Question

I am getting an unexpected output from Properties.contains()…

This is my code…

File file = new File("C:\\ravi\\non-existing.no");
Properties pro = System.getProperties();
pro.put("file", file);
System.out.println(pro.contains(file)); //PRINTS TRUE , AS EXPECTED

File file2 = file;
System.out.println(pro.contains(file2)); //PRINTS TRUE , AS EXPECTED

File file3 = new File("C:\\ravi\\non-existing.no");
System.out.println(pro.contains(file3)); //EXPECTED FALSE , BUT PRINTS TRUE

File file4 = new File("C:\\ravi\\non.no");
System.out.println(pro.contains(file4)); //PRINTS FALSE , AS EXPECTED

I am expecting the Properties to check for the existance of the File, however this doesn’t seem to work. Could someone please help me explain why file3 doesn’t work as I expect.

Answer 1

I think your problem lies here :

 pro.put("file", file);

From the Java docs:

Because Properties inherits from Hashtable, the put and putAll methods can be applied to a Properties object. Their use is strongly discouraged as they allow the caller to insert entries whose keys or values are not Strings. The setProperty method should be used instead.

And when you call contains() on it, according to the Java docs:

returns true if and only if some key maps to the value argument in this hashtable as determined by the equals method; false otherwise.

You see your problem now?

To clarify further:

When you do :System.out.println(pro.contains(file3)); you end up doing file.equals(file3) , hence true.

And when you do :System.out.println(pro.contains(file4)); you end up doing file.equals(file4) , hence false.