在 Java 中打印 2D ArrayList 导致空括号

Question

I'm trying to read a file and put each 2 lines into an ArrayList of strings. My program has a counter that starts at 1 and keeps track of the lines, and if it is on line 2 it will do the following:

• Add the ArrayList of strings named line into a 2D ArrayList named list2D
• Clear line using .clear()
• Reset the line counter to 1

So the idea is to store every 2 lines of text as a 2D ArrayList of strings. Here is an expected input file and the expected output:

INPUT FILE: cat
dog
mouse
duck (every word is on a different line idk why SO put it on the same line)

OUTPUT: [[cat, dog],[mouse, duck]]

However, when I print my 2D ArrayList using w.println(list2D) , the program outputs emptiness:

[ [ ], [ ] ]

Here is my code:

ArrayList<ArrayList<String>> list2D = new ArrayList<ArrayList<String>>();
ArrayList<String> lines = new ArrayList<String>();

Integer lineNum = 1;

for (String line = r.readLine(); line != null; line = r.readLine()) {
    lines.add(line);
    if (lineNum == 2) { //line counter is at the end of block
        list2D.add(lines);
        lines.clear();
        lineNum = 1; //reset line counter
    } else {
        lineNum++;
    }
}
w.println(list2D);

I've checked other SO posts regarding how to create a 2D ArrayList, so I'm pretty sure I got everything from the for loop right. When I test print the lines ArrayList before adding it to lines2D , it prints the desired result (eg [cat, dog]), so at least I'm correct up until that point.

I'm also not sure if w.println(list2D) is the right method for printing, but again I've seen multiple other SO posts that use this method.

Any help/criticism is appreciated, thank you.

Answer 1

the program outputs emptiness: [ [ ], [ ] ]

That is because you only have a single ArrayList.

You add a reference of that ArrayList to the 2D ArrayList multiple times, but there is no data in the ArrayList because you add two entries, then you clear the entries.

You need to create an new ArrayList for every two lines of text.

There is no need for the "lineNum" variable.

You just add a line of text to the ArrayList. When the size of the ArrayList is 2, you add it to the 2D ArrayList and create an new empty ArrayList for the next 2 lines of text.