java.lang.*中Integer类源代码详解

Myth丶恋晨 2023-02-21 04:58 5阅读 0赞

# java.lang.\*中Integer类 源代码详解 #

**核心方法：**

int parseInt(String s) 字符串转为int
    Integer valueOf(String s) 字符串转为Integer对象

## parseInt方法 ##

public static int parseInt(String s) throws NumberFormatException { 
            return parseInt(s,10); //默认转化为10进制数
    }
    /* * @param s 目标字符串 * @param radix 转化的进制 * @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");
            }
    		//最小转化为2进制
            if (radix < Character.MIN_RADIX) { 
                throw new NumberFormatException("radix " + radix +
                                                " less than Character.MIN_RADIX");
            }
    		//最大转化为36进制
            if (radix > Character.MAX_RADIX) { 
                throw new NumberFormatException("radix " + radix +
                                                " greater than Character.MAX_RADIX");
            }
    
            int result = 0;//接收计算结果
            boolean negative = false;//标记符号
            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 "-" //判断符号
                    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; 这样更好理解
                }
            } else { 
                throw NumberFormatException.forInputString(s);
            }
            return negative ? result : -result;
    }

## valueOf方法 ##

> 1.它本身底层调用的还是parseInt方法  
> 2.区别于parseInt，它返回的是Integer对象  
> 3.对于-128-127的值，它默认缓存了，执行速度更快

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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