Question

我是java语言的新手，这是我第一次使用 scanner lib 。我很确定我在方法中阻止了空指针，但是当我运行main时，堆栈跟踪说：

线程中的异常＆＃34; main＆＃34; java.lang.NullPointerException at Big_data_package.sort_test_main.readintoarr（sort_test_main.java:22）在Big_data_package.sort_test_main.main（sort_test_main.java:60）

它发生在nextInt方法的scanner函数上。你能帮我指出我的代码中的错误吗？非常感谢。

public class sort_test_main{

static File inputfile = new File("E:\\Java_workspace\\Big_Data_Sort\\io\\input.txt");
static mergeSort merge = new mergeSort();
static Scanner scanner;
static int[] inputarr;

public static int[] readintoarr (int[] list, Scanner scanner) throws IOException{
    int i = 0;
    boolean endfile = false;

    scanner = new Scanner(inputfile);

        while(!endfile)
        {
            if(scanner.hasNextInt()){
             list[i++] = scanner.nextInt();
            }
            else{
                endfile = true;
            }
        }
        scanner.close();

    return list;
}// end of readintoarr

public static void arrtofile (int[]x) throws IOException{

    File file = new File("E:\\Java_workspace\\Big_Data_Sort\\io\\output.txt");

    // if file doesnt exists, then create it
    if (!file.exists()) {
        file.createNewFile();
    }

    BufferedWriter outputWriter = null;


      outputWriter = new BufferedWriter(new FileWriter(file));
      for (int i = 0; i < x.length; i++) {
        // Maybe:
        outputWriter.write(x[i]+"");
        // Or:
        outputWriter.write(Integer.toString(x[i]));
        outputWriter.newLine();
      }
      outputWriter.flush();  
      outputWriter.close();  
}//end of arrtofile

//test
public static void main(String[] args)throws IOException {

    readintoarr(inputarr,scanner);
    merge.mergeSort1(inputarr,0,inputarr.length);
    arrtofile(inputarr);

}}

Answer 1

您尚未为list / inputarr分配内存。试试这个：

import java.util.Scanner;

public class sampleMethods {

    public static void main(String[] args) {
        printYikes();
        int x = 12;

        x = getAnInt("Please enter an integer min and a max seperated by a space: ",y, z);

        System.out.println("\nThat was a valid Number.");
        System.out.println("You entered " + x + "!");
        System.out.println( "x is " + x );
        x = doubleMyNumber(x);
        System.out.println( "x is " + x );
        x = tripleMyNumber(x);
        System.out.println( "x is " + x );

    }



    static void printYikes() {
        System.out.println("Yikes");
    }

    static int doubleMyNumber(int a) {
        int b = a * 2;
        return b;
    }

    static int tripleMyNumber(int a) {
        int c = a * 3;
        boolean odd = isOddOrEven(c);
        if (odd) {
            System.out.println("Number is Odd!!");
        } else {
            System.out.println("Number is Even!!");
        }
        return c;
    }

    static boolean isOddOrEven (int d) {
        boolean isOdd = false;
        if (d % 2 == 0) {
            isOdd = false;
        } else {
            isOdd = true;
        }
        return isOdd;
    }

    static int getAnInt(String prompt, int min, int max) {
        Scanner keyboard = new Scanner(System.in);
        boolean numberError = false;
        int enteredNumber = 0;
        String enteredString = "";
        do {
            try {
                System.out.print(prompt);
                enteredString = keyboard.nextLine();  //Read into a string
                enteredNumber = Integer.parseInt(enteredString.trim());  //then cast as a integer
                numberError = false;  //if we haven't bailed out, then the number must be valid.
                if (enteredNumber < min || enteredNumber > max) {
                    numberError = true;
                    System.out.println("Your entry: \"" + enteredNumber + "\" is out of range...Please try again");
                }
            } catch(Exception e) {
                System.out.println("Your entry: \"" + enteredString + "\" is invalid...Please try again");
                numberError = true;  //Uh-Oh...We have a problem.
            }
        } while (numberError == true );  //Keep asking the user until the correct number is entered.

        return enteredNumber;

    }
}

scanner.nextint（）上的Java Null指针异常

1 个答案: