为什么这段代码不等待用户输入？

Question

I just wrote some code in java which is supposed to let the user input data bit by bit. Once all data is captured, it will do its thing and throw out some output. Now here's the problem: The program does not wait for user input at all. Here's a minimized version of my code:

import java.util.Scanner;
import java.util.Arrays;
import java.util.ArrayList;
import java.util.List;
public class MyClass {
    // initialize array variable in global namespace
    static boolean[][] array;
    public static void main(String[] args) {
        // Input numbers
        Scanner text = new Scanner(System.in);
        int[] first_in = new int[2];

        System.out.println("Please start data entry: \n");
        // Take in first variables (numbers)
        for(int i = 0; i<first_in[0]; i++) {
            first_in[i] = text.nextInt();
        }
        int n = first_in[0];

        // Initialize reference array
        String[] reference = new String[n];
        System.out.println("Please give names: \n");
        // Ask for reference names
        for(int i = 0; i<n; i++) {
            reference[i] = text.next();
        }

        System.out.println("Please give other data: \n");
        // Ask for edges
        String edgein;
        for(int i = 0; i<first_in[1]; i++) {
            edgein = text.nextLine();
            /* do some stuff */
        }
        // Be nice and close the scanner
        text.close();
        System.out.println("Scanner closed.");

The output is simply

Please start data entry: 

Please give vertices: 

Please give edges: 

Scanner closed.

Why does the program not wait for user input in either one of the three loops? And how can I fix it?

Answer 1

You need to initialize the array first_in . The default value is [0,0] so it doesn't go inside the loop since the comparison i<first_in[0] will be 0<0 .

    Scanner text = new Scanner(System.in);
    int[] first_in = new int[2];

    System.out.println("Please start data entry: \n");
    // Take in first variables (numbers)
    for(int i = 0; i<first_in[0]; i++) {
        first_in[i] = text.nextInt();
    }
    int n = first_in[0];