whats the difference between try...catch and if...throw when dealing with exception?

Great question.

It's probably better to take a specific example.

Suppose you have a program that needs to read a file. There's always the possibility that the file won't be where it's supposed to be.

So if you have code like this:

Scanner input = new Scanner(new File('someFile.txt');

the program won't compile unless you do something. The compiler will report an unreported exception: FileNotFoundException. The compiler "knows" that if the file doesn't exist it won't have a file to read from. So it complains.

In this case, because a file I/O error is what is known as a checked exception -- an exception that must be either caught or declared in the method header that might generate it -- you must deal with it. As the definition says, you can either (1) catch it, or (2) declare it in the appropriate method header.

For option (1), you use try... catch...

For option (2), you include a throws clause in the method header, viz.., throws FileNotFoundException, and by doing that you pass the problem on and you don't need the try... catch...

So you can do this:

public .... methodName() throws FileNotFoundException {
    . . . 
    Scanner input = new Scanner(new File('someFile.txt');
    . . .
}

or this:

```java
public .... methodName() {
    try {
        Scanner input = new Scanner(new File('someFile.txt');
    } catch (FileNotFoundException e) { 
        . . .
    }

There are many other considerations, including unchecked expressions, and other varieties of try... catch.... syntax, who deals with the thrown exception, etc., but this is the basics.