java 出现NullPointerException的原因及解决办法

package java.lang;

/**
 * Thrown when a program tries to access a field or method of an object or an
 * element of an array when there is no instance or array to use, that is if the
 * object or array points to {@code null}. It also occurs in some other, less
 * obvious circumstances, like a {@code throw e} statement where the {@link
 * Throwable} reference is {@code null}.
 */
public class NullPointerException extends RuntimeException {

  private static final long serialVersionUID = 5162710183389028792L;

  /**
   * Constructs a new {@code NullPointerException} that includes the current
   * stack trace.
   */
  public NullPointerException() {
  }

  /**
   * Constructs a new {@code NullPointerException} with the current stack
   * trace and the specified detail message.
   *
   * @param detailMessage
   *      the detail message for this exception.
   */
  public NullPointerException(String detailMessage) {
    super(detailMessage);
  }

}

void main() {
  int* i = NULL;
  printf("%#x\n", i);
  printf("%#x\n", &i);
  system("pause");
}