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BIT 115: Introduction
To Programming
Professor: Dr. Baba Kofi Weusijana
(say Doc-tor Way-oo-see-jah-nah,
Doc-tor, or Bah-bah)
[email protected]
http://edutek.net/cascadia/bit115
BIT 115: Introduction To Programming
Quiz
• https://catalysttools.was
hington.edu/webq/survey
/babaw/56674
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Today
• Ch 7.2, 7.3.1-7.3.2, 7.3.3
– Numeric Types
– Non-Numeric Types
– Strings & toString()
BIT 115: Introduction To Programming
Next Lecture
• Wednesday (6/4):
– Ch 8.2 (Reference Variables)
– A4 and J5 will be due BEFORE CLASS
BIT 115: Introduction To Programming
String Objects
• This class seeks to model stuff that you can
write down.
• Text: Words, usually, but also numbers,
punctuation, random symbols
• This is done with objects of a class called
java.lang.String
• String is basically short for: a string of
characters
– example: the letter ‘a’ is a character
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Java API
• Included with the Java language is a library
of classes that seek to model common
programming things (like strings)
• The public classes and methods you can use
are called the Java API (Application
Programming Interface)
– It's got so much stuff, that it tries to group
related stuff together.
• So everything that belongs in the language goes
under java.lang
– E.g., java.lang.String
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Packages in Java API
• java.lang is called a package
– The java.lang package is automatically
available to your programs
– Everything else has to be imported
• To tell the Java compiler what you're going to use
• You include a package using the import
keyword/command
– import java.applet.AudioClip;
// To use just the "AudioClip"
class in the java.applet
package
– import java.applet.*; // To use
anythingBIT 115:
inIntroduction
the Tojava.applet
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JavaDocs of Java API
• Immediate implication:
– You'll need to look for documentation under
java.lang.String
– Use the link to the online HTML JavaDocs
called “API/JavaDoc of Java 5 (JDK 1.5)” on
the course website
– You should be able to get to the doc .zip file
version on Sun's java.sun.com site
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String seeks to model an
immutable (unchanging) string
• java.lang.String class contains a number of
methods (behaviors) that are common to all
strings.
• Yet each individual String has attributes
which are unique to it – most importantly,
the characters in it's string.
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2 ways to create an instance of
the String class
• String s = new String("Baba");
• I like this way, because it's clear that you're actually
creating a new instance (instantiating a new object)
of the String class. This does the same thing:
• String s = "Baba";
• Whenever you write a literal like "some text" in
your code, at compilation time, the Java compiler
changes this to be a reference to a String object that
it creates for you
• So the above twoBITlines
are Tosemantically
identical
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4 Important Points
• You can't leave out the semi-colon at the end!!
; is required
• You have to first list the type (String), then the name of
the variable (like s).
– Java is strongly typed, meaning that it (mostly) won't convert
from, say, a String to an integer. JavaScript is loosely typed,
meaning that it will attempt to convert from, say, a String to an
integer.
• Optionally, you can immediately assign a value to the new
variable (initialize it)
• s is a reference variable – it doesn't actually contain the
entire string inside it, but instead it is a reference to the
object that actually does, that’s why it’s immutable (can’t
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change)
Calling methods on a String
• Let's say that you want to see your name, in
all caps.
– The point of providing an API to a library is
that it'll do work for you; you don't have to
figure out all the details
– It just so happens that the String class can do
this for you, using the toUpperCase method
– So, we'd like to send a message to the String
object referred to by the reference variable s,
telling the object to produce an upper-case
version of itself:
•s.toUpperCase();
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• However, Strings are immutable, meaning
that they can't change.
– Thus, the object that s refers to will be exactly
the same after the method call is finished.
– Instead of changing the object that s refers to,
toUpperCase actually produces a brand-new
String object, who's value is a copy of s's,
except that the new String is in all uppercase.
– If we don't do anything with it, then we'll lose
it. In order to keep track of it, we'll need a
reference:
•String s2;
•s2 = s.toUpperCase();
•System.out.println(s2);
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Vocabulary review
• We're using a single class (String), but
we've got 2 instances of the String class
(2 objects). We refer to these objects using
references – the variables s, and s2. We
created s2 by sending a message to s
(a.k.a. "calling a method on s"). Even
though both objects are of the same class,
each object has different attributes – in this
case, the characters(letters) that it contains
– yes, upper & lower case letters are different
• You can think of the String class as
being a cookie-cutter
for individual objects.
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Composing a Message
–String s2;
–s2 = s.toUpperCase();
–System.out.println(s2);
• Notice that we never actually use s2 again, so we
can avoid using an s2 with:
–System.out.println(s.toUpperCase());
• This is sometimes called composing a message the result of the first message (toUpperCase) is
used as an argument to the second message.
• Note that there is an implicit order here – you can't
print the string until you've first changed it to
uppercase, so the toUpperCase happens first.
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Working with user input Strings
• This will get a String from the user
• TextInput in = new TextInput();
String input = "";
input = in.readLine();
• Then you can compare it to other Strings
• if(input.equals(s))
• {// do something}
• if(input.equalsIgnoreCase(“jim”))
• {// do something}
• DON’T use == or !=
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ICE 17
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