17- Synchronized Collections

Jul 03, 2026 22:42 · 3:24 · English · Whisper Turbo · 2 Altofalantes

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Sometimes we need to share a collection across many threads.

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So let's create a collection object collection of

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integers.

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We call it collection and set it to a new array

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list.

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Now let's create two threads and have each thread

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add three items to this collection.

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So thread one,

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let's set this to a new thread.

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Now here I'm going to use a lambda expression to represent a

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runnable object.

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Because this runnable interface has a single abstract method that

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is run, so it's a functional interface and we can represent it using a lambda expression.

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Now this method doesn't have any parameters and returns void.

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So over here,

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I'm going to pass empty parenthesis,

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an arrow,

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and a block.

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Now in the body of this lambda,

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we're going to call collection .addAll

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Here we want to pass three items.

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So let's use arrays that has list and pass one

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two and three Now let's duplicate this code

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So we create another thread and

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call this one thread two and with this we're going to add four

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five and six Next we start both

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these threads.

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So thread one does start and thread two does start then

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we join with them

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So thread one the join and thread to the join that

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we should wrap this inside the try catch block.

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There you go Let's move this line over here

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as well.

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So once they're finished,

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let's print what we have in the collection Take

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a look.

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So we have four five six one of

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our insertions was lost This is because we have a race condition two threads

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are racing or competing to modify our collection concurrently

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Now, to solve this problem,

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we can use a synchronized collection,

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which is a wrapper around a regular collection type in Java.

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So instead of setting this collection to a new array list,

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we're going to call collections.

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This class has a bunch of static factory methods for creating

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synchronized collections.

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So we have synchronized collection.

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Let me put this on a new line so we can see clearly.

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Take a look.

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We have synchronized collection,

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you have synchronized list,

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synchronized set,

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synchronized map,

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and so on.

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So let's call synchronized collection.

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Here we should pass an object that implements the collection

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interface, like an array list.

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So new array list.

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Internally, our objects are stored inside this array list,

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but when we call this method,

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this method is going to wrap this array list inside a synchronized collection.

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In that synchronized collection,

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all the methods like add,

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remove and so on,

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they have synchronization code.

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So synchronized collection essentially wraps a regular collection like

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an array list,

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but it makes it synchronized.

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Okay, now with this simple change,

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when we run our program,

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we can see that two threads

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modify our collection concurrently.

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So,

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this is how we can create a synchronous collection.

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