标准输入 - java中Scanner的hasNext()的疑问

Answer 1

The first question is that the difference between the two pieces of code lies in the different blocking positions. You can clearly see the difference by adding a line of output code.

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());
        }
    }
}

In addition, the hasNext() method will block, but it does not mean that the next() method will not block.

/**
 * 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();
    }
}

Second question, if you want to end the loop, you need to enter Ctrl+Z in the Windows environment; and in the Unix environment, you need to enter Ctrl+D. Note that this is input, not manipulation of the console. This is equivalent to entering a character into the console, which represents EOF. At this time, the hasNext() method returns false and the loop ends.

Answer 2

The blocking positions are different