题目：字符串转换为整数。
思路：将字符串转化为整数首先是遍历字符串中的每一个字符，有三种情况：首字符是正号，首字符是负号，首字符非正负号；然后遍历每一个字符进行num = num * 10 + charArray[i] - '0'，在进行这个工作之前首先需要对charArray[i]进行是否为数字字符的判断，循环遍历直到结束，输出结果；
注意事项（思考点）：（1）字符串中包含有非数字字符；（2）字符串中包含正负符号；（3）考虑最大的正整数；（4）考虑最小的负整数；（4）溢出。
开始时我的想法只考虑了第一，第二种情况：
/*** 将字符串转换为整数 1:非数字字符 2:正负号* @author Peter*/public class Main {public static void main(String[] args) {Scanner sc = new Scanner(System.in);String str = sc.nextLine();int length = str.length();int number = 0;char[] charArray = str.toCharArray();if (charArray[0] == '-') {for (int i = 1; i < length; i++) {if (charArray[i] > '9' || charArray[i] < '0') {System.out.println("输入的字符串中包含非数字字符");return;} else {number = number * 10 + charArray[i] - '0';}}System.out.println(-number);} else if (charArray[0] == '+') {for (int i = 1; i < length; i++) {if (charArray[i] > '9' || charArray[i] < '0') {System.out.println("输入的字符串中包含非数字字符");return;} else {number = number * 10 + charArray[i] - '0';}}System.out.println(number);} else {for (int i = 0; i < length; i++) {if (charArray[i] > '9' || charArray[i] < '0') {System.out.println("输入的字符串中包含非数字字符");return;} else {number = number * 10 + charArray[i] - '0';}}System.out.println(number);}}}
源码（考虑了上面所有情况）：
package com.chap01.offer01;import java.util.Scanner;/*** 字符串转整形源码*/public class Main1 {/*** 将字符串转化为整形* @param s* @return* @throws NumberFormatException*/public static int parseInt(String s) throws NumberFormatException {return parseInt(s, 10);}/*** 将字符串转化为整形，其中radix代表基数，即代表你输入的是几进制的字符串。* @param s* @param radix* @return* @throws NumberFormatException*/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.*///判断是否为空if (s == null) {throw new NumberFormatException("null");}//判断不能小于2进制if (radix < Character.MIN_RADIX) {throw new NumberFormatException("radix " + radix+ " less than Character.MIN_RADIX");}//判断不能大于36进制 0-9，a-z一共36进制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; //-0x7fffffff即：-2147483647int multmin;int digit;if (len > 0) {char firstChar = s.charAt(0); //取首字符if (firstChar < '0') { // Possible leading "+" or "-"if (firstChar == '-') {negative = true;limit = Integer.MIN_VALUE; //0x80000000} else if (firstChar != '+')throw NumberFormatException.forInputString(s);if (len == 1) // Cannot have lone "+" or "-"throw NumberFormatException.forInputString(s);i++;}multmin = limit / radix; //-214748364while (i < len) {// Accumulating negatively avoids surprises near MAX_VALUEdigit = Character.digit(s.charAt(i++), radix); //返回指定基数的数值if (digit < 0) {throw NumberFormatException.forInputString(s);}if (result < multmin) {throw NumberFormatException.forInputString(s);}result *= radix; //字符abc: a*100 + b*10 + cif (result < limit + digit) {throw NumberFormatException.forInputString(s);}result -= digit;}} else {throw NumberFormatException.forInputString(s);}return negative ? result : -result;}public static void main(String[] args){Scanner sc = new Scanner(System.in);String str = sc.nextLine();int num = parseInt(str);System.out.println(num);}}
package com.chap01.offer01;//异常类public class NumberFormatException extends IllegalArgumentException {static final long serialVersionUID = -2848938806368998894L;/*** Constructs a <code>NumberFormatException</code> with no detail message.*/public NumberFormatException() {super();}/*** Constructs a <code>NumberFormatException</code> with the specified detail* message.** @param s* the detail message.*/public NumberFormatException(String s) {super(s);}/*** Factory method for making a <code>NumberFormatException</code> given the* specified input which caused the error.** @param s* the input causing the error*/static NumberFormatException forInputString(String s) {return new NumberFormatException("For input string: \"" + s + "\"");}}