3.6. Strings¶
Strings in Java are objects of the String
class that hold sequences of characters (a, b, c, $, etc). Remember that a class (or classification) in Java defines the data that all objects of the class have (the fields) and the behaviors, the things that objects know how to do (the methods).
You can declare a variable to be of type String
.
Note
Class names in Java, like String
, begin with a capital letter. All primitive types: int
, double
, and boolean
, begin with a lowercase letter. This is one easy way to tell the difference between primitive types and class types.
Run the following code. What does it print?
The code above declares an object variable named greeting
and sets the value of greeting to the Java keyword null
to show that it doesn’t refer to any object yet. So System.out.println(greeting);
will print null
.
Object variables refer to objects in memory. A reference is a way to find the actual object, like adding a contact to your phone lets you reach someone without knowing exactly where they are. The value of greeting is null since the string object has not been created yet.
![../_images/greeting.png](../_images/greeting.png)
Figure 1: Initial value for an object reference¶
In Java there are two ways to create an object of the String
class. You can use the new
keyword followed by a space and then the class constructor and then in parentheses you can include values used to initialize the fields of the object. This is the standard way to create a new object of a class in Java.
String greeting = new String("Hello");
In Java you can also use just a string literal, which is a set of characters enclosed in double quotes ("
), to create a String
object.
String greeting = "Hello";
In both cases an object of the String
class will be created in memory and the value of the variable greeting will be set to an object reference, a way to find that object.
Here is an active code sample that creates two greeting strings: one using a string literal and the other using new and the String constructor. Change the code to add 2 new strings called firstName and lastName, one using a string literal and the other using new, and print them out with the greetings.
Now that greeting refers to an actual object we can ask the object what class created it. Try the following. What does it print?
The code above will first print class java.lang.String
since greeting
was created by the String
class. The full name for the String
class is java.lang.String
. The java.lang
part is the package name. Every class in the Java language is in a package and the standard classes like String
are in the java.lang
package. Every object in Java knows the class that created it. Also, every class knows its parent class. Yes, a class can have a parent class, just as people have parents. But, in Java a class can only have one parent. A class can inherit
object fields and methods from a parent class, just like you might inherit musical ability from a parent. The last print statement will print class java.lang.Object
because the parent class (superclass) of the String class is the Object class. All classes in Java inherit from the Object class at some point in their ancestry.
![../_images/stringObject.png](../_images/stringObject.png)
Figure 2: Object variable of type String with a reference to a String object which has a reference to the String class which has a reference to the Object class.¶
3.6.1. String Operators - Concatenation¶
String
s can be added to each other to create a new string using the +
or +=
operator . This is also called appending or concatenating. You
can also add any other kind of value to a String
with +
or +=
and
the other value will be converted to a String
automatically. Objects are
converted by calling their toString
method which we’ll talk about in section
5.4.
Remember, however, that String
s are immutable, just like int
s and
double
s. So when we add two String
s (or a String
and another
value converted to a String
) we get a new String
without making any
change to the values being added together just like when we add 1 + 2 the
original 1 and 2 aren’t changed. When we use +=
we are making a new
String
by adding something to the current value of a variable and then
assigning that new value back into the variable, again just like with numbers.
Try the following code. Add another variable for a lastname that is “Hernandez”. Use += or + to add the lastname variable after name to the result. Use += or + to add 2 more exclamation points (!) to the end of the happy birthday greeting in result.
Note
Note that spaces are not added between strings automatically. If you want a space between two strings then add one using + “ “ +. If you forget to add spaces, you will get smushed output like “HiJose” instead of “Hi Jose”. And remember that variables are never put inside the quotes (“”) since this would print the variable name out letter by letter instead of its value.
- xyz
- s1 will equal "xy" plus another "xy" then z at the end.
- xyxyz
- s1 contains the original value, plus itself, plus "z"
- xy xy z
- No spaces are added during concatenation.
- xy z
- No spaces are added during concatenation, and an additional "xy" should be included at the beginning.
- z
- s1 was set to "xy" initially, so the final answer will be "xyxyz"
2-6-5: Given the following code segment, what is in the string referenced by s1?
String s1 = "xy";
String s2 = s1;
s1 = s1 + s2 + "z";
You can even add other items to a string using the +
operator. Primitive
values like int
and boolean
will be converted to a String
like what
you would type into a Java program and objects will be converted to String
using the
toString
method discussed in the previous section. All objects inherit a
toString
method that returns a String
representation of the object and
many classes override it to produce a useful human-readable value. (We’ll
talk about overriding in detail in Unit 9.)
What do you think the following will print? Guess before you hit run. If you want the addition to take place before the numbers are turned into a string what should you do? Try to modify the code so that it adds 4 + 3 before appending the value to the string. Hint: you used this to do addition before multiplication in arithmetic expressions.
Note
If you are appending a number to a string it will be converted to a string first before being appended.
Since the same operators are processed from left to right this will print 1243
. First 4 will be turned into a string and appended to 12 and then 3 will be turned into a string and appended to 124. If you want to do addition instead, try using parentheses!
What if you wanted to print out a double quote “ character? Since the double quote “ is a special character with meaning in Java, we put in a backslash in front of the quote to signal that we want just the character. This is called a backslash escape sequence. And if you wanted to print out a backslash, you would have to backslash it too in order to escape its special meaning. Another useful backslashed character is backslash \n which will put in a newline.
Here are the escape sequences that may be used in the AP course.
3.6.2.
Programming Challenge : Mad Libs¶
Have you ever played MAD LIBS? In this game, you first choose a bunch of words without looking at the story and then those words are filled into the story to make it sound very wacky! Fill in the variables below with Strings for each word, and then run to see the wacky story.
Then, working in pairs, come up with another silly story that uses at least 5 new String variables. When you’re done, try another team’s mad libs code. Your teacher may ask you to create this program in a Java IDE like replit.com so that you can use input to read in the words (see input examples using the Scanner class).
If you used replit.com for this challenge, copy the url of your repl here to turn in.
3.6.3. Summary¶
Strings in Java are objects of the
String
class that hold sequences of characters.String objects can be created by using string literals (String s = “hi”;) or by calling the String class constructor (String t = new String(“bye”);).
new is used to create a new object of a class.
null is used to indicate that an object reference doesn’t refer to any object yet.
String objects can be concatenated using the
+
or+=
operator, resulting in a new String object.Primitive values can be concatenated with a String object. This causes implicit conversion of the values to String objects.
Escape sequences start with a backslash
\
and have special meaning in Java. Escape sequences used in this course include\"
,\\
, and\n
to print out a quote, backslash, and a new line.