To start writing to a file, you just need to create an instance of java.io.File, for example:

File file = new File('output.txt')

Now let's see which writing operations we can perform on a File object.

In the first and second code snippets, we used the setText method. It is actually absolutely equivalent to the write method that also replaces the file content with the string that is passed. Those methods exist to give the developer the freedom to choose the best wording to describe his or her intent.

The functionality of the leftShift method is exactly the same as for the append method. We used both in steps 4 and 5. There are other special method names in Groovy such as plus for + or minus for - , which you can add to your own classes to be able to write more concise expressions with the help of well-known arithmetical operators.

Each call to the append or leftShift methods opens and closes the file every time you execute them. That's, of course, not very efficient if you need to perform many write operations. To get more control, you can operate on the java.io.Writer object within a closure passed to the withWriter method that we used in the sixth example.

As you might have noticed, we also make use of the left shift (<<) operator on the writer object. That operator is made available by Groovy on all the java.io.Writer instances, just like with java.io.File, and even with java.io.OutputStream.

The withWriter method creates a new instance of java.io.BufferedWriter and ensures it is flushed and closed upon return. The file content will be fully replaced. In a similar way, you can use the withWriterAppend method to use the writer object to add content to the end of the file. Another available method is withPrintWriter, which gives you access to the java.io.PrintWriter instance within a closure.

Another way of writing to a file implies using the withOutputStream method, which is similar to withWriter with the exception that the closure operates on the java.io.BufferedOutputStream instance as you can see in the seventh snippet.

The leftShift operator is used again to append data to a stream. The stream object will be opened and closed automatically in a similar way to the withWriter method.

You can also wrap file access using the withDataOutputStream and withObjectOutputStream methods. This gives you access to the java.io.DataOutputStream and java.io.ObjectOutputStream instances within a closure that allows you to write out primitive and serialized Java types respectively.

If you want to control the writer or stream instances yourself, you can use convenient constructor methods such as newWriter, newOutputStream, newDataOutputStream, and newObjectOutputStream.

Many of the writing methods of java.io.File are also available in the java.io.Writer and java.io.OutputStream classes that allow using Groovy goodies for virtually any output target.