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The Thing About Strings

10:53 with Craig Dennis

Strings can be confusing and seem to work differently than you thought you understood. Let's explore the common misconceptions and remove the confusion.

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String.equals Javadoc
Most IDEs generate that code for you. See IntelliJ IDEA (search equals).
String.intern Javadoc

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[MUSIC] 0:00

Hi. 0:04

I'm Craig and I'm a developer. 0:05

Now we've all been there. 0:07

We think we understand the basics of a programming language and 0:08

then we encounter a line of code that blows our mind. 0:11

And it makes us question everything we thought we knew up until that point. 0:14

Well a huge culprit of that sort of experience is strings in Java. 0:18

Strings have several characteristics about them that, when combined, 0:23

can definitely go cross-eyed with confusion. 0:26

And you'll find yourself not alone in your confusion. 0:28

Hitting the search engines, you'll find hundreds of thousands of questions about 0:30

them and why they behave the way they do. 0:34

I thought that this format would be a good way to go and 0:35

explore why strings seem to be so mysterious. 0:39

And try to demystify and 0:42

expose the common misunderstandings that happen when we use string in our code. 0:43

Are you ready? 0:46

Let's dive in. 0:48

Okay. So I've written a little program here 0:50

called Thing.java. 0:52

And it's gonna help us explore some concepts about strings. 0:53

Now I've added a method here called explore. 0:57

And it takes an assumption about what we are going to be exploring and 1:00

an expression. 1:03

The expression is expected to always be true. 1:04

It will generate a result and let us know if our assumption is correct. 1:07

So let's get right into some of those explorations. 1:11

We have explored equality a little bit already. 1:13

And we know that when we want to compare primitives together we can use the double 1:16

equal sign. 1:19

So let's make a couple primitives. 1:20

So we'll have int firstNumber equals 12 and int secondNumber equals 12. 1:22

Hey, let's go ahead and call our explorer method. 1:30

That's what it looks like. 1:32

We say primitives use double equals. 1:33

Okay, and then the expression that we wanna say, 1:39

wanna say is firstNumber use double equals to the secondNumber. 1:41

All right, well let's just run this and 1:47

make sure that our explorer method's working. 1:49

So we'll say clear and then javac. 1:51

We compile Thing.Java, and then we're gonna run it. 1:53

Cool. 1:59

So we get a yea. 2:00

So we know that it's working. 2:00

So that's primitives, but what about objects? 2:02

Let's create two brand new objects exactly the same way. 2:04

Now since we didn't change anything, they must be equal, right? 2:07

So, let's make two new objects. 2:11

We'll say Object, firstObject equals new Object. 2:13

And secondObject equals new Object. 2:17

Okay, so let's explore that. 2:26

Well let's say, explore objects work just like primitives. 2:28

So we should be able to use the double equals, right? 2:36

[BLANK_AUDIO] 2:38

Let's run that. 2:43

Okay, and we got a what. 2:45

you're assumption is incorrect. 2:47

Our first what. 2:49

So objects work a little bit different. 2:50

When we create an object, the variable represents an object reference. 2:52

Now you can use double equals for objects, but what that's saying is that 2:56

the object references are actually referring to the same object. 3:00

Let's explore that. 3:05

So, first of all, let's fix our exploration statement here, 3:05

our assumption, and let's say, Objects, references use double equals to check 3:08

if they refer to the same object in memory. 3:16

It's gonna wrap seemingly equal objects are not equal. 3:21

So we can actually use the not equals sign. 3:28

So let's do that. 3:31

I know, let's go ahead and make a new object that refers to the first object. 3:33

So we'll just say Object, the sameObject equals, and 3:38

we'll just literally put the object reference in there. 3:43

So now let's ex, let's do another exploration. 3:47

Let's say, let's say explore object references 3:49

must refer to the same object to use double equals. 3:54

So, firstObject equals sameObject. 4:02

Let's go ahead and run that and see. 4:05

There we go, two YAYs. 4:10

So this holds true for all objects. 4:11

Now the thing to remember is this, 4:13

strings are objects, everything inherits from objects. 4:15

So double equals only works if the variable is referring to the same object. 4:19

So you might have seen some double equals seemingly work for strings. 4:24

Let's explore that just a bit. 4:28

We'll make a new string, firstString this equals, [INAUDIBLE] my name and 4:30

we'll say, String secondString is equal to Craig. 4:37

And let's just go ahead and we'll say explore 4:42

strings are objects and work like objects. 4:47

So therefore, they're two separate objects, so they're not the same. 4:53

So we'll say firstString is not equal to secondString. 4:56

Let's go ahead and run that. 5:05

And we get a what. 5:10

Huh, how could those two be equal? 5:11

Aren't they two completely different objects? 5:13

What is the deal? 5:15

I thought I understood how this works, and now I'm not so sure. 5:17

Ok, so 5:20

the good news here is you're not going crazy, those are actually the same object. 5:20

