9- Throwing Exceptions

Jun 18, 2026 04:37 · 4:41 · English · Whisper Turbo · 2 स्पीकर

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So far we've only been handling or catching exceptions,

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but sometimes we want to throw an exception.

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Let me show you an example.

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So in this package,

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the exceptions package,

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I'm going to add a new class called account.

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Here we need a method for depositing

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money into this account.

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So public void deposit,

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we give it a parameter of type float called value.

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Now this value has to be a positive value.

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It doesn't make sense to pass 0 or negative 1 here.

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So we can validate this argument and throw an exception.

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If value is less than or equal to 0,

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we throw,

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so we type the throw keyword,

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then we create an exception object.

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In this case,

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new illegal argument exception.

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We're telling the caller of this method that they have passed an illegal argument.

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Now,

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back to our demo class.

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Let's create an account object and then

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call account .deposit with negative 1.

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When we run this program,

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our program crashes because we have an illegal

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argument exception.

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This is a runtime or unchecked exception which indicates a

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programming error.

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So we shouldn't handle this with a try -catch block.

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We should come back and fix the source of the problem.

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So we pass a positive 1.

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Now the exception is gone.

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So this is how we can throw an exception in Java.

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Now this technique that we have used here validating this argument and

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throw an exception is called defensive programming.

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So if a negative value here can cause a serious problem down the

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road, we throw an exception and prevent the rest of our code from getting executed.

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This is why this technique is called defensive programming because we're

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coding in a defensive manner.

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So our code is immediately telling that it cannot work with a negative value.

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This is better than letting this negative value get some serious problem

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down the road.

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For example,

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you may store it in a database and sometime in the future,

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you read it and then something really weird happens in your application.

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So this is the benefit of defensive programming.

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Having said that,

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as I always say,

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too much of a good thing can be a bad thing.

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Adding this validation logic and throwing exception adds a bit of noise

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to our code.

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It makes our code a bit longer and more verbose.

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This is a very simple example,

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but what if we had a method with three parameters?

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With this approach,

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the first six lines of this method would just be data validation logic.

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So I would say it's better to use defensive programming if

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you're building a library or a framework for others to use.

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If you're building an application,

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you should have a level of trust in your code.

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Your methods should trust each other.

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So don't pollute every single method with data validation logic.

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Perform this kind of validation only when you receive input from the user or

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external systems.

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So at the boundary of your application,

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but not within the application itself.

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Now here we're throwing a runtime or an unchecked exception,

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but what if you want to throw a checked exception?

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Let me show you.

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So I'm going to replace this with IO exception.

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Now we have a compilation error.

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Unhandled exception,

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IO exception.

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So the Java compiler is saying we haven't handled this exception.

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Now, should we wrap this inside the try catch block?

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No,

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it doesn't make sense to throw this exception and handle it right here.

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We want to throw this exception in this method and have the caller of

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this method handle that exception.

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Just like how we use the file reader class.

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So the file reader class throws an exception that we have to explicitly handle.

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So,

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to get rid of this compilation error,

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Over here in the declaration of this method,

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we should specify that this method may throw an IO exception.

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So we type throws,

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note that this is throws,

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but here we're using the throw keyword.

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So throws IO exception.

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We're basically telling the caller of this method that this method may

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throw an IO exception.

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So this is part of the API of this method.

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API is short for Application Programming Interface.

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So anyone who wants to use our account class should know that the deposit method

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takes a float and may throw an exception.

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These details represent the interface or the API of

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this method.

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Okay?

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Now,

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back to our demo class,

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the Java compiler is saying that we haven't handled this exception.

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So this is where we wrap our code inside the try catch

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block.

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There you go.

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