Question

我找到了许多关于如何将ArrayList从一个类正确传递到另一个类的网站。我已经尝试了所有这些方法，但不知何故，列表中的信息不会传递给其他类。

public class Final {

public static void main(String[] args) {
    Scanner input = new Scanner(System.in);
    boolean correctInput;
    int numQuestions;
    String again;

    Part1 part1 = new Part1();
    Part2 part2 = new Part2();

    do{

    System.out.println("Welcome to the What If game.");
    do{
    try{
        correctInput = false;
    System.out.println("How many questions and answers would you like? There's a max of 15.");
    Scanner filein = new Scanner(System.in);
        numQuestions = filein.nextInt();
        part1.numQuestions(numQuestions);

    while(numQuestions > 15){
        System.out.println("Sorry, you can only have up to 15. Try again.");
        numQuestions = input.nextInt();
        part1.numQuestions(numQuestions);
    }
    }
    catch(InputMismatchException e){
        System.out.println("You entered in an invalid answer.");
        correctInput = true;
    }
    }while(correctInput);

    part1.Random();

    System.out.println("Would you like the questions and answers printed to the screen or a separate file? Enter 'screen' or 'file'.");
    String screenFile = input.next();

    if(screenFile.equalsIgnoreCase("screen")){
        part2.PrinttoScreen();
    }
    else if(screenFile.equalsIgnoreCase("file")){
        System.out.println("Please input the location you want to write to. Example: C:\\temp\\WhatIf.txt");
        String writeFile = input.next();
        part2.writeFile(writeFile);
        part2.PrinttoFile();
    }
    else{
        System.out.println("Invalid input. Try again.");
        screenFile = input.next();
    }



    System.out.println("Do you want to play again? y/n");
    again = input.next();
    again.toLowerCase();


    }while(again == "y");
}

首先支持班级：

public class Part1 {
ArrayList<String> arrayQuestions = new ArrayList<String>();
ArrayList<String> arrayAnswers = new ArrayList<String>();
ArrayList<String> randomPairs = new ArrayList<String>();
int numQuestions;

public void numQuestions(int numQuestions){
     this.numQuestions = numQuestions; 
 }

private void ReadFile1(){
    try {
        Scanner fileIn = new Scanner(Paths.get("C:\\temp\\Questions.txt"));

        while(fileIn.hasNextLine()){
            arrayQuestions.add(fileIn.nextLine());

            }

    } catch (IOException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    } 
}

private void ReadFile2(){
    try {
        Scanner fileIn2 = new Scanner(Paths.get("C:\\temp\\Answers.txt"));

        while(fileIn2.hasNextLine()){
            arrayAnswers.add(fileIn2.nextLine());

            }
    } catch (IOException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }
}


public void Random(){
    ReadFile1();
    ReadFile2();

    Random randnum1 = new Random();
    Random randnum2 = new Random();
    randomPairs = new ArrayList<String>();

    for(int x=0; x<this.numQuestions; x++){
        int rand1 = randnum1.nextInt(arrayQuestions.size());
        String randquestion =arrayQuestions.get(rand1);
        randomPairs.add(randquestion);

        int rand2 = randnum2.nextInt(arrayAnswers.size());
        String randanswer = arrayAnswers.get(rand2);
        randomPairs.add(randanswer);
    }   


}

public ArrayList<String> getPairs(){
    return randomPairs;
}

第二支持班级：

public class Part2 {

PrintWriter fileout;

String writeFile;   
Part1 part1 = new Part1();
ArrayList<String> randomPairs = part1.getPairs();

public void writeFile(String writeFile){
    this.writeFile = writeFile;
}

public void PrinttoScreen(){

    for(int j=0; j<randomPairs.size(); j++){
    System.out.println(randomPairs.get(j));
    }
}

public void PrinttoFile(){

    try {
        fileout = new PrintWriter(this.writeFile);

        for(int x=0; x<randomPairs.size(); x++){
            fileout.println(randomPairs.get(x));
        }

        fileout.close();
    } catch (FileNotFoundException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }


}
public static void main(String[] args){
    Part2 test = new Part2();
    test.PrinttoScreen();
}

Answer 1

班级part2中的以下一行：

ArrayList<String> randomPairs = part1.getPairs();

不起作用 - 您应该将初始化移到构造函数中：

public class Part2 {

    PrintWriter fileout;
    String writeFile;   
    Part1 part1 = null;
    ArrayList<String> randomPairs = null;

    Part2(){
        part1 = new Part1();
        randomPairs  = part1.getPairs();
    }
    ...

在另一个类中调用ArrayList不起作用

1 个答案: