包装类Integer和String互相转换
本文为joshua317原创文章,转载请注明：转载自joshua317博客 https://www.joshua317.com/article/202
一、包装类Integer和String互相转换
package com.joshua317;
public class Main {
public static void main(String[] args) {
Integer i = 100;
//包装类Integer ---> String
//方式一：直接后面跟空字符串
String str1 = i + "";
//方式二：调用String类的静态方法valueOf()
String str2 = String.valueOf(i);
//方式三：调用Integer类的成员方法toString()
String str3 = i.toString();
//方式四：调用Integer类的静态方法toString()
String str4 = Integer.toString(i);
System.out.println(str1);
System.out.println(str2);
System.out.println(str3);
System.out.println(str4);
//String ---> 包装类Integer
String str5 = "12345";
//方式一：调用Integer类的静态方法parseInt()
Integer i2 = Integer.parseInt(str5);
//方式二：调用Integer类的静态方法valueOf()
Integer i3 = Integer.valueOf(str5);
//方式三：调用Integer类的静态方法valueOf()返回一个Integer，然后intValue()拆箱返回int，再自动装箱
Integer i4 = Integer.valueOf(str5).intValue();
//方式四：调用Integer类的构造方法
Integer i5 = new Integer(str5);
System.out.println(i2);
System.out.println(i3);
System.out.println(i4);
System.out.println(i5);
}
}
二、拓展Integer.parseInt（String str）方法的原理
Integer.parseInt（String str）方法
/**
* Parses the string argument as a signed decimal integer. The
* characters in the string must all be decimal digits, except
* that the first character may be an ASCII minus sign {@code '-'}
* ({@code '\u005Cu002D'}) to indicate a negative value or an
* ASCII plus sign {@code '+'} ({@code '\u005Cu002B'}) to
* indicate a positive value. The resulting integer value is
* returned, exactly as if the argument and the radix 10 were
* given as arguments to the {@link #parseInt(java.lang.String,
* int)} method.
*
* @param s a {@code String} containing the {@code int}
* representation to be parsed
* @return the integer value represented by the argument in decimal.
* @exception NumberFormatException if the string does not contain a
* parsable integer.
*/
public static int parseInt(String s) throws NumberFormatException {
//内部默认调用parseInt(String s, int radix)，radix默认设置为10，即十进制
return parseInt(s,10);
}
Integer.parseInt（String s, int radix）方法
/**
* Parses the string argument as a signed integer in the radix
* specified by the second argument. The characters in the string
* must all be digits of the specified radix (as determined by
* whether {@link java.lang.Character#digit(char, int)} returns a
* nonnegative value), except that the first character may be an
* ASCII minus sign {@code '-'} ({@code '\u005Cu002D'}) to
* indicate a negative value or an ASCII plus sign {@code '+'}
* ({@code '\u005Cu002B'}) to indicate a positive value. The
* resulting integer value is returned.
*
* <p>An exception of type {@code NumberFormatException} is
* thrown if any of the following situations occurs:
* <ul>
* <li>The first argument is {@code null} or is a string of
* length zero.
*
* <li>The radix is either smaller than
* {@link java.lang.Character#MIN_RADIX} or
* larger than {@link java.lang.Character#MAX_RADIX}.
*
* <li>Any character of the string is not a digit of the specified
* radix, except that the first character may be a minus sign
* {@code '-'} ({@code '\u005Cu002D'}) or plus sign
* {@code '+'} ({@code '\u005Cu002B'}) provided that the
* string is longer than length 1.
*
* <li>The value represented by the string is not a value of type
* {@code int}.
* </ul>
*
* <p>Examples:
* <blockquote><pre>
* parseInt("0", 10) returns 0
* parseInt("473", 10) returns 473
* parseInt("+42", 10) returns 42
* parseInt("-0", 10) returns 0
* parseInt("-FF", 16) returns -255
* parseInt("1100110", 2) returns 102
* parseInt("2147483647", 10) returns 2147483647
* parseInt("-2147483648", 10) returns -2147483648
* parseInt("2147483648", 10) throws a NumberFormatException
* parseInt("99", 8) throws a NumberFormatException
* parseInt("Kona", 10) throws a NumberFormatException
* parseInt("Kona", 27) returns 411787
* </pre></blockquote>
*
* @param s the {@code String} containing the integer
* representation to be parsed
* @param radix the radix to be used while parsing {@code s}.
* @return the integer represented by the string argument in the
* specified radix.
* @exception NumberFormatException if the {@code String}
* does not contain a parsable {@code int}.
*/
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.
*/
//判断字符是否为null
if (s == null) {
throw new NumberFormatException("null");
}
//进制是否小于最小进制2，Character.MIN_RADIX值为2
if (radix < Character.MIN_RADIX) {
throw new NumberFormatException("radix " + radix +
" less than Character.MIN_RADIX");
}
//进制是否大于最大进制36，Character.MAX_RADIX值为36
if (radix > Character.MAX_RADIX) {
throw new NumberFormatException("radix " + radix +
" greater than Character.MAX_RADIX");
}
int result = 0;
//是否是负数
boolean negative = false;
//char字符数组下标和长度
int i = 0, len = s.length();
//限制
int limit = -Integer.MAX_VALUE;
int multmin;
int digit;
//判断字符长度是否大于0，否则抛出异常
if (len > 0) {
//第一个字符是否是符号
char firstChar = s.charAt(0);
//根据ascii码表看出加号(43)和负号(45)对应的,十进制数小于'0'(48)的
if (firstChar < '0') { // Possible leading "+" or "-"
//是负号
if (firstChar == '-') {
//负号属性设置为true
negative = true;
limit = Integer.MIN_VALUE;
} else if (firstChar != '+')//不是负号也不是加号则抛出异常
throw NumberFormatException.forInputString(s);
//如果有符号（加号或者减号）且字符串长度为1，则抛出异常
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
//返回指定基数中字符表示的数值。（此处是十进制数值）
digit = Character.digit(s.charAt(i++),radix);
//小于0，则为非radix进制数
if (digit < 0) {
throw NumberFormatException.forInputString(s);
}
//这里是为了保证下面计算不会超出最大值
if (result < multmin) {
throw NumberFormatException.forInputString(s);
}
result *= radix;
if (result < limit + digit) {
throw NumberFormatException.forInputString(s);
}
result -= digit;
}
} else {
throw NumberFormatException.forInputString(s);
}
//根据上面得到的是否负数，返回相应的值
return negative ? result : -result;
}
本文为joshua317原创文章,转载请注明：转载自joshua317博客 https://www.joshua317.com/article/202
