Java - 有沒有辦法打開一個文本文件，如果它不存在，並附加到它，如果它確實存在？

Question

我對Java很新，而且遇到了這個問題。如果它不存在，我想讓java代碼生成一個txt文件，但如果它確實存在，我希望PrintWriter使用FileWriter將其附加到它。這裏是我的代碼：

編輯：我試圖解決我的代碼，但現在我得到IOException錯誤。我在這裏做錯了什麼？ 編輯2：我認爲我的代碼是獨一無二的，因爲我試圖在文件不存在的情況下創建一個新文件，並且在現有文件已經存在的情況下添加它。

import java.util.Scanner; 
import java.io.File; 
import java.io.FileWriter; 
import java.io.BufferedWriter; 
import java.io.IOException; 

/** 
* Created by FakeOwl96 on 3/28/2017. 
*/ 
public class AreaOfCircle { 
    private static double PI = Math.PI; 
    private double radius; 
    private static double area; 
    public static void main(String[] args) { 
     Scanner keyboard = new Scanner(System.in); 
     AreaOfCircle a = new AreaOfCircle(); 
     System.out.print("Type in the radius of circle: "); 
     a.radius = keyboard.nextDouble(); 
     getArea(a.radius); 
     System.out.print("Name of the txt file you want to create:"); 
     String fileName = keyboard.nextLine(); 
     keyboard.nextLine(); 
     try { 
      File myFile = new File(fileName); 
      if (!myFile.exists()) { 
       myFile.createNewFile(); 
      } 
      FileWriter fw = new FileWriter(myFile, true); 
      BufferedWriter bw = new BufferedWriter(fw); 
      bw.write("The area of the circle is " + area + ".\n"); 
      bw.close(); 
     } 
     catch (IOException e) { 
      System.out.println("IOException Occured"); 
      e.printStackTrace(); 
     } 
    } 

    public static void getArea(double n) { 
     area = n * PI; 
    } 
} 

Answer 1

import java.io.BufferedWriter; 
import java.io.File; 
import java.io.FileWriter; 
import java.io.IOException; 
import java.util.Scanner; 

public class AreaOfCircle { 
    private static double PI = Math.PI; 
    private double radius; 
    private static double area; 
    public static void main(String[] args) { 
     Scanner keyboard = new Scanner(System.in); 
     AreaOfCircle a = new AreaOfCircle(); 
     System.out.print("Type in the radius of circle: "); 
     a.radius = keyboard.nextDouble(); 
     getArea(a.radius); 
     System.out.print("Name of the txt file you want to create:"); 
     String fileName = keyboard.next(); 
     keyboard.nextLine(); 
     try { 
      File myFile = new File(fileName); 
      if (!myFile.exists()) { 
       myFile.createNewFile(); 
      } 
      FileWriter fw = new FileWriter(myFile, true); 
      BufferedWriter bw = new BufferedWriter(fw); 
      bw.write("The area of the circle is " + area + ".\n"); 
      bw.close(); 
     } 
     catch (IOException e) { 
      System.out.println("IOException Occured"); 
      e.printStackTrace(); 
     } 
    } 

    public static void getArea(double n) { 
     area = n * PI; 
    } 
} 

我所做的唯一變化是 字符串文件名= keyboard.next（）;來自//keyboard.nextLine（） 上面的代碼適用於我。希望這可以幫助。

Answer 2

添加以下初始化myFile後線：

myFile.createNewFile(); // if file already exists will do nothing 

Answer 3

這是文件附加行的例子，並創建新的文件，如果文件不存在。

import java.io.BufferedWriter; 
import java.io.File; 
import java.io.FileWriter; 
import java.io.IOException; 
public class AppendFileDemo { 

    public static void main(String[] args) { 
     try { 
      String content = "This is my content which would be appended " 
        + "at the end of the specified file"; 
      //Specify the file name and path here 
      File file = new File("myfile.txt"); 

      /* This logic is to create the file if the 
     * file is not already present 
      */ 
      if (!file.exists()) { 
       file.createNewFile(); 
      } 

      //Here true is to append the content to file 
      FileWriter fw = new FileWriter(file, true); 
      //BufferedWriter writer give better performance 
      BufferedWriter bw = new BufferedWriter(fw); 
      bw.write(content); 
      //Closing BufferedWriter Stream 
      bw.close(); 

      System.out.println("Data successfully appended at the end of file"); 

     } catch (IOException ioe) { 
      System.out.println("Exception occurred:"); 
      ioe.printStackTrace(); 
     } 
    } 
} 

Answer 4

這是文件追加行的另一個示例，如果文件不存在，則創建新文件。

import java.io.BufferedWriter; 
import java.io.File; 
import java.io.FileWriter; 
import java.io.IOException; 
import java.io.PrintWriter; 
public class AppendFileDemo2 { 

    public static void main(String[] args) { 
     try { 
      File file = new File("myfile2.txt"); 
      if (!file.exists()) { 
       file.createNewFile(); 
      } 
      FileWriter fw = new FileWriter(file, true); 
      BufferedWriter bw = new BufferedWriter(fw); 
      PrintWriter pw = new PrintWriter(bw); 
      //This will add a new line to the file content 
      pw.println(""); 
      /* Below three statements would add three 
      * mentioned Strings to the file in new lines. 
      */ 
      pw.println("This is first line"); 
      pw.println("This is the second line"); 
      pw.println("This is third line"); 
      pw.close(); 

      System.out.println("Data successfully appended at the end of file"); 

     } catch (IOException ioe) { 
      System.out.println("Exception occurred:"); 
      ioe.printStackTrace(); 
     } 
    } 
}