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Single threaded applications run everything on one thread.

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By default, every application has a main thread.

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Any single threaded application runs everything on the main thread.

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In this lesson, you will learn about the limitations of a single threaded app.

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Open up the folder for this lesson by following this path in your course of resources.

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OK, let's talk about the main thread.

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Every Java app has a default, a mean thread and a single threaded app runs everything on the main thread.

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If I run this app the way it is, it should be relatively fast.

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All of the code we have in Maine runs on the main thread, and it runs relatively fast.

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OK, simple enough.

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Let's uncomment this function call.

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And inside of the same thread, we're going to be performing a really long task and then we're going

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to be doing some other work.

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If I were on the up.

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Wow, that took really long.

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The problem is this task takes three whole seconds to process.

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Here we have all of our code running on the main threat.

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The first task takes three whole seconds to process.

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In other words, the task is blocking the thread for three seconds.

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That's really bad.

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And only after the three seconds this, our other tasks get processed.

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That being said, time intensive tasks should not block the main thread, time intensive tasks should

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run on another thread.

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And that's where concurrency comes into play.

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Let's recap you saw the limitation of a single threaded up time intensive tasks can block the main thread.

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A single threaded app runs everything on the main thread.

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If I run this app, the first task takes really long to process and blocks the main thread for three

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whole seconds.

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Then our other task can run.

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That being said, time intensive tasks such as this one should never block the main thread.

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They should be run on another thread, which is what we're going to look at in the next video.