Java的类常量和文件导入

Question

I'm pretty much trying to use a class constant to declare as a file but get an error.

import java.awt.*;
import java.util.*;
import java.io.*;

public class BabyNames {
   public static final Scanner NAME=new Scanner(new File("names.txt")); //specifically this part 
   public static final int YEAR=1900;                                   //generates the error
   public static final int LS=11;
   public static final int WIDTH=50;
   public static void main(String[] args) throws FileNotFoundException{
      intro();
      personName();
      graph();
   }

// Error contained:
// BabyNames.java:6: error: unreported exception FileNotFoundException; must be caught or
// declared to be thrown                 
//   public static final Scanner NAME=new Scanner(new File("names.txt"));
                                ^
//1 error

this is not all of the program but dont think the rest is required. sorry if my method of asking was funky, first time. thanks much.

Answer 1

You need a try catch around that code.

That's what "must be caught or declared to be thrown" means.

Read this : doc oracle

Answer 2

There are two types of Exception : 1) checked Exception: those should be handle at compile time (by using try,catch,throws) 2) unchecked Exception: No compiler error for these exception but at run time exception may occurs.

Your case is first one FileNotFoundException is checked exception so either you have to write that line inside try catch block or you have to use keyword throws. to pass exception handling to calling method.

Answer 3

Technically you can use a static initializer block to set your staic final Scanner variable. Inside the initializer block you can use try/catch:

public static final Scanner NAME; 

static {
  // Be sure scanner is initialized even in the case of an exception
  Scanner scanner = null;

  try {
    new Scanner(new File("names.txt"));
  } catch (FileNotFoundException e) {
    e.printStackTrace();
  }
  NAME = scanner;
}

BUT: It is bad style to hold an instance of a class like Scanner as a static constant. A Scanner is a one way object. You are about to consume the file content and that's it. It doesn't make sense to hold such an object as a constant and much less if it is public . A static variable will be intialized as soon as the class is loaded and lives as long as your program. In most cases this is not what you want.

A better approach would be to make it an instance variable and initilize it in the constructor. And make it private to the class. You can hold the file name as a constant and create the constructor to accept any file name. The methods intro() , personName() and graph() can be made instance methods, too.

public class BabyNames {
  public static final String NAMES_FILE_NAME = "names.txt";

  public static final int YEAR=1900;                                   
  public static final int LS=11;
  public static final int WIDTH=50;

  private final Scanner name; 

  public BabyNames(String fileName) throws FileNotFoundException {
    name = new Scanner(new File(fileName));

  }

  public static void main(String[] args) throws FileNotFoundException {
    BabyNames babyNames = new BabyNames(NAMES_FILE_NAME);
    babyNames.intro();
    babyNames.personName();
    babyNames.graph();
  }

  public void graph() {
    // ...    
  }

  public void personName() {
    // ...    
  }

  public void intro() {
    // ...    
  }

  // ...
}

As a consequence the Scanner variable is only known to the BabyNames class and lives as long as the particular BabyNames instance. This is more modular, better to maintain and easier to test. Eg you can write a unit test and initialize your class with a test file.