Java，如何检查文件是否存在

Question

I have this small code

    File source;
    if ( !source.exists() ) {
        source = new File("instances/student"+student.getStudentID()+".data");
    }

The problem is, source is not initialized. Since the whole point is to check if it exists, how do I avoid this?

Answer 1

Create a File object.

File source = new File(...);

What constructor you use depends on how you want to locate a file. A simple path String could be enough.

EDIT: just realized the source of your confusion may be that you think creating the File object will try to locate the file or create it on the file system. That's not the case. Just calling new File(...) won't check its existence or try to create it. A File object is simply an abstraction for a path in your file system. It could be a directory as well.

Answer 2

You can do:

File f = new File(somepathhere);
if ( !f.exists() ) {
    f = new File("instances/student"+student.getStudentID()+".data");
}

Or you can check if f.isFile()

Answer 3

You seem to have a misunderstanding. Creating a File doesn't create a file on the file system. A File object only really represents a filename. If you want to see whether a file exists, create a File of the appropriate name and check exists() .

But then if you want to overwrite or append to a file you don't even need all that. Just create a new FileOutputStream(...) , with the append parameter set to true if you want to append. No need to check beforehand, in fact it is more than a waste of time.