先来看段代码
public class IntegerDemo {
public static void main(String[] args) {
String num = null;
System.out.println( Integer.parseInt(num));// Exception java.lang.NumberFormatException
System.out.println( Integer.valueOf(num));// Exception java.lang.NumberFormatException
System.out.println( String.valueOf(num)); //输出null
num = "";
System.out.println( Integer.parseInt(num)); // Exception java.lang.NumberFormatException
System.out.println( Integer.valueOf(num)); // Exception java.lang.NumberFormatException
System.out.println( String.valueOf(num));//空串，啥也不输出
}
}
先看一下 String.valueOf() 里面是怎么写的
String.valueOf（） 在遇到 null 和 空串的情况下 ，都能正常输出，所以不抛错
再来看下 包装类型 Integer里面又是如何处理的
这两个方法里面都需要先 parseInt( s，10)，就是将字符串s先转成 十进制的 int基本类型，，但是 valueOf（）会根据int范围从[-127,127]的内部缓存中去取（用到设计模式中的 享元模式）
一起来看下 parseInt(s, 10)，，在方法里面会判断字符串是否是合法的数字，会去校验null, 空串等其他格式，所以会抛错
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;
}