Java文件读写

import java.io.*;
import java.util.Random;
import java.util.Scanner;

public class lecture {
    public static void main(String[] args) throws IOException {
        // writing data to text file
        try{
            //overwrite "w"
            PrintWriter out = new PrintWriter(new FileOutputStream("data.txt"));
            out.println("Welcome to JAVA");
            out.println(2020);
            out.println(23.12);
            out.close();
            //add a number 100 into data.txt
            PrintWriter out1 = new PrintWriter(new FileOutputStream("data.txt",true));
            out1.println("Welcome to JAVA");
            out1.println(2020);
            out1.println(23.12);
            out1.close();
        }catch (IOException e){
            System.out.println("Problem with file output");
        }


        // reading data from text file
        try {
            Scanner in = new Scanner(new FileInputStream("data.txt"));
            while (in.hasNextLine()){
                System.out.println(in.nextLine());
            }
            in.close();
        }catch(FileNotFoundException e){
            System.out.println("File was not found");
        }

        // write a program to store the exchange rate to the text file
        //named currency.txt
        //USD 0.245 EUR 0.205 GBP 0.184 AUD 0.332 THB 7.41
        try{
            //overwrite "w"
            PrintWriter out = new PrintWriter(new FileOutputStream("currency.txt"));
            out.println("USD 0.245\nEUR 0.205\nGBP 0.184\nAUD 0.332\nTHB 7.41\n");
            out.close();
        }catch (IOException e){
            System.out.println("Problem with file output");
        }

        // write a program to calculate the following based on the exchange rate
        //RMB1234 = USD
        //RMB456 = AUD
        //RMB999 = THB
        System.out.println("RMB1234 = USD" + getCurrency(1234, "USD"));
        System.out.println("RMB1234 = AUD" + getCurrency(456, "AUD"));
        System.out.println("RMB1234 = THB" + getCurrency(999, "THB"));


        // write data to binary file
        try{
            ObjectOutputStream out = new ObjectOutputStream(new FileOutputStream("binary.dat"));
            out.writeUTF("Welcome to JAVA");
            out.writeInt(2020);
            out.writeDouble(23.12);
            out.close();
        }catch (IOException e){
            System.out.println("Problem with file output");
        }

        // read data from binary file
        try{
            ObjectInputStream in = new ObjectInputStream(new FileInputStream("binary.dat"));
            // SEQUENCE is important!!
            System.out.println(in.readUTF());
            System.out.println(in.readInt());
            System.out.println(in.readDouble());
            in.close();
        }catch  (FileNotFoundException e){
            System.out.println("File was not found");
        }catch (IOException e) {
            System.out.println("Problem with file input");
        }

        // generate N random numbers within 10-100, N is within (20-30)
        // and then store in a binary file number.dat
        Random r = new Random();
        final int MIN = 10, MAX = 100;
        int N = r.nextInt(11) + 20;
        try{
            ObjectOutputStream out = new ObjectOutputStream(new FileOutputStream("number.dat"));
            for(int i=1; i<=N; i++){
                    out.writeInt(r.nextInt(MAX - MIN + 1) + MIN);
                }
            out.close() ;
        }catch (IOException e){
            System.out.println("Problem with file output");
        }

        // read all the random numbers from number.dat
        // display all the numbers, maximum and minimum
        int num=0, max=0, min=0;
        try {
            ObjectInputStream in = new ObjectInputStream(new FileInputStream("number.dat"));

            num = in.readInt();
            min = num;
            max = num;
            System.out.print(num + " ");
            try {
                while (true) {
                    num = in.readInt();
                    if (min > num) {
                        min = num;
                    }
                    if (max < num) {
                        max = num;
                    }
                    System.out.print(num + " ");
                }

            } catch (EOFException e) { }
            in.close();
        }catch (FileNotFoundException e){
            System.out.println("File was not found");
        }catch(IOException e){
            System.out.println("Problem with file input");
        }
        System.out.println("\nThe max is " + max);
        System.out.println("\nThe min is " + min);

        // File Class
        File f = new File("data.txt");
        File f1 = new File("data1.txt");
        File f2 = new File("data2.txt");
        if (f.exists()){
            System.out.println("The file already exists");
            System.out.println("Rename file to data1.txt");
            System.out.println(f.renameTo(f1));
        }
        if (f2.exists()){
            System.out.println("The file already exists");
            System.out.println("Delete the file");
            System.out.println(f2.delete());
        }

    }
    public static double getCurrency(double a, String b){
        double result = 0;
        try{
            Scanner in = new Scanner(new FileInputStream("currency.txt"));
            while(in.hasNextLine()){
                String currency = in.next();
                double rate = in.nextDouble();
                if (b.equals(currency)){
                    result = a*rate;
                    break;
                }
                in.nextLine();
            }
            in.close();
        }catch (FileNotFoundException e){
            System.out.println("File was not found");
        }
        return result;
    }
}