如何从独立方法打印ArrayList？电厂数据分析程序打印功能实现求助

Let's walk through the issues in your code and fix them so the PrintData method works correctly and your program runs as expected.

Key Issues in the Original Code #

• UploadData doesn't return the loaded data: Right now it creates an ArrayList<Entry> but returns null, so PrintData has no data to print.
• Main loop logic is broken: Using separate while loops for each choice won't handle repeated menu selections correctly.
• Missing parameter when calling PrintData: You're calling PrintData() without passing the ArrayList<Entry> it needs.
• Case-sensitive variable mismatch: userChoice vs userchoice causes undefined variable errors.
• Scanner newline leftover: After nextInt(), the newline character stays in the buffer, causing nextLine() to read an empty string when asking for the filename.

Fixed Full Code #

import java.io.File;
import java.io.IOException;
import java.util.ArrayList;
import java.util.Scanner;

public class PowerPlantAnalyzer {

    public static void main(String[] args) throws IOException {
        Scanner keyboard = new Scanner(System.in);
        ArrayList<Entry> dataList = null; // Store the loaded data here
        int userChoice;

        do {
            showMenu();
            userChoice = keyboard.nextInt();
            keyboard.nextLine(); // Clear the newline after nextInt()

            switch (userChoice) {
                case 1:
                    dataList = UploadData(keyboard); // Save the returned data
                    break;
                case 2:
                    if (dataList != null) {
                        PrintData(dataList); // Pass the loaded data to PrintData
                    } else {
                        System.out.println("Please upload data first!");
                    }
                    break;
                case 3:
                    System.out.println("Download Statistics feature coming soon!");
                    break;
                case 4:
                    System.out.println("Print Month feature coming soon!");
                    break;
                case 5:
                    System.out.println("Exiting program...");
                    break;
                default:
                    System.out.println("Invalid choice, please try again.");
            }
        } while (userChoice != 5); // Loop until user chooses exit

        keyboard.close();
    }

    public static void showMenu() {
        System.out.println("Welcome to the Power Plant Analyzer program. Please choose from the following options:\n" +
                " 1. Upload Data\n" +
                " 2. View Data \n" +
                " 3. Download Statistics \n" +
                " 4. Print Month \n" +
                " 5. Exit the program");
    }

    // Changed return type to ArrayList<Entry> and pass the existing Scanner
    public static ArrayList<Entry> UploadData(Scanner keyboard) throws IOException {
        System.out.println("What is the name of the file that contains the data?");
        String inputFileName = keyboard.nextLine();

        File f = new File(inputFileName);
        Scanner inputFile = new Scanner(f);

        ArrayList<Entry> monthList = new ArrayList<>();

        while (inputFile.hasNext()) {
            String m = inputFile.next();
            String d = inputFile.next();
            String y = inputFile.next();
            float p = inputFile.nextFloat();

            Entry entry = new Entry(m, d, y, p);
            monthList.add(entry);
        }

        inputFile.close();
        System.out.println("Data uploaded successfully!");
        return monthList; // Return the loaded list
    }

    // Now correctly accepts the ArrayList<Entry> parameter
    public static void PrintData(ArrayList<Entry> monthList) {
        System.out.println("\n--- Power Plant Data ---");
        for (Entry entry : monthList) {
            // Format output to match your example: Date: January 10, 2018 Output: 236.9
            System.out.printf("Date: %s %s, %s Output: %.1f%n", 
                entry.getMonth(), entry.getDay(), entry.getYear(), entry.getPowerOutput());
        }
        System.out.println("------------------------\n");
    }
}

class Entry {
    private String month;
    private String day;
    private String year;
    private float powerOutput;

    public Entry() {}

    public Entry(String m, String d, String y, float p) {
        month = m;
        day = d;
        year = y;
        powerOutput = p;
    }

    // Added getters to access private fields in PrintData
    public String getMonth() { return month; }
    public String getDay() { return day; }
    public String getYear() { return year; }
    public float getPowerOutput() { return powerOutput; }

    // Optional: Keep your original print method if needed
    public void print() {
        System.out.println("Month: " + month + " Day: " + day + " Year: " + year + " Power Output: " + powerOutput);
    }
}

What Changed & Why #

1. Stored loaded data in main: Added ArrayList<Entry> dataList to hold the data returned by UploadData.
2. Fixed UploadData return type: Changed from String[] to ArrayList<Entry> and return the populated list instead of null. Also re-used the main Scanner to avoid newline issues.
3. Redesigned main loop: Used a do-while loop with a switch statement for cleaner, more maintainable menu handling.
4. Passed data to PrintData: Now calls PrintData(dataList) and checks if data exists first to avoid null errors.
5. Added getters to Entry: Allowed PrintData to access private fields to format the output exactly as your example shows.
6. Formatted output correctly: Used printf to match the requested output style (Date: January 10, 2018 Output: 236.9).

How to Test #

1. Run the program, choose option 1 and enter your data file name.
2. Choose option 2—you'll see the data printed in the exact format you wanted.
3. The menu will re-display after each action until you choose option 5 to exit.

内容的提问来源于stack exchange，提问作者Nitro