Handling exceptions in Java

When building software, there’s always a chance that your code will be used in some unexpected way, or that some resource that your code relies on will suddenly become unavailable. These events are not “normal” scenario for your code, and are called exceptions.

So what do we do?

Should we let exceptions run wild in our applications, and cross our fingers that everything will turn out find in the end? NO! We should stand tall, look the exceptions in the eye and HANDLE them.

When we try to use a method or class that has a risk of failing at runtime, we need to be able to catch any errors, and give the application a way to get out of it’s current situation.

What does that look like in code?

Imagine that you’ve written an application that will fill the gas tank of your car if pulls up to a pump at a gas station. It uses a fillGasTank method that makes interacts with a pump instance to get gas.

public void fillGasTank() {
  try {
    Pump pump = new Pump();
    pump.getGas();
    System.out.print("Attempting to fill the tank");
  } catch (OutOfFuelException ex){
    System.out.print("The pump is empty");
  } finally {
    pump.detachHose();
  }
}

When the fillGasTank method is called, it runs the logic inside the try block first, creates and new pump instance, calls getGas, and prints alerts the driver that it’s “Attempting to fill the tank”.

The prints statement inside the catch block will only run if the getGas method throws an OutOfFuelException meaning that this pump is completely empty.

The finally block will execute whether an exception was thrown or not, and makes sure that the hose detaches from the car.

Summary

When using “risky” code, always use a try/catch to protect yourself from unexpected behavior. Try/catch blocks aren’t meant to protect you from logic errors in your code, but will shield you from many errors that are outside of your control.