问题
Hey guys sorry i'm a noob at java but im trying to make something and i need a multidimensional array list. It must be implemented into the following code:
public static void OpenFile() {
ArrayList<ArrayList<String>> array = new ArrayList<ArrayList<String>>();
int retrival = chooser.showOpenDialog(null);
if (retrival == JFileChooser.APPROVE_OPTION) {
try (BufferedReader br = new BufferedReader(new FileReader(
chooser.getSelectedFile()))) {
String sCurrentLine;
while ((sCurrentLine = br.readLine()) != null) {
if (sCurrentLine.equals("")) {
continue;
} else if (sCurrentLine.startsWith("Question")) {
System.out.println(sCurrentLine.split(":")[1]);
//add to [0] in array ArrayList
} else if (sCurrentLine.startsWith("Answer")) {
System.out.println(sCurrentLine.split(":")[1]);
//add to [1] in array ArrayList
} else if (sCurrentLine.startsWith("Category")) {
System.out.println(sCurrentLine.split(":")[1]);
//add to [2] in array ArrayList
} else if (sCurrentLine.startsWith("Essay")) {
System.out.println(sCurrentLine.split(":")[1]);
//add to [3] in array ArrayList
}
}
} catch (Exception ex) {
ex.printStackTrace();
}
}
}
All the [1] index's of the strings i split need to go into a multidimesional array in this order: question, answer, category, essay.
So i am currently using a normal multidimensional array but you cant change the values of it easily. What i want my multidimensional arraylist to look like is this:
MultiDimensional ArrayList
- [0]: questions(it may be over 100 of them.)
- [1] answers(it may be over 100 of them.)
- [2] category(it may be over 100 of them.)
- [3] essay(it may be over 100 of them.)
回答1:
Seems like an assignment with List of List.
Here is the psuedo code. Try to implement rest.
ArrayList<ArrayList<String>> array = new ArrayList<ArrayList<String>>();
First thing you have to do is take 4 fresh ArrayLists
ArrayList<String> qtns= new ArrayList<String>>();
ArrayList<String> answrs= new ArrayList<String>>();
--
--
Add all of them to main list
array.add(qtns);
--
--
Then now filling them like
else if (sCurrentLine.startsWith("Question")) {
System.out.println(sCurrentLine.split(":")[1]);
//add to [0] in array ArrayList
array.get(0).add(sCurrentLine.split(":")[1]);//get first list(which is qtns list) and add to it.
} else if (sCurrentLine.startsWith("Answer")) {
System.out.println(sCurrentLine.split(":")[1]);
//add to [1] in array ArrayList
array.get(1).add(sCurrentLine.split(":")[1]);//get second list(which is answers list) and add to it.
}
-- -
---
So that in the end you'll have a list of list which contains the data.
回答2:
array is an array of four elements (or will be, after you set it up--see below). Each of the elements is a reference to another ArrayList, i.e. an ArrayList<String>. To modify the inner ArrayList, first you need to get the reference to it, which you do with the get method:
array.get(0)
for the [0] list, or array.get(1) for the [1] list, and so on. The result will be an ArrayList<String>. Now you can perform methods on that; to add a String to the end of the ArrayList<String>:
array.get(0).add("String to add");
etc.
Actually, now that I look at it, you'll need to set up the four elements of array first. Each element will be set to a new ArrayList<String>:
for (int i = 0; i < 4; i++)
array.add(new ArrayList<String>());
Do this at the top of the method.
回答3:
This is a really bad design. You should create a custom class, with the separate types of rows as fields (maybe call it Question), and then use a List<Question> to store the individual question objects. This will make it clearer, and safer, to work with in the future.
回答4:
You can have a HashMap with key as (Questions,Answers..) and ArrayList as value
HashMap <String,ArrayList<String>> array = new HashMap<String,ArrayList<String>>();
if(array.containsKey("Question")){
array.get("Question").add(sCurrentLine.split(":")[1]);
}else{
ArrayList<String> questions = new ArrayList<String>();
questions.add(sCurrentLine.split(":")[1]);
array.put("Question",questions );
}
Modification In your Code
public static void OpenFile() {
HashMap <String,ArrayList<String>> array = new HashMap<String,ArrayList<String>>();
int retrival = chooser.showOpenDialog(null);
if (retrival == JFileChooser.APPROVE_OPTION) {
try (BufferedReader br = new BufferedReader(new FileReader(
chooser.getSelectedFile()))) {
String sCurrentLine;
while ((sCurrentLine = br.readLine()) != null) {
if (sCurrentLine.equals("")) {
continue;
} else if (sCurrentLine.startsWith("Question")) {
System.out.println(sCurrentLine.split(":")[1]);
if(array.containsKey("Question")){
array.get("Question").add(sCurrentLine.split(":")[1]);
}else{
ArrayList<String> questions = new ArrayList<String>();
questions.add(sCurrentLine.split(":")[1]);
array.put("Question",questions );
}
//add to [0] in array ArrayList
} else if (sCurrentLine.startsWith("Answer")) {
System.out.println(sCurrentLine.split(":")[1]);
//add to [1] in array ArrayList
//Do the same as above if condition with Answer as Key
} else if (sCurrentLine.startsWith("Category")) {
System.out.println(sCurrentLine.split(":")[1]);
//add to [2] in array ArrayList
} else if (sCurrentLine.startsWith("Essay")) {
System.out.println(sCurrentLine.split(":")[1]);
//add to [3] in array ArrayList
}
}
} catch (Exception ex) {
ex.printStackTrace();
}
}
}
回答5:
Once you created a 2D array, you also need to create 4 1D arrays.
ArrayList<ArrayList<String>> array = new ArrayList<ArrayList<String>>();
// 1D array for "Question"
array.add(new ArrayList<String>());
// 1D array for "Answer"
array.add(new ArrayList<String>());
// 1D array for "Category"
array.add(new ArrayList<String>());
// 1D array for "Essay"
array.add(new ArrayList<String>());
Let's say if you need to add one item to the question array, then you should do:
array.get(i).add(...);
where i = 0. The range of i is from 0 to 3, which corresponds to the 4 enumerations.
回答6:
You should use another data structure to achieve that, Use a Map<String,List<String>> where the possible keys are Question,Answers,Category,Essay
Then in your code you can remove all unnecessary if-else
Example with your code:
public static void openFile() {
Map<String,List<String>> map = new HashMap<>();
// here you put the keys, you can do it when you read the file based in your input too dynamically
map.put("Question",new ArrayList<>());
map.put("Answers",new ArrayList<>());
map.put("Category",new ArrayList<>());
map.put("Essay",new ArrayList<>());
int retrival = chooser.showOpenDialog(null);
String[]array = null;
if (retrival == JFileChooser.APPROVE_OPTION) {
try (BufferedReader br = new BufferedReader(new FileReader(
chooser.getSelectedFile()))) {
String sCurrentLine;
while ((sCurrentLine = br.readLine()) != null) {
if (sCurrentLine.equals("")) {
continue;
}
array = sCurrentLine.split(":");
if(map.containsKey(array[0])){
map.get(array[0]).add(array[1]);
}
}
} catch (Exception ex) {
ex.printStackTrace();
}
}
}
You can see this way is much more cleaner and readable.
来源：https://stackoverflow.com/questions/20862515/how-to-work-with-multidimensional-arraylist