在 .txt 中找不到文本，但它已经存在

Question

我正在学习 Mooc.fi Java 课程，但我不知道如何不写如果文件已包含字符串，则将其写入文件。我只尝试了一个字符串，并尝试不使用“”（空 space），并且不使用另一个字符串，但即使文件已经包含它，它仍然会添加该字符串。

并且 translate() 方法不会 find/return 它找到给定单词的整行。

public class main {

    public static void main(String[] args) throws Exception {
        MindfulDictionary dict = new MindfulDictionary();
        dict.add("apina", "monkey");
        dict.add("banaani", "banana");
        dict.add("apina", "apfe");

        System.out.println( dict.translate("apina") );
        System.out.println( dict.translate("monkey") );
        System.out.println( dict.translate("programming") );
        System.out.println( dict.translate("banana") );

    }
}

public class MindfulDictionary {
    File file;
    FileWriter writer; 
    Scanner imeskanera;
    
    public MindfulDictionary() throws Exception {
        this.file  = new File("C:\Users\USER\Desktop\test.txt");
        this.imeskanera = new Scanner(this.file, "UTF-8");
        
    }
        
    public void add(String word, String translation) throws Exception {
        boolean found = false;
        while(this.imeskanera.hasNextLine()) {
            String lineFromFile = this.imeskanera.nextLine();
            if(word.contains(lineFromFile)) {
                found = true;
                break;
            }
        }
        if(!found) {
            this.writer = new FileWriter(this.file,true);
            this.writer.write(word +" " + translation +"\n");
            this.writer.close();
        }
    }
    
    public String translate(String word) throws Exception {
        
        String line = null;
        while(this.imeskanera.hasNextLine()) {
            String data = this.imeskanera.nextLine();
            if(data.contains(word)) {
                line = data;
                break;
            }
        
    }
        return line;
    }
}

Answer 1

问题是您的 Scanner 对象已被 add() 方法使用。您需要重新打开输入流才能读取文件的内容。如果您添加

this.imeskanera = new Scanner(this.file, "UTF-8");

translate()方法开头，应该是word。这基本上告诉你 Scanner 没有必要成为一个全球领域。在每个方法中本地使用它。以前我是这样解释文件流的概念的：

Think about file streams (for reading and writing) logically. You cannot allow for such a stream to be "circular". Otherwise, when you try to get the "next line", there will always be a next line and you will never be able to stop reading (or writing). The stream is consumed when it reach the end, and once that is done, to go back to the beginning of the stream, you will need to open a new one; not reuse the old one.

我认为即使在答案被接受后我也需要添加这个解释，因为我知道新的开发人员对这个概念有困难，因此，有必要详细解释它。

话虽如此，您的 MindfulDictionary class 应该如下所示：

public class MindfulDictionary {
    File file;
    FileWriter writer;
//    Scanner imeskanera;

    public MindfulDictionary() throws Exception {
        this.file = new File("test.txt"); // I changed the path to the file to make it work for me. You can change it back if you want to.
        file.createNewFile();

    }

    public void add(String word, String translation) throws Exception {
        Scanner imeskanera = new Scanner(this.file, "UTF-8");
        boolean found = false;
        while (imeskanera.hasNextLine()) {
            String lineFromFile = imeskanera.nextLine();
            if (word.contains(lineFromFile)) {
                found = true;
                break;
            }
        }
        if (!found) {
            this.writer = new FileWriter(this.file, true);
            this.writer.write(word + " " + translation + "\n");
            this.writer.close();
        }
        imeskanera.close();
    }

    public String translate(String word) throws Exception {
        Scanner imeskanera = new Scanner(this.file, "UTF-8");
        String line = null;
        while (imeskanera.hasNextLine()) {
            String data = imeskanera.nextLine();
            if (data.contains(word)) {
                line = data;
                break;
            }

        }
        imeskanera.close();
        return line;
    }
}

我运行你的代码经过我的修改，现在输出是

apina monkey
apina monkey
null
banaani banana

Answer 2

除了@hfontanez 的回答提到的 Scanner 问题，还有以下变化。

if (word.contains(lineFromFile))

这会检查第一个单词是否包含该行，这是不正确的。该文件包含第一个单词和翻译。所以这可以更改为

if (lineFromFile.contains(word))

正如@ghostCat 提到的，可以重构搜索关键字（词）。具有这些更改的代码。

public class MindfulDictionary {
    File file;
    FileWriter writer;
//    Scanner imeskanera;

    public MindfulDictionary() throws Exception {
        this.file = new File("test.txt"); 
        file.createNewFile();

    }

    public void add(String word, String translation) throws Exception {
        
        if (get(word) == null) {
            this.writer = new FileWriter(this.file, true);
            this.writer.write(word + " " + translation + "\n");
            System.out.println("Out>>:"+word + " " + translation + "\n");
            this.writer.close();
        }
    }
    private String get(String word) throws Exception {
        Scanner imeskanera = new Scanner(this.file, "UTF-8");
        boolean found = false;
        String retStr= null;
        while (imeskanera.hasNextLine()) {
            String lineFromFile = imeskanera.nextLine();
            if (lineFromFile.contains(word)) {
                found = true;
                retStr=lineFromFile;
                break;
            }
        }
        imeskanera.close();
        return(retStr);
    }

    public String translate(String word) throws Exception {
        return get(word);
    }


    public static void main(String[] args) throws Exception {
        MindfulDictionary dict = new MindfulDictionary();
        dict.add("apina", "monkey");
        dict.add("apina", "monkey");
        dict.add("banaani", "banana");
        dict.add("apina", "apfe");

        System.out.println( dict.translate("apina") );
        System.out.println( dict.translate("monkey") );
        System.out.println( dict.translate("programming") );
        System.out.println( dict.translate("banana") );

    }

}

在 .txt 中找不到文本，但它已经存在

Text not found in .txt , but it's already there

java

file