如何访问在“if”语句中创建的 ArrayList？ [关闭]

Question

有人可以帮助我吗，我试图在声明它的 if 语句之外使用名为 prop 的数组列表，并在其中添加了一个对象。

这是 if 语句。代码要求用户输入信息，然后声明一个 Property 对象和一个 ArrayList 属性，它将我创建的属性对象添加到 ArrayList 中。

if(option.equals("one")){

    int x = 1;
    do{
        try{
            System.out.println("Enter owner name");
            String ownerName = scan.nextLine();
            System.out.println("Enter address");
            String address = scan.nextLine();
            System.out.println("Are you the principle private residant?");
            String principlePrivateResidance = scan.nextLine();
            System.out.println("Enter property location(city, large town, small town, vilage, countryside) ");
            String location = scan.nextLine();
            System.out.println("would you like to pay the propery tax now?(yes//no)");
            String paid = scan.nextLine();
            System.out.println("Enter your filename");
            String fn = scan.nextLine();
            System.out.println("Enter the estimated property value");

            int propertyEstimatedVal = scan.nextInt();
            // i make the object here 
            Property property = new Property(ownerName, address, paid, principlePrivateResidance,location, propertyEstimatedVal);

            // Then make the arraylist and add the object to it take this arraylist and put it....

            ArrayList<Property> prop = new ArrayList<Property>;
            prop.add(property);
            CalculateTax tax = new CalculateTax(property.getLocation(),property.getPrinciplePrivateResidance(),property.getPropertyEstimatedVal(), property.getPaid());

            System.out.println("owner:" + property.getOwnerName());
            System.out.println("address:" +property.getAddress());
            System.out.println("propertyEstimatedVal:" +property.getPropertyEstimatedVal());
            System.out.println("principlePrivateResidance:" +property.getPrinciplePrivateResidance());
            System.out.println("location:" +property.getLocation());
            System.out.println("paid:" +property.getPaid());
            System.out.println("Your property tax is" + CalculateTax.getSumoftax() );
            System.out.println("your details have been taken");

            FileWriter fstream = new FileWriter(fn,true);
            BufferedWriter out = new BufferedWriter(fstream);
            out.write(property.getOwnerName() + " | " + property.getAddress() + " | " + property.getPropertyEstimatedVal() + " | " + property.getPrinciplePrivateResidance() + " | " + property.getLocation() + " | " + property.getPaid() + " | " + CalculateTax.getSumoftax());
            out.newLine();
            out.close();
            x= 2;
        }
        catch(Exception e){
            System.out.println("error has occured");
        }
    }
    while(x == 1);

}
else if(option.equals("two")){
    // this just opens a file
    System.out.println("Please enter you file name");
    String filename = scan.nextLine();

    try{
        // Open the file that is the first
        // command line parameter
        FileInputStream fstream = new FileInputStream(filename);
        // Get the object of DataInputStream
        DataInputStream in = new DataInputStream(fstream);
        BufferedReader br = new BufferedReader(new InputStreamReader(in));
        String strLine;

        //Read File Line By Line
        while ((strLine = br.readLine()) != null)   {
            // Print the content on the console
            System.out.println (strLine);
        }
        //Close the input stream
        in.close();
    }
    catch (Exception e){//Catch exception if any
        System.err.println("Error: " + e.getMessage());
    }
}
else if(option.equals("three")){
    //i need to get that arraylist with the objects i added to it here but
    //i just dont know how
    System.out.print("Service is currently available ");

}
else{
    System.exit(0);
}

Answer 1

如果您需要在if 语句之外访问ArrayList，您需要将变量声明移到if 语句之前。比如这里去掉这行...

Property property = new Property(ownerName, address, paid, principlePrivateResidance,location, propertyEstimatedVal);

// Then make the arraylist and add the object to it take this arraylist and put it....

ArrayList<Property> prop = new ArrayList<Property>; // REMOVE THIS LINE
prop.add(property);

然后在此处添加它...

ArrayList<Property> prop = new ArrayList<Property>; // ADD THIS LINE

if(option.equals("one")){

    int x = 1;
    do{

现在ArrayList 定义在if 语句之前，因此您可以在if 语句内外的任何位置使用它。

无论如何，您可能希望将ArrayList 移到do-while 循环之外，否则每次通过do-while 循环时都会创建一个新的ArrayList。我确定您真正想要的是创建一个 ArrayList 并添加一些项目 - 因此您需要将 ArrayList 的声明移到 do-while 循环之外，这也可以通过我上面的代码更改了。

您需要做的下一件事是检测项目是否已添加到您的ArrayList。如果您不执行if(option.equals("one")){ 语句中的代码，则不会向ArrayList 添加任何内容，因此它将为空。因此，如果您以后想使用您的ArrayList，您应该首先检查其中是否有任何项目，如果它们存在则使用它们 - 可能是这样的......

if (prop.size() > 0){
    // there are items
    doSomething();
}
else {
    // no items
    doSomethingElse();
}

Answer 2

您需要将您的数组列表 decalration/insntantiation 移到您的 if 语句之外，如下所示：

        List<Property> prop = new ArrayList<Property>;

        if(option.equals("one")){
             //your if block code
            do{
               //rest of your code except list declaration/initialization
              }while(x==1);
              ....
        }else if(option.equals("two")){
           .....
        } else if(option.equals("three")){
           //your list `prop` is available here
           ....
        }

这是必需的，原因有两个：

1. 目前，您正在 do-while 循环的每次迭代中重新实例化道具列表，并丢失之前添加的数据。
2. 只有当您需要在 if 之外（即在 else 块中）访问它时，您的列表才在 if 块的范围内。

通过将列表初始化移到 if 块之外，您将解决这两个问题。