一、明来路：

public final class Scanner implements Iterator<String>, Closeable {

    // Internal buffer used to hold input
    private CharBuffer buf;

    // Size of internal character buffer
    private static final int BUFFER_SIZE = 1024; // change to 1024;

    // The index into the buffer currently held by the Scanner
    private int position;

    // Internal matcher used for finding delimiters
    private Matcher matcher;

    // Pattern used to delimit tokens
    private Pattern delimPattern;

    // Pattern found in last hasNext operation
    private Pattern hasNextPattern;

    // Position after last hasNext operation
    private int hasNextPosition;

    // Result after last hasNext operation
    private String hasNextResult;

    // The input source
    private Readable source;

    // Boolean is true if source is done
    private boolean sourceClosed = false;

    // Boolean indicating more input is required
    private boolean needInput = false;

    // Boolean indicating if a delim has been skipped this operation
    private boolean skipped = false;

    // A store of a position that the scanner may fall back to
    private int savedScannerPosition = -1;

    // A cache of the last primitive type scanned
    private Object typeCache = null;

    // Boolean indicating if a match result is available
    private boolean matchValid = false;

    // Boolean indicating if this scanner has been closed
    private boolean closed = false;

    // The current radix used by this scanner
    private int radix = 10;

    // The default radix for this scanner
    private int defaultRadix = 10;

    // The locale used by this scanner
    private Locale locale = null;

    // A cache of the last few recently used Patterns
    private PatternLRUCache patternCache = new PatternLRUCache(7);

    // A holder of the last IOException encountered
    private IOException lastException;

    // Number of times this scanner's state has been modified.
    // Generally incremented on most public APIs and checked
    // within spliterator implementations.
    int modCount;

    // A pattern for java whitespace
    private static Pattern WHITESPACE_PATTERN = Pattern.compile(
                                                "\\p{javaWhitespace}+");

    // A pattern for any token
    private static Pattern FIND_ANY_PATTERN = Pattern.compile("(?s).*");

    // A pattern for non-ASCII digits
    private static Pattern NON_ASCII_DIGIT = Pattern.compile(
        "[\\p{javaDigit}&&[^0-9]]");

    // Fields and methods to support scanning primitive types

    /**
     * Locale dependent values used to scan numbers
     */
    private String groupSeparator = "\\,";
    private String decimalSeparator = "\\.";
    private String nanString = "NaN";
    private String infinityString = "Infinity";
    private String positivePrefix = "";
    private String negativePrefix = "\\-";
    private String positiveSuffix = "";
    private String negativeSuffix = "";

二、知用处

---

import java.util.Scanner;

public class ATestForScanner {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        if (scanner.hasNextInt()) {
            int i = scanner.nextInt();
            System.out.println(i);
        }
//hasNextInt为true时
//才会执行nextInt的读入
//两个大多数配套使用
//此外使用 next（） 和 next八种基本数据冲突，会吞输入,因为具体的nextInt是一块一块读取的
//nextInt 输入 每次输入一个数字，用回车表示输出结束
        System.out.println(scanner.next());
        scanner.close();
    }
}

此外用完记得关闭，这是个好习惯