只输出循环的最后一行

Question

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

public class LargeDataset {
public static void main(String[] args) throws Exception {
    File file = new File("src/Salary.txt");
    if (file.exists()) {
        System.out.print("Sorry this file already exists.");
        System.exit(0);
    }
    String firstName = "";
    String lastName = "";
    String rank = "";
    double salaryRange = 0.0;

    for (int i = 1; i <= 1000; i++) {
        try (PrintWriter output = new PrintWriter(file))
        {
            firstName = "FirstName" + i;
            lastName = "LastName" + i;
            rank = generateRandomRank();
            if (rank == "assistant")
                salaryRange = generateSalary(50000.00, 80000.00);
            else if (rank == "associate")
                salaryRange = generateSalary(60000.00, 110000.00);
            else
                salaryRange = generateSalary(75000.00, 130000.00);
            output.printf("%s %s %s $%.2f", firstName, lastName, rank, salaryRange);
            output.println();
        }
    }
}

public static String generateRandomRank() {
    String[] rank = {"assistant", "associate", "full"};
    Random random1 = new Random();
    return rank[random1.nextInt(3)];
}

public static double generateSalary(double minSalary, double maxSalary) {
    double randomSalary = minSalary + Math.random() * (maxSalary - minSalary);
    return randomSalary;
 }  
}

Hi everyone. I have a program that generates 1000 lines of text and saves it into a file named Salary. The format of each line is: firstNamei, lastNamei, a random rank, and a random salary that is suited to the rank. However when I run this program it only outputs the 1000th line of the loop. I noticed however, when I don't put the PrintWriter in the try statement and close it after the loop by myself, it runs fine and generates all 1000 lines. Why is it only generating the last line based on how it is right now though?

Answer 1

You should open your PrintWriter once , and then write to it many times from your loop, not the other way around:

try (PrintWriter output = new PrintWriter(file)) {
    for (int i = 1; i <= 1000; i++) {
        firstName = "FirstName" + i;
        lastName = "LastName" + i;
        rank = generateRandomRank();
        if (rank == "assistant")
            salaryRange = generateSalary(50000.00, 80000.00);
        else if (rank == "associate")
            salaryRange = generateSalary(60000.00, 110000.00);
        else
            salaryRange = generateSalary(75000.00, 130000.00);
        output.printf("%s %s %s $%.2f", firstName, lastName, rank, salaryRange);
        output.println();
    }
}

You should use the above pattern instead of what you have. If you want an exact fix to your current code, then you may try opening the PrintWriter in append mode:

for (int i=1; i <= 1000; i++) {
    try (PrintWriter output = new PrintWriter(new FileOutputStream(file, true)) {
        // same logic
    }
}

This should also work, because now, even though you create a new PrintWriter for each iteration of the loop (inefficient), you open the underlying file in append mode, so each new line should get written properly.

Answer 2

Every time that you are iterating through your 1000 you are creating a new file

for (int i = 1; i <= 1000; i++) {
    try (PrintWriter output = new PrintWriter(file))
    ...
}

move it before the loop

 try (PrintWriter output = new PrintWriter(file)) {
   for (int i = 1; i <= 1000; i++) {
   }
 }