1. Testing and debugging
A short interlude
17-Apr-19

2. Print statements
An old-fashioned, but still very useful, debugging technique is putting in print statements
However, it’s a nuisance to remove them again (then add them again, then remove them again...)
Here’s a helpful technique:
boolean debugging = true;
debug("Some debugging message...");
private void debug(Object obj) { if (debugging) System.out.println(obj); }

3. Debugging output methods, I
JUnit testing should not produce any output that you have to look at--it should be fully automatic
First, consider whether you can separate the output method into one method that computes the output, and another method that actually prints it
Then you can test the former method
Since a String can contain newlines, you are not restricted to single-line output
Separation of concerns is good practice in any case

4. Debugging output methods, II
You can also change (maybe temporarily) where the output goes
PrintStream usualSystemOut = System.out; // save old stream
System.setOut(someOtherPrintStream); // use new stream
System.setOut(usualPrintStream); // change it back
You can write to a file, then test whether the file has the right contents
Here's a more sophisticated approach:
Use a ByteArrayOutputStream to capture the output in a byte array
Use the ByteArray's toString() method to examine the output
Complete sample code is on the next slide

5. Program to capture output
public static void main(String[] args) { PrintStream originalOut = System.out; OutputStream os = new ByteArrayOutputStream(); PrintStream ps = new PrintStream(os); System.setOut(ps); System.out.print("Hello, output!"); System.setOut(originalOut); System.out.println("I got: [" + os.toString() + "]"); }
I got: [Hello, output!]

6. toString()
In general, you want to avoid unnecessary methods in your classes, but...
...It’s usually a good idea to override toString()
Having a toString() method often simplifies debugging, because you can see what your objects really are
Don’t use your toString() methods to compare objects in your JUnit tests, unless you are very sure you will never change the string representation of your objects
For JUnit testing (especially with assertEquals), you should have a good version of public boolean equals(Object o)
The default equals method tests identity, not equality, and this is probably not what you want

7. Review: Overriding methods
To override a method is to write a method that has the exact same signature as an inherited method
The names of parameters are not part of the signature, so they can be different if you really want
You cannot override a method with a less public method (but it can be more public if you want)
You cannot throw any checked exceptions that the inherited method doesn’t throw
In Java 5, you should precede your method
Examples:
public String toString() { ... }
public boolean equals(Object o) { ... } // Notice type of parameter!

8. The End