Assertion not working

Question

I am trying to write an Assertion to check if the size the user gives is a positive value, if not then make it positive, this statement is inside the class constructor which takes the size value and then makes an array[size]. I have written the below code which i believe to be correct.

    public Grid(int size) {       try{         assert size > 0 ;     }     catch(AssertionError e){         size = Math.abs(size);     }      setLayout(new GridLayout(size, size));     grid = new JButton[size][size]; } 

Though I never seems to evaluate my assertion and continues the program then causes the NegativeArraySize error( which i am trying to avoid)

I also tried just

assert size>0; 

And the program fails to stop for negative values..

I have had a few problems with running java on mac recently, so i don't know if my code is right or if it is just one of those odd mac quirks!! and should just use

size=Math.abs(size); 

Thanks Sam,

Answer 1

You need to run your program with the -ea switch (enable assertions), otherwise no assert instructions will be run by the JVM at all. Depending on asserts is a little dangerous. I suggest you do something like this:

public Grid(int size) {     size = Math.max(0, size)      setLayout(new GridLayout(size, size));     grid = new JButton[size][size]; } 

Or even like this:

public Grid(int size) {     if(size < 0) {         throw new IllegalArgumentException("cannot create a grid with a negative size");     }      setLayout(new GridLayout(size, size));     grid = new JButton[size][size]; } 

The second suggestion has the benefit of showing you potential programming errors in other parts of your code, whereas the first one silently ignores them. This depends on your use case.

Answer 2

Assertions can be enabled or disabled when the program is started, and are disabled by default.

See Enabling and Disabling Assertions

In short, to enable assertions in all classes, except System classes, use the -enableassertions, or -ea, switch when you run your class.