问题

以下是翻译好的内容：

我有一个像这样写入 `List<String>` 到文件的方法：

```java
public static void main(String[] args) {
    List<String> data = Arrays.asList("1", "2", "3", "4", "5");
    writeObjectToFile(data, "demo.dat");
    
    List<String> result = null;
    readObjectFromFile(result, "demo.dat");
    System.out.println(result);
}

使用 writeObjectToFile 方法写入数据：

public static <T> void writeObjectToFile(List<T> obj, String fileName) {
    File file = new File(fileName);
    FileOutputStream fos = null;
    ObjectOutputStream oos = null;

    try {
        fos = new FileOutputStream(file);
        oos = new ObjectOutputStream(fos);
        oos.writeObject(obj);
    } catch (Exception e) {
        e.printStackTrace();
    } finally {
        // 关闭 fos，oos 等...
    }
}

以及 readObjectFromFile 方法：

public static <T> void readObjectFromFile(List<T> obj, String fileName) {
    File file = new File(fileName);
    FileInputStream fis = null;
    ObjectInputStream ois = null;

    if (file.exists()) {
        try {
            fis = new FileInputStream(file);
            ois = new ObjectInputStream(fis);
            Object readObject = ois.readObject();
            obj = (List<T>) readObject;
            System.out.println(obj);
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            // 关闭 fis，ois 等...
        }
    }
}

写入文件的步骤是正确的，但读取步骤未按预期工作。我得到的 result 列表为 null：

// 控制台输出
[1, 2, 3, 4, 5] // --> 这是 System.out.println(obj) 的结果;
null            // --> 这是 System.out.println(result) 的结果;

但是当我将读取方法更改为返回列表时：

public static <T> List<T> readObjectFromFile(String fileName) {
    File file = new File(fileName);
    FileInputStream fis = null;
    ObjectInputStream ois = null;

    if (file.exists()) {
        try {
            fis = new FileInputStream(file);
            ois = new ObjectInputStream(fis);
            Object readObject = ois.readObject();
            List<T> obj = (List<T>) readObject;
            System.out.println(obj);
            return obj;
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            // 关闭 fis，ois 等...
        }
    }
    return null;
}

那么我就得到了预期的结果：

// 控制台输出
[1, 2, 3, 4, 5] // --> 这是 System.out.println(obj) 的结果;
[1, 2, 3, 4, 5] // --> 这是 System.out.println(result) 的结果;

我不知道为什么会这样。为什么当我将 result 列表作为参数传递时，我无法获得预期的结果？请帮助我。

<details>
<summary>英文:</summary>

I have a method writing `List&lt;String&gt;` into file like this:

```java
public static void main(String[] args) {
	List&lt;String&gt; data = Arrays.asList(&quot;1&quot;, &quot;2&quot;, &quot;3&quot;, &quot;4&quot;, &quot;5&quot;);
	writeObjectToFile(data, &quot;demo.dat&quot;);
	
	List&lt;String&gt; result = null;
	readObjectFromFile(result, &quot;demo.dat&quot;);
	System.out.println(result);
}

with the writeObjectToFile method:

public static <T> void writeObjectToFile(List<T> obj, String fileName) {
	File file = new File(fileName);
	FileOutputStream fos = null;
	ObjectOutputStream oos = null;

	try {
		fos = new FileOutputStream(file);
		oos = new ObjectOutputStream(fos);
		oos.writeObject(obj);
	} catch (Exception e) {
		e.printStackTrace();
	} finally {
		// close fos, oos, ... 
	}
}

and readObjectFromFile method:

public static <T> void readObjectFromFile(List<T> obj, String fileName) {
	File file = new File(fileName);
	FileInputStream fis = null;
	ObjectInputStream ois = null;

	if (file.exists()) {
		try {
			fis = new FileInputStream(file);
			ois = new ObjectInputStream(fis);
			Object readObject = ois.readObject();
			obj = (List<T>) readObject;
			System.out.println(obj);
		} catch (Exception e) {
			e.printStackTrace();
		} finally {
			// close fis, ois,...
		}
	}
}

The writing data to file is OK, but the reading step is not working as expected. I got the result list is null:

// console output
[1, 2, 3, 4, 5] // --> this is result of System.out.println(obj);
null            // --> this is result of System.out.println(result);

But when I change the reading method to return a list:

public static <T> List<T> readObjectFromFile(String fileName) {
	File file = new File(fileName);
	FileInputStream fis = null;
	ObjectInputStream ois = null;

	if (file.exists()) {
		try {
			fis = new FileInputStream(file);
			ois = new ObjectInputStream(fis);
			Object readObject = ois.readObject();
			List<T> obj = (List<T>) readObject;
			System.out.println(obj);
			return obj;
		} catch (Exception e) {
			e.printStackTrace();
		} finally {
			// close fis, ois,...
		}
	}
	return null;
}

Then I get the expected result:

// console output
[1, 2, 3, 4, 5] // --> this is result of System.out.println(obj);
[1, 2, 3, 4, 5] // --> this is result of System.out.println(result);

I don't know why is it. Why when I passed the result list as a parameter, I could not get the expected result? Please help me.

答案1

得分: 1

你正在改变它的引用值，Java 是按值传递的，当你将一个对象作为参数传递给一个方法时，Java 会传递一个保存引用值的变量，你不能改变那个值（确切地说，你只能在方法内部改变那个值）。

想象一下，在你将它传递到方法之前，它指向 null。

在方法内部，你试图将它指向另一个对象（分配新的引用值）。
当方法结束时，这个变化将被丢弃，它仍然指向 null。

答案2

得分: 0

您的List obj仅是对您在main()中初始化的列表的引用。只要方法正在执行，此引用就在堆栈上。当您将一个新值赋给obj时，obj指向该值。您没有改变result的值，就像您可能期望的那样。您需要做的是类似于

obj.addAll(readObject)

obj.addAll(readObject)