java - .hasNext() 无法正常工作

Question

    Scanner scanner = new Scanner(System.in);
    // check if the scanner has a token
    System.out.println(scanner.hasNext());

    // print the rest of the string
    System.out.println(scanner.nextLine());

    // check if the scanner has a token after printing the line
    System.out.println(scanner.hasNext());

当我运行这段代码并输入:

Hi

在控制台打印这些:

true
Hi

但程序从未结束或打印false。问题是什么？

Answer 1

/**
 * 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()

hasNext() 在等待输入时阻塞。这就是为什么第二次调用 System.out.println(scanner.hasNext()); 什么也不打印，程序也没有结束。

如果您的 Scanner 从文件而不是标准输入读取数据，hasNext() 会在到达文件末尾时返回 false。