Transcript Data Structures

Classes/Objects
Creating Objects
• Much like creating variables of primitive types
– String name;
– type name;
• Object variables hold references, not values
– can be initialized to null
• Use new to instantiate new objects/initialize
object variables
– String name = new String(“Mickey”);
– invokes Constructor of String class
Example
int num = 5;
String name = new String(“Mickey”);
String name2 = “Minnie”; //String shortcut
num
5
name
Mickey
name2
Minnie
Dot Operator
• Use dot operator to access methods of an
object
– name.length()
Aliases
• Two object variables can point to the same
object
• With primitive types, a copy is made
int a = 5;
a
12
b
12
int b = 12;
a = b;
Aliases
String name = “Mickey”;
String name2 = “Mickey”;
name
Mickey
name2
Mickey
Aliases
name = name2;
name
name2
Mickey
Strings
• Strings are immutable
• String methods
– char charAt(int index)
– String replace(char oldChar, char newChar)
Strings
String s = “Hello”
String t = s.replace('l', 'p');
s
Hello
t
Heppo
Packages
• Classes in the standard Java library are
grouped into packages
– java.util
– java.math
– java .text
• Using the API
import
• To use a class from the library you must
either use the fully-qualified name or import
the class
java.util.Scanner s = new java.util.Scanner(System.in)
import java.util.Scanner;
import java.util.*; //import all classes in java.util.
Exercises
• Complete PP 3.1
– Write an application that prompts for and reads
the user's first and last name (separately).
Then, print a string composed of the first letter
of the user's first name, followed by (no more
than) the first five characters of the user's last
name, followed by a random number in the
range 10 to 99.
• Hint: You will need to use java.util.Random
Java Classes
• Contain data and methods
• Methods enable user to access and modify
data
Encapsulation
• Data should be hidden from the user
– Access to/modification of data is controlled by
methods
• Modifiers
– Enable programmer to specify which data and
methods are private and which are public
• private are accessible within the class
• public are accessible from anywhere
Name.java
public class Name {
private String first;
private String last;
//Constructor
public Name(String thefirst, String thelast) {
first = thefirst;
last = thelast;
}
public void printName() {
System.out.println("First: " + first + " Last: " + last);
}
}
Driver Classes
• The driver typically contains the main
method
– this is the starting point for the program
• The driver creates instances of other
classes
NameTest.java
public class NameTest {
public static void main(String[] args) {
Name n = new Name("Sami", "Rollins");
n.printName();
}
}
Methods
public void printName() {
...
}
• modifier return_type name(type name, ...)
• You must specify the return type and the
type of each parameter
More Method Headers
• For each data member, consider whether you
should create setter and getter methods
public String getFirstName()
public void setFirstName(String newname)
public String getLastName()
public void setLastName(String newname)
Constructors
• Called when a new object is created
• Like a method with no return type
• Should initialize all relevant data
• Should take as input initial values for any variables that do
not have a logical default value
• A default constructor takes no parameters
public Name(String thefirst, String thelast) {
first = thefirst;
last = thelast;
}
Scope
public Name(String first, String last) {
this.first = first;
this.last = last;
}
• this provide access to class variable
• first/last are local to the constructor
Exercises
• Complete exercise PP4.4
– Design and implement a class called Book that
contains instance data for the title, author, publisher,
and copyright date. Define the Book constructor to
accept and initialize this data. Include setter and getter
methods for all instance data. Include a toString
method that returns a nicely formatted , multi-line
description of the book. Create a driver class called
BookShelf, whose main method instantiates and
updates several Book objects.