java中Scanner的hasNext()的疑问

Question

请问下面代码有什么区别，该如何输入才能使第一段代码不满足条件而跳出循环？

Scanner s = new Scanner(System.in);
        while(s.hasNext())
        {
            System.out.println(s.next());
        }

Scanner s = new Scanner(System.in);
        while(true)
        {
            System.out.println(s.next());
        }

Answer 1

第一个问题，两段代码的区别在于阻塞的位置不同，加上一行输出代码就可以很明显地看到差别。

Test.java

import java.util.Scanner;

public class Test {
    public static void main(String[] args) {
        Scanner s = new Scanner(System.in);
        while(s.hasNext())
        {
            System.out.print("You inputted: ");
            System.out.println(s.next());
        }
    }
}

另，hasNext()方法会阻塞，不代表next()方法就不会阻塞。

/**
 * Returns true if this scanner has another token in its input.
 * This method may block while waiting for input to scan.
 * The scanner does not advance past any input.
 *
 * @return true if and only if this scanner has another token
 * @throws IllegalStateException if this scanner is closed
 * @see java.util.Iterator
 */
public boolean hasNext() {
    ensureOpen();
    saveState();
    while (!sourceClosed) {
        if (hasTokenInBuffer())
            return revertState(true);
        readInput();
    }
    boolean result = hasTokenInBuffer();
    return revertState(result);
}

/**
 * Finds and returns the next complete token from this scanner.
 * A complete token is preceded and followed by input that matches
 * the delimiter pattern. This method may block while waiting for input
 * to scan, even if a previous invocation of {@link #hasNext} returned
 * <code>true</code>.
 *
 * @return the next token
 * @throws NoSuchElementException if no more tokens are available
 * @throws IllegalStateException if this scanner is closed
 * @see java.util.Iterator
 */
public String next() {
    ensureOpen();
    clearCache
    while (true) {
        String token = getCompleteTokenInBuffer(null);
        if (token != null) {
            matchValid = true;
            skipped = false;
            return token;
        }
        if (needInput)
            readInput();
        else
            throwFor();
    }
}

第二个问题，想要结束循环，在Windows环境下，需要输入Ctrl+Z；而在Unix环境下，需要输入Ctrl+D。注意，这是输入，而不是对控制台进行操作。这相当于向控制台输入一个字符，这个字符代表EOF，此时hasNext()方法返回false，循环结束。

Answer 2

阻塞位置不一样