java - 迭代器实现只读取字符串中的第一个标记？

Question

对于这个程序，我需要实现一个 token 迭代器，它从一个字符串中扫描并存储一个有效的 token 。这似乎是一个简单的过程，但我的代码只返回字符串中的第一个有效标记。我需要调用 hasNext(); 吗？在 next() 方法的不同位置？

这是我的代码:

public class TokenIter implements Iterator<String>{

    //input line to be tokenized
    private String line;

    // the next Token, null if no next Token
    private String nextToken;

    //Edit
    private Scanner scan;

    public TokenIter(String line){
        this.line = line;
    }

    @Override
    public boolean hasNext() {
        //Edit
        scan = new Scanner(line);
            if(scan.hasNext()){
                nextToken = scan.next();
                return true;
            }
            scan.next();
            nextToken = null;
            return false;
    }

    @Override
    public String next() {
        if(!hasNext()){
            throw new NoSuchElementException();
        }
        else{
            if(nextToken.equals("true")){
                return nextToken;
            }
            if(nextToken.equals("false")){
                return nextToken;
            }
            if(nextToken.equals("or")){
                return nextToken;
            }
            if(nextToken.equals("and")){
                return nextToken;
            }
            if(nextToken.equals("not")){
                return nextToken;
            }
            if(nextToken.equals("(")){
                return nextToken;
            }
            if(nextToken.equals(")")){
                return nextToken;
            }
        }
        return "";      
    }

    @Override
    public void remove() {
        throw new UnsupportedOperationException();
    }

    public static void main(String[] args){
        String line;
        args = Debug.init(args);
        if(args.length>0)
            line = args[0];
        else
            line = " not (true or error false) @# $% ";
        System.out.println("line: [" + line + "]");
        TokenIter tokIt = new TokenIter(line);
        while(tokIt.hasNext()){
            System.out.println("next token: [" + tokIt.next() + "]");
        }
    }

这是我运行程序的输出:

line: [ not (true or error false) @# $% ]
next token: [not]
next token: [not]
next token: [not]
next token: [not]
next token: [not]
next token: [not]

这是我想要的输出:

line: [ not (true or error false) @# $% ]
next token: [not]
next token: [(]
next token: [true]
next token: [or]
next token: [false]
next token: [)]

Answer 1

您的hasNext() 方法不正确，您在每次调用时扫描行，因此您将始终返回第一个标记:

 @Override
    public boolean hasNext() {
        Scanner scan = new Scanner(line);
            if(scan.hasNext()){
                nextToken = scan.next();
                return true;
            }
            scan.next();
            nextToken = null;
            return false;

解决方法: 变量 scan 需要成为一个类变量，以便您保持 hasNext() 调用之间的状态。