So here's the thing, when you create a string like that, you know, 5:25

putting it in double quotes? 5:27

We're creating what is known as a string literal. 5:29

Now since strings are immutable which means, if you remember, 5:32

they can't be changed, the compiler does some tricks to optimize. 5:35

Since it knows the string won't change it just creates a single instance of that 5:40

string and makes all variables refer to that single instance. 5:43

It keeps these special references in a special place in memory called 5:47

the string pool. 5:51

So it's essentially like we saw in the previous object equality, right? 5:52

They're pointing to the same thing. 5:55

So let's fix our exploration here. 5:56

String literals are actually referring 5:59

to the same object, all right. 6:05

So at compile time that becomes the same object. 6:11

So we're gonna put double equals there. 6:13

Okay. 6:15

And let's add a new one that uses a dynamic string, 6:16

one that isn't there at compile time. 6:18

So you can kind of imagine this as input coming in from the user. 6:20

Now a handy way to make sure that we're creating a unique string is 6:24

by using its constructor that actually takes a string. 6:27

So let's take a look at that. 6:30

So let's say, string differentString is equal to a new String. 6:32

They have the same, what seems like to be the same value, so let's do this. 6:41

All right, this exploration string objects 6:45

that contain the same characters but 6:51

point to different objects cannot use double equals. 6:55

So let's prove that really quick. 7:03

FirstString does not equal a differentString. 7:05

Okay, so let's check those assumptions. 7:13

Awesome, those are both YAY. 7:18

So, one final trick that you might see used is a process called string interning. 7:20

You can actually take a user imputed string and add it to the same string pool, 7:25

and you do that by using a method on stings called intern. 7:30

So let's explore that, just for a quick second. 7:33

So let's add a string called anotherString. 7:36

And we use that same constructor method, okay. 7:40

And let's make a new assumption and we'll say string interning 7:47

adds to the same String Pool where literals live, 7:53

so you get back the same reference, if it exists. 8:00

Okay, so we can say anotherString and 8:06

if we do .intern is equal, now we can use double equals. 8:11

Let's go ahead and make sure that that works. 8:18

Awesome. 8:22

So because you almost always are checking object equality and 8:25

not concerning yourself with object reference equality, 8:28

this is why you almost always should use the equals method, like this. 8:31

[BLANK_AUDIO] 8:34

And this is true of all objects, so 8:40

all objects should use equals to check equality. 8:43

So we'll say firstString equals differentString. 8:48

And you remember different string was the one that was created with the new. 8:55

So see differentString was created with the new string, Craig, and 9:01

the first string was a literal. 9:04

So they're two separate, they're in, they're in different memory pools. 9:06

Let's just make sure that worked. 9:09

YAY. 9:12

So how does that equals method work? 9:14

Well for strings, 9:16

it must check that every single character is the same in the other string. 9:16

So the way this works is that string has overridden the equals method 9:21

on this class definition. 9:24

Now I have added a link in the teacher's notes about how you might want to go about 9:25

writing your own equals method in your own classes. 9:29

All right, let's review the output of your exploration here together one more time. 9:31

So primitives use double equals. 9:36

Object references use double equals to check if they refer to the same object in 9:38

memory, seemingly equal objects are not equal. 9:42

Object references must refer to the same object to use double equals. 9:46

So that's the only time you should use double equals. 9:49

Now string literals are actually referring to the same object. 9:52

Remember they go into the string pool together. 9:56

String objects that contain the same characters but 9:58

point to different objects can not use double equals. 10:01

String interning adds to the same string pool where literals live. 10:04

So you can get back the same reference, if it exists of course. 10:08

And, all objects should use equals to check for equality, and 10:11

you can write your own. 10:15

So I hope this knowledge will arm you, 10:16

as you come up against confusing encounters with strings. 10:18

And help remind you that you are, in fact, not going crazy. 10:21

So there you go. 10:27

I hope that cleared things up for you and 10:28

if nothing else it should give you something to refer back to 10:30

if you do in fact end up thinking you're losing your mind about strings. 10:33

If you like this format and 10:37

would like to see more like this please speak up in the forum. 10:38

And we'll tackle some more of the more nuance problems that 10:40

all we developers encounter from time to time. 10:43

As always, your suggestions are very welcome and 10:46

really help to shape what we're building together here. 10:48

Thanks for hanging out, and see you soon. 10:51

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