Question

我试图理解java中的扫描器类，是不是为什么这个代码只打印第一部分中的第一行而不是第2部分中的所有信息。

import java.io.ByteArrayInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.io.OutputStreamWriter;
import java.io.PrintWriter;
import java.util.NoSuchElementException;
import java.util.Scanner;

public class BadParse {

public static String getNextEntry(InputStream in) {
    Scanner sin = new Scanner(in);

    try {
        String ssn = sin.next();
        String name = sin.next();
        int age = sin.nextInt();

        return name + "(" + ssn + ") is " + age + " years old.";
    } catch (NoSuchElementException e) {
        return null;
    }
}

public static void putNextEntry(String ssn, String name, int age, OutputStream out) {
    PrintWriter pout = new PrintWriter(new OutputStreamWriter(out));

    pout.print(ssn + " ");
    pout.print(name + " ");
    pout.print(age + " ");
}

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

    // Part I
    String input = "1234567890 John 20\n0987654321 Beth 18\n2468101214 Jack 19\n";
    InputStream in = new ByteArrayInputStream(input.getBytes());

    String entry;
    while ((entry = getNextEntry(in)) != null) {
        System.out.println(entry);
    }

    // Part II
    OutputStream out = new FileOutputStream("data");

    putNextEntry("1234567890", "John", 20, out);
    putNextEntry("0987654321", "Beth", 18, out);
    putNextEntry("2468101214", "Jack", 19, out);

    out.close();
}

}

我尝试使用分隔符，但没有成功。

Answer 1

如果您运行调试器并逐步执行getNextEntry方法，则会在第二次调用时看到Scanner抛出NoSuchElementException。发生这种情况的原因是因为第一个Scanner消耗了部分ByteArrayInputStream。

您可以执行的一项修复是在Scanner循环之前创建while并将Scanner传递给getNextEntry，而不是传递ByteArrayInputStream。

public static String getNextEntry(Scanner sin) {

...

Scanner sin = new Scanner(in);
String entry;
while ((entry = getNextEntry(sin)) != null) {

就第2部分而言，PrintWriter未被刷新。如果您将pout.flush()添加到putNextEntry的底部，这会导致数据发送到您的文件。对于PrintWriter个对象，只有在您拨打println，printf，format或flush时才会刷新数据。

public static void putNextEntry(String ssn, String name, int age, OutputStream out) {
    PrintWriter pout = new PrintWriter(new OutputStreamWriter(out));
    pout.print(ssn + " ");
    pout.print(name + " ");
    pout.print(age + " ");
    pout.flush();
}

我建议创建并传入类似于扫描仪的PrintWriter。减少创建的资源，您可以多次调用putNextEntry后清除所有资源。

Answer 2

问题是您在Scanner内创建了getNextEntry对象，当该对象被销毁时，它会关闭该流。尝试将Scanner作为参数传递而不是像流一样（对于PrintWriter也是如此）：

 import java.io.ByteArrayInputStream;

import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.io.OutputStreamWriter;
import java.io.PrintWriter;
import java.util.NoSuchElementException;
import java.util.Scanner;

public class BadParse {
public static String getNextEntry(Scanner sin) {

    try {
        String ssn = sin.next();
        String name = sin.next();
        int age = sin.nextInt();

        return name + "(" + ssn + ") is " + age + " years old.";
    } catch (NoSuchElementException e) {
        return null;
    }
}

public static void putNextEntry(String ssn, String name, int age, PrintWriter pout) {

    pout.print(ssn + " ");
    pout.print(name + " ");
    pout.print(age + " ");
}

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

    // Part I
    String input = "1234567890 John 20\n0987654321 Beth 18\n2468101214 Jack 19\n";
    InputStream in = new ByteArrayInputStream(input.getBytes());
    Scanner sin = new Scanner(in);

    String entry;
    while ((entry = getNextEntry(sin)) != null) {
        System.out.println(entry);
    }

    // Part II
    OutputStream out = new FileOutputStream("data");
    PrintWriter pout = new PrintWriter(new OutputStreamWriter(out));

    putNextEntry("1234567890", "John", 20, pout);
    putNextEntry("0987654321", "Beth", 18, pout);
    putNextEntry("2468101214", "Jack", 19, pout);

    pout.close();
}
}

扫描仪跳过新线

2 个答案: