Java String返回方法不返回

I have a method to return a string of the folder path. CSVFile method calls another method ConfigFilePath to extract the value from a config file and return it back to be used for another condition.

我有一个方法来返回文件夹路径的字符串。 CSVFile方法调用另一个方法ConfigFilePath从配置文件中提取值并将其返回以用于另一个条件。

public CSVFile(ManagedElement_T[] meInfo) {
    String path = null;
    ConfigFilePath(path);
    system.out.print("CSV "+path);
}

public String ConfigFilePath(String path) {
    final String directory = System.getProperty("user.dir");
    final String filename = "config.properties";
    final File configFile = new File(directory+"/"+filename);

    final Properties prop = new Properties();
    InputStream input = null;

    try {
        input = new FileInputStream(configFile);
        prop.load(input);
        path = prop.getProperty("diretory_csv");
        System.out.println("PATH " + path);
    } catch (IOException ex) {
        ex.printStackTrace();
    } finally {
             // Close the file
    }

    File checkPath = new File(path.toString());
    System.out.println("CHECK " + checkPath);
    if (checkPath.exists()) {
        System.out.println("Directory already exists ...");
    } else {
           //mkdir if it does not exist
    }


    return path;
}

Problem is, it doesn't return any var path? just says null when I print it but inside the ConfigFilePath method it seems its getting the right values based from the prints in eclipse.

问题是,它没有返回任何var路径?当我打印它时只是说null但是在ConfigFilePath方法中它似乎从eclipse中的打印得到了正确的值。

System.out.println("PATH " + path); = C:/opt/CORBA/input
System.out.println("CHECK " + checkPath); = C:\opt\CORBA\input
System.out.print("CSV "+path); = null

4 个解决方案

#1

Java is actually pass by value not pass by reference.

Java实际上是通过值传递而不是通过引用传递。

so when you initialized it here:

所以当你在这里初始化时:

path = prop.getProperty("diretory_csv");

path wont actually reference that value but get destroyed when out of scope.

path实际上不会引用该值,但在超出范围时会被销毁。

solution:

Get the returned value of the ConfigFilePath method

获取ConfigFilePath方法的返回值

path = ConfigFilePath(path);

#2

The method ConfigFilePath is returning a value. You have to assign that returned value to the variable path:

ConfigFilePath方法返回一个值。您必须将返回的值分配给变量路径:

path = ConfigFilePath(path);

Note: Try to follow Java naming conventions. Use mixedCase for methods/variables and use CamelCase for classes/interfaces

注意:尝试遵循Java命名约定。将mixedCase用于方法/变量,并将CamelCase用于类/接口

#3

ConfigFilePath is returning a value. You should get the result from it and put it in the path:

ConfigFilePath返回一个值。您应该从中获取结果并将其放在路径中:

public CSVFile(ManagedElement_T[] meInfo) {
    String path = ConfigFilePath(path);
    system.out.print("CSV "+path);
}

#4

The problem you face is that java is passing method-parameters by value, not by reference. It can be confusing, since when using object types the passed value is actually an object-reference.

您遇到的问题是java是按值传递方法参数,而不是通过引用传递。它可能令人困惑,因为在使用对象类型时,传递的值实际上是对象引用。

Let's have a look at some lines of your code:

我们来看看代码的某些行:

public String ConfigFilePath(String path)

Here your method gets passed a reference to some String and keeps it in a local variable called path.

在这里,您的方法传递对某些String的引用,并将其保存在名为path的局部变量中。

path = prop.getProperty("diretory_csv");

Now you're changing the local variable path to point to a new String-object.

现在,您要将局部变量路径更改为指向新的String对象。

return path;

This is passing out the reference as a return-value. ConfigFilePath's local path-variable goes out of scope.

这会将引用作为返回值传递出去。 ConfigFilePath的本地路径变量超出范围。

ConfigFilePath(path);

As the other answers said: in this line you're calling the method passing a reference to a String that is referenced by CSVFile's local path-variable. But since passing is by value this path-variable will not be changed by the method-call. Your result is passed via the return-value - and you are ignoring that instead of assigning it to your path-variable.

正如其他答案所说:在这一行中,您调用的方法是将引用传递给由CSVFile的本地路径变量引用的String。但由于传递是按值,因此方法调用不会更改此路径变量。您的结果通过返回值传递 - 您忽略了它而不是将其分配给路径变量。

So to sum it up:

总结一下:

ConfigFilePath() doesn't need a parameter since it's not using the value that is passed in

ConfigFilePath()不需要参数,因为它没有使用传入的值

You have to assign the returned value to some variable in your calling method for it to be of any use.

您必须将返回的值分配给调用方法中的某个变量,以使其具有任何用途。

#1