java - 扫描仪在尝试获取下一个时抛出空指针

Question

public class ScannerTest {

    protected Scanner scan;

    public ScannerTest(String s) {

        Scanner scan = new Scanner(s);

    }

    public void getone() {

        if (scan.hasNext()) {
            String temp = scan.next();
            temp = temp.replaceAll("[\\[\\](){}]", "");
            System.out.println(temp);
        }
    }

    public static void main(String[] args) {

        String s = "(abcd) (defg) (w)";
        ScannerTest test = new ScannerTest(s);
        Scanner rando = new Scanner(s);
        System.out.println(rando.next());
        System.out.println(rando.hasNext());

        test.getone();
    }
}

Answer 1

你是所谓的变量阴影的受害者。带线

Scanner scan = new Scanner(s); 

您正在声明一个局部变量 scan，而不是在 ScannerTest 类中初始化您的成员 scan。将提到的行更改为

scan = new Scanner(s); 

它将按预期工作...

完整的源代码

import java.util.Scanner;

public class SomeClass {

    protected Scanner scan;

    public SomeClass(String s) {
        scan = new Scanner(s);
    }

    public void getone() {

        if (scan.hasNext()){
            String temp = scan.next();
            temp = temp.replaceAll("[\\[\\](){}]","");
            System.out.println(temp);
        }

    }

    public static void main(String[] args) {

        String s = "(abcd) (defg) (w)";
        SomeClass test = new SomeClass(s);
        Scanner rando = new Scanner(s);
        System.out.println(rando.next());
        System.out.println(rando.hasNext());

        test.getone();

    }

}