前言
2023-10-9 14:29:39
以下内容源自《【JDK源码分析】》
仅供学习交流使用
推荐
Integer类
parseInt()
测试代码
源码分析在Solution类中
其巧妙之处在于：
使用负数保存运算结果：
// Accumulating negatively avoids surprises near MAX_VALUE 负积累可避免在MAX_VALUE附近出现意外 int result=0; result -= digit;
预先判断1
if (result < multmin) {//保证不能 <-214748364 ，否则*radix之后必然越界 throw forInputString(s); } result *= radix;
预先判断2
if (result < limit + digit) {//保证不能result-digit<limit，否则-digit;之后必然越界 throw forInputString(s); } result -= digit;//这里是负数保存
测试代码
package testlang; public class TestInteger1 { public static void main(String[] args) { int max=Integer.MAX_VALUE;//2147483647 int min=Integer.MIN_VALUE;//-2147483648 try { String minS="-2147483649"; int m = Integer.parseInt(minS); System.out.println(m); } catch (NumberFormatException e) { e.printStackTrace(); } try { String maxS="2147483648"; int n = Integer.parseInt(maxS); System.out.println(n); } catch (NumberFormatException e) { e.printStackTrace(); } } static class Solution{ public static int parseInt(String s) throws NumberFormatException { return parseInt(s,10); } public static int parseInt(String s, int radix) throws NumberFormatException { 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"); } //result是按照负数来处理结果的 //result -= digit; int result = 0; boolean negative = false; int i = 0, len = s.length(); int limit = -Integer.MAX_VALUE;//-2147483647 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;//-2147483648 } else if (firstChar != '+') throw forInputString(s); if (len == 1) // Cannot have lone "+" or "-" throw forInputString(s); i++; } multmin = limit / radix;//-214748364 while (i < len) { // Accumulating negatively avoids surprises near MAX_VALUE digit = Character.digit(s.charAt(i++),radix); if (digit < 0) { throw forInputString(s); } if (result < multmin) {//保证不能 <-214748364 ，否则*radix之后必然越界 throw forInputString(s); } result *= radix; if (result < limit + digit) {//保证不能result-digit<limit，否则-digit;之后必然越界 throw forInputString(s); } result -= digit;//这里是负数保存 } } else { throw forInputString(s); } return negative ? result : -result; } //NumberFormatException.forInputString() static NumberFormatException forInputString(String s) { return new NumberFormatException("For input string: \"" + s + "\""); } } }
parseByte()
源码是调用parseInt()之后判断是否溢出
可以借鉴parseInt()来自己实现此代码
代码
package testlang; public class TestByte { public static void main(String[] args) { try { String str="-129"; byte b = parseByte(str); System.out.println(b); } catch (Exception e) { e.printStackTrace(); } try { String str="+128"; byte b = parseByte(str); System.out.println(b); } catch (Exception e) { e.printStackTrace(); } String str="+127"; byte b = parseByte(str); System.out.println(b); str="-128"; b = parseByte(str); System.out.println(b); } public static byte parseByte(String s){ int radix=10; if (s==null){ throw new NumberFormatException("null"); } byte result=0;//结果按负数保存 boolean negative = false; int i = 0, len = s.length(); int limit = -Byte.MAX_VALUE;//-127 int multmin; int digit; if (len>0){ char firstChar=s.charAt(0); if (firstChar<'0'){ if (firstChar=='-'){ negative=true; limit=Byte.MIN_VALUE; }else if (firstChar!='+'){//非法的符号 throw forInputString(s); } if (len==1){//不能只包含符号 throw forInputString(s); } i++; } multmin=limit/radix; while (i<len){ digit=Character.digit(s.charAt(i++),radix); if (digit<0){ throw forInputString(s); } if (result<multmin){ throw forInputString(s); } result*=radix; if (result<limit+digit){ throw forInputString(s); } result-=digit; } return negative?result: (byte) -result; }else { throw forInputString(s); } } static NumberFormatException forInputString(String s) { return new NumberFormatException("For input string: \"" + s + "\""); } }
最后
2023-10-9 14:38:43
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