Transcript Using Data Types

3.1 Objects
Data Types
Data type. Set of values and operations on those values.
Data Type
Set of Values
Operations
boolean
true, false
not, and, or, xor
int
-231 to 231 - 1
add, subtract, multiply
double
any of 264 possible reals
add, subtract, multiply
Primitive/Built-In types.
Usually single values
Can build arrays but they can still store only the same type of data
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You don’t have to know HOW the data types and their operations are
implemented -> Abstraction
You may want to define your own data types.
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Objects
Object. An entity that can take on a data type value.
Data Type
Set of Values
Operations
Color
24 bits
get red component, brighten
Picture
2D array of colors
get/set color of pixel (i, j)
String
sequence of characters
length, substring, compare
An object’s “value” can be returned to a client or be changed by one of
the data type’s operations.
Impact. Enables us to create our own data types; define operations on
them; and integrate into our programs.
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Objects and Arrays are Similar
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What distinguishes creating arrays from creating
variables of basic data types?
Explicit memory management
Arrays and objects are both “reference” types
They are allocated a chunk of memory in the address space
The memory needs to be initialized
Assigning one object/array to another object/array results in an alias
Key difference
Arrays are part of the core Java language/standard library. Objects
are not.
“Class” vs. “Object”
Similar to difference between “type” vs. “variable”
An object is an instance of a class
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A Simple First Class
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Height
Data:
– Feet (as an integer value)
– Inches (as a double value)
Methods:
– Set height
– Display height
API:
Height(int feet, double inches) //constructor
void displayHeight() //instance method
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Constructors
Objects are reference types => Need to initialize their memory
Can use the new operator, just like arrays
One special instance method in the API:
Height(int feet, double inches) //constructor
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This method is called when you invoke “new” to create a new object
You can pass parameters to control initialization.
The class’ author defines the logic for this method to carry out
proper initialization.
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Constructors vs. Methods
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Height(int feet, double inches)
How is this signature different from a
method signature?
No return type
The constructor manipulates the data in
the object
No “return” statement
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Instance Methods vs. Static Methods
Instance methods are called “on” an object (a variable)
System.out.println(); // call println on System.out
String s = aCharge.toString(); // call toString on aCharge
In fileIn = new In(“file.txt”); fileIn.readInt();
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Ways to think about this:
Send a message to an object.
Tell an object to do something.
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Constructors and Methods
To construct a new object: Use keyword new and name of data type.
To apply an operation: Use name of object, the dot operator, and
the name of the method.
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Height.java
public class Height {
// data
int feet;
double inches;
// constructor
Height(int setFeet, double setInches) {
feet = setFeet;
inches = setInches;
}
//method
void displayHeight() {
System.out.println("Height: "+feet+"
feet and "+inches+ " inches");
}
}
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Using Height.java
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Compile .java file into a .class file
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Create objects of that class in other
programs
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Dr. Java Coding Demo
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Extending the Height Class
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double getHeightInches()
Returns the height in inches
Dr. Java Coding Demo
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Adding More Constructors
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May want to initialize an object in
more than one way
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E.g., allow initialization when only the
feet value is provided
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Height(int setFeet)
Set “feet” to “setFeet”; set “inches” to
zero
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Use the “this” keyword to invoke the
original constructor
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Dr. Java Coding Demo
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String Data Type
String data type. Basis for text processing.
Set of values. Sequence of Unicode characters.
API.
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http://java.sun.com/j2se/1.5.0/docs/api/java/lang/String.html
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Typical String Processing Code
How do we implement a recursive version of this
function?
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String Data Type
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http://java.sun.com/j2se/1.5.0/docs/api/java/lang/String.html
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Recursive Palindrome Check
static boolean isPalindrome(String s) {
if(s.length() < 1) //base
return true;
if(s.charAt(0) == s.charAt(s.length()-1)) //reduction
return isPalindrome(s.substring(1,s.length()-1));
return false;
}
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