串转换

接着，程序会对字符串进行遍历，将每个字符转换为数字，根据 radix 参数的值来确定转换规则。在转换过程中，会检查每个字符是否有效，如果无效，则抛出 NumberFormatException 异常。最后，将转换后的数字相加，得到最终的结果，并返回。