Scanner类的nextLine()方法

1. Overview

In this quick tutorial, we’ll briefly look at the nextLine() method of java.util.Scanner class, of course, with a focus on learning how to use it in practice.

2. Scanner.nextLine()

The nextLine() method of the java.util.Scanner class scans from the current position until it finds a line separator delimiter. The method returns the String from the current position to the end of the line.

Consequently, after the operation, the scanner’s position is set to the beginning of the next line that follows the delimiter.

The method will search through the input data looking for a line separator. It may scan all input data, searching for the line to skip if no line separators are present.

The signature of the nextLine() method is:

public String nextLine()

The method takes no parameters. It returns the current line, excluding any line separator at the end.

Let’s look at its usage:

try (Scanner scanner = new Scanner("Scanner\nTest\n")) {
    assertEquals("Scanner", scanner.nextLine());
    assertEquals("Test", scanner.nextLine());
}

As we have seen, the method returns the input from the current scanner position until the line separator is found:

try (Scanner scanner = new Scanner("Scanner\n")) {
    scanner.useDelimiter("");
    scanner.next();
    assertEquals("canner", scanner.nextLine());
}

In the above example, the call to next() returns ‘S’ and advances the scanner position to point to ‘c’.

Therefore, when we call the nextLine() method, it returns the input from the current scanner position until it finds a line separator.

The nextLine() method throws two types of checked exceptions.

Firstly, when no line separator is found, it throws NoSuchElementException:

@Test(expected = NoSuchElementException.class)
public void whenReadingLines_thenThrowNoSuchElementException() {
    try (Scanner scanner = new Scanner("")) {
        scanner.nextLine();
    }
}

Secondly, it throws IllegalStateException if the scanner is closed:

@Test(expected = IllegalStateException.class)
public void whenReadingLines_thenThrowIllegalStateException() {
    Scanner scanner = new Scanner("");
    scanner.close();
    scanner.nextLine();
}

3. Conclusion

In this to-the-point article, we looked at the nextLine() method of Java’s Scanner class.

Furthermore, we looked at its usage in a simple Java program. Finally, we looked at the exceptions thrown by the method and sample code illustrating it.

As always, the full source code of the working examples is available on GitHub.