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Java Java Objects (Retired) Meet Objects Welcome Back

Can you not use the sign "=" when referring to strings?

It seems you can use if (age = 40) but you can't say if (name = "Craig") instead you have to put if(name.equals("Craig")

is that just because it's a string value? Does it just evaluate it differently based on type?

Because it seems odd considering you use "=" to set the value in the first place...

I'm confused by the difference in {String name = "Craig"} vs {if (name = "Craig")}

3 Answers

OK, Jennifer has explained = vs ==, but what about equals()?

equals() is used to compare objects. It's a method inherited by all classes from Object.

Consider this code:

String a = new String("Hi")
String b = new String("Hi")
a == b  //false
a.equals(b)  //true

Here we create two String variables, a and b, each of which points to a new String object, "Hi". The two object variables, a and b, refer/point to separate objects, so a == b is false. But the two String objects that a and b refer to are the same (have the same character sequences) and so a.equals(b) returns true.

Note that equals(), unless overridden, asks if the two objects' memory locations are the same. But since it is overridden in the String class, it returns true if the state of the two Strings are the same (and it ignores their memory locations). You've probably also heard that if you create a class of your own you should override equals() -- and that is correct. That's because you will also want to compare the state of your class's objects, not their memory locations.

Just to make things interesting, though, consider this code:

String x = "Hi"
String y = "Hi"
x == y
true
x.equals(y)
true

Here we have used the shortcut method to create two Strings. As before we have two String variables, x and y, both of which point to a String object, "Hi".

In this case, however, the memory locations in x and y (the locations they point to) must be the same, since x == y is true. Java has taken a shortcut itself. Since it already had a "Hi" String, it didn't create a second one. It just made y a second name for the same object.

There's a more detailed explanation of this gotcha, and several others, at: https://www.owasp.org/index.php/Java_gotchas

Jennifer Nordell
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STAFF
.a{fill-rule:evenodd;}techdegree
Jennifer Nordell
Treehouse Teacher

Hi there Faithe Yates! This is a common question when starting with a programming language.

A single equals sign is called an "assignment operator". And there's a reason for this. It's because we're assigning a value to that variable. You've probably heard by now that you can think of a variable as a box that holds a piece of data. The equals here puts a value in the box.

x = 10;
y = 15;
name = "Jane Doe";

These are all assignments. We're telling the variable what it has stored in there... not asking.

We use the double equals in a conditional expression. Here we're asking what their value is, because we don't really know.

if( x == 10) {

}

Here we're saying: Ok, if x is equal to 10 please run the code inside these brackets. Hope that clarifies things a bit! :smiley:

edited for additional note

:information_source: Some languages can directly compare strings to each other. However, Java implements this a little differently with the .equals method.

taylornajjar
taylornajjar
15,807 Points

In Java a single = sign is always used to assign a value to a variable. You wouldn't type if (age = 40), as you say, but rather if (age == 40) to make a comparison.

= is used to assign a value to a variable

== is used for object comparisons. It checks the memory addresses of your objects, meaning it checks to see whether it's the same object.

.equals is a method that allows you to compare whether the characters in your given string are the same as the characters in the string you're comparing against, even if they're not actually the same object.