字符串和数字转换

public class Test {
    public static void main(String[] args) {
        String str = "123";
 
        Integer num1 = new Integer(str);
 
        int num2 = Integer.parseInt(str);
 
        Integer num3 = Integer.valueOf(str);
 
        System.out.println(num1 + "\t" + num2 + "\t" + num3);//123	123	123
    }
}

在上面代码中，演示了三个方式的转化，分别是new了新对象和调用静态方法，我们查看内部的变化

Interger 类的构造函数

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public Integer(String s) throws NumberFormatException {
        this.value = parseInt(s, 10);
    }

从代码中看的，构造函数时调用了一个parseInt(s,10)的内部函数

parseInt(s)

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public static int parseInt(String s) throws NumberFormatException {
        return parseInt(s,10);
    }

从代码中看到也是调用了一个parseInt(s,10)的内部函数

Integer.valueOf(s)

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public static Integer valueOf(String s) throws NumberFormatException {
        return Integer.valueOf(parseInt(s, 10));
    }

从代码中看到也是调用了一个 parseInt(s,10) 的内部函数
我们可以看出其实都调用了 parseInt(String s,int radix); 让我们看看他内部实现

parseInt(String s,int radix)

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++;
          }
      ......
      ......
      ......
       } else {
          throw NumberFormatException.forInputString(s);
      }
      return negative ? result : -result;
  }

从代码中看出，==字符串不能为空值以及字符串得为数字==，不然会报异常（ NumberFormatException ）