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Oh, boy, the reference trap.

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This is my favorite lesson because I've seen the best developers get caught in the trap.

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That's because nobody told them about it and they had to learn the hard way by falling into the trap.

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But because you're my student, I'm going to make sure that you don't fall into any traps.

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I told you earlier that a variable cannot store an array, it stores a reference that points to it because

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of this, another variable can actually store reference that points to the same array.

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Isn't that weird?

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As far as I know, there isn't an official name to describe this phenomenon.

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So I call it the reference trap.

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In this lesson, we're going to talk about the reference drop, the violation that causes it and the

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solution that prevents it.

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First things first, inside Section six, make a new file named reference, drop Java and inside the

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class, make sure it has a main method.

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This trap happens with many variables, Cheryl, reference to one array.

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This is about and I call it the reference strap, because if I change the area through one variable,

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the other variable gets affected as well.

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In this example, I set numbers equal to an array of three values.

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But notice that I can update the array from another variable numbers to.

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This is bad.

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This is a trap, and to avoid the reference trap, do not set array variables equal to each other.

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That's because, well, suppose you have a variable that stores an array of integers, numbers, stores,

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a reference that points to the array.

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If I set another variable equal to numbers, it's going to copy what's inside and what's inside is the

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reference.

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So now both variables share a reference to the same array.

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And so the reference trap happens when we set array of variables equal to each other.

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No stores a reference that points to the array and by setting numbers to equal to numbers, it copies

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the value inside and what's inside is the reference.

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Now, both variables share a reference to the same array.

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So it's not a coincidence that number two is reference is also a pink circle.

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I want to commit this violation and show you the trappin code.

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So let's assume you're a manager at the Java Cafe and you have the staff list from last year.

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I'm going to set a variable string staff last year.

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Is equal to an array of three values, Tommy.

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Joel and Ellie.

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And now I'm going to print the contents of staff last year as one long string.

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Good.

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Nothing too hard, we're just printing an array of string values, and now this year, only one staff

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member changed and you're too lazy to rewrite another array.

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So you decide to set another array variable equal to this one string stuff this year is equal to staff

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last year.

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And you're going to print the contents of the story as well.

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OK, we're off to a good start.

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And now, unfortunately, Joel went missing in a new employee named Abby took his place.

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The staff this year changed.

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So we're going to change the next one from the second variable.

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We were on the code.

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Oh, that's not good.

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I meant the only update this year staff list, but last year, staff list updated as well.

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This is very bad because now last year's data is ruined.

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This is the reference trap I was telling you about.

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When you set a variable equal to another eight copies, the value inside, which is a reference.

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If I update this right through one variable, it will affect the other because now they both point to

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the same array.

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This is bad.

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The state of a variable should not change because you updated another.

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In a perfect world, every variable should point to its own array.

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So what's the solution if I want to copy an array into another variable, how do I do it?

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The solution is to make a new era.

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This trap occurs when you set array variables equal to each other, so don't be lazy, create a new

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array and then copy every evalu into the new array using a for loop.

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So back in our code, we're going to do just that, I'm going to set stuff this year equal to a new

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string array with a length of three elements.

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Then I'm going to create for a loop that starts at ENTI equal to zero.

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And it's going to run through every element in one of the arrays, they both have a length of three,

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so it doesn't matter which length you use.

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Plus.

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And during each run, I'm going to update each element and stuff this year by setting it equal to an

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element from staff last year with the same index.

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Now I can rerun the code.

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Okay, perfect.

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We successfully copied every element from the staff last year to the staff this year.

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Now what happens if we change the next one from the second variable?

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So I'm going to change the next one, I'm going to set that equal to Abby.

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And the world makes sense again, sweet.

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That's because you stuff that you're equal to a brand new array.

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The loot copies every value from staff last year to the same index stuff this year, and now each variable

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stores a unique reference to its own array.

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Updating the second variable should have no effect on the first.

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Now, here's a better solution, erase that copy of because for something so simple, the for loop is

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so much code.

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The erased utility class that we've been using to call the two string method has another useful method

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called copy of.

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It takes two arguments the area that you want to copy and how much of it you want to copy.

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This looks a lot easier and it seems like a much better solution.

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Someone to a better table.

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And don't worry, this will be in your cheat sheet.

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So in your code, you remove this monstrosity.

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And replace it with a raised copy of.

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And erase that copy of requires two arguments, the you want to copy stuff last year and how much of

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it you want to copy.

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We're going to copy the full length of the array.

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All right, and we get the same result.

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That's because stuff this year is equal to a copy of the old array, I left out the second argument

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in this animation just because there wasn't any room.

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But you get the point.

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Each variable points to a unique array.

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I hope now you realize that working with a raise is like playing with knives, but as long as you're

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careful and you follow my table, you should be fine.

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In this video, you learned about the reference drop, the reference trap happened when you mishandle

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the race, do not separate variables equal to each other.

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This doesn't copy the array.

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It copies the reference.

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The right way to copy arrays is to use erase that copy of it sets the variable equal to a copy of the

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original array.

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And now each variable points to a unique array.

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And you don't have to worry about the state of one variable changing because you updated another.