JAVA中Integer.valueOf， parsetInt() String.valueOf的區別和結果

先來看段代碼

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)); //輸出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，啥也不輸出
    }
}

先看一下 String.valueOf() 裡面是怎麼寫的

String.valueOf（） 在遇到 null 和 空串的情況下 ，都能正常輸出，所以不拋錯

再來看下 包裝類型 Integer裡面又是如何處理的

這兩個方法裡面都需要先 parseInt( s，10)，就是將字元串s先轉成  十進位的 int基本類型，，但是 valueOf（）會根據int範圍從[-127,127]的內部緩存中去取（用到設計模式中的 享元模式）

一起來看下 parseInt(s, 10)，，在方法裡面會判斷字元串是否是合法的數字，會去校驗null, 空串等其他格式，所以會拋錯

 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;
    }