CMSC 132 Material

Encapsulation

the hiding of implementation details

Data Encapsulation

way data is stored not visible and don't know data structure (ArrayList, Array? idk)

Procedural Encapsulation

doesnt give instructions for procedure / method

Java Interface

contains method prototypes for classes to implement

can interface have instance variables?

no

can interface have static final variables?

yes

can interface have static method?

yes can have implementation (InterfaceName.method());

can interface have instance methods?

yes, need DEFAULT **public default void land(){...}

why do we use interfaces?

polymorphism,,, can have interface as parameter for method, polymorphic collections

polymorphic collections (interface)

an interface name can be for an array ... CanEat[] eaters \= {new Dog(), new Owl(), ...};

wrapper class

a class designed to contain a primitive data type so that the primitive type can behave like a reference type (EX ARRAY LISTS)

initiating wrapper objects (integer with number 7)

Integer x \= Integer.valueOf(7); or Integer x \= 7

autoboxing

the automatic conversion between a primitive value and a corresponding wrapper object

ex. Integer x \= 7;

**7 is primitive but boxed / wrapped in object

Autounboxing

the automatic conversion between wrapper object to corresponding primitive value

ex.

Integer y \= 3;
Integer z \= y+ 2;

**unbox obj y, add to 2, box again to make object z

collections class

contains static methods that work on most collections

collections method to shuffle list

Collections.shuffle(list);

collections method to find max in list

Collections.max(list)
must be comparable

collections method to find min in list

Collections.min(list)
must be comparable

collections method to sort list

Collections.sort(list)
must be comparable

collections method to complete a binary search (index where thing is in list)

Collections,binarySearch(list, num to be found)

list must already be sorted

collections method to reverse a list

Collections.reverse(list);

Java enumerations

-a lot like a class
-has small number of pre-defined instances with symbolic constants

how do we compare enum variables?

\==

how to access enum variable

EnumerationName.VARIABLE

how to create array from enums

EnumerationName.values()

how to get index of enum

EnumerationName.Variable.ordinal();

how are enumerated types like a class?

can have instance members(variables & methods)

can have static members(variables and methods)

how are enumerated types different from a class?

constructors PRIVATE

how to write enumerated types with enums and variables
ex. SUN
name variable "Sunday"
int workHours \= 0

SUN("Sunday",0);
private String name;
private int workHours;

\---contructor

writing a copy contructor for class Party
with variables name, max guests

public Party(Party party){
this.name \= party.name;
this.maxGuests \= party.maxGuests;
}

**REMEMBER MAY NEED TO CREATE COPY FOR PRIVACY LEAKS

Iterator

an object containing data and methods to iterate through a collection of data, allowing processing of one data item at a time.

Iterable

interface that has ability to provide you with an interator, implemented by collection

is a for each loop an interator?

YES

why do we need iterator / iterable types?

only way to cycle through ANY Java Collection

create an iterator to go through a Integer ArrayList

Interator

what methods do you have to use to iterate through a list and remove elements

while(iterator.hasNext)

iterator.next()
iterator.remove();

Immutable Class/Object

state (instance variables) set during contruction but cannot be modified later

Mutable Class / Object

state is set during contruction

privacy leak

When someone OUTSIDE a class has access to a private reference INSIDE the class

defensive copy

Technique when getter method returns a copy of the mutable object.

public Mutable(StringBuilder b) {
builder \= new StringBuilder(b);
}
public StringBuilder getBuilder() {
return new StringBuilder(builder);
}

writing comparable method

implement Comparable

implement vs extend

ONLY EXTEND 1 CLASS

CAN IMPLEMENT MULTIPLE INTERFACES

Student is a subclass of Person. Which is correct?

this vs super

this --\>reference to current object

super --\> reference to superclass

**super does same thing as this except the class right above on inheritance tree

inheritance memory diagram

\

subclass constructor

call super constructor

equals method

check if this \== obj
check if ! instanceOf
cast as class
compare and return

object class

-object class extended if no other specified
-always root of inheritance tree
-object methods: toString() equals() getClass()

class vs type

-object object has exactly one instance of a class
-class of an object cannot change
-objects may satisfy many types

Person p \= new GradStudent();

What is the type of the variable?

person

Person p \= new GradStudent();

What is the class of the object?

GradStudent

Person p \= new GradStudent();

What are the type(s) of the obect? (IS-A)

Gradstudent, Student, person

overloading

2 methods same name dif parameters (return type doesn't matter)

overriding

same name, same parameter types
-in subclass, "replaces" inherited method
-return type may be more specific (ex original returned is Person, now is Student)
-visibility may be increased(public --\> adding extra feature)

Early (static) binding

makes decision at compile time, only sees type of variable

Late (dynamic) binding

makes decision at runtime and looks at the class of the object
what java uses

dynamic dispatch

what to invoke is decided during run time based on type.

casting...when will it compile?

if cast is above or below on inheritance tree

casting...when will it run?

when object satisfies type (IS-A)

Upcasting

referencing an object as an instance of a more general parent class, do not have to specify, done implicitly.

Downcasting

casting a superclass to a subclass...never done implicitly!

Class Cast Exception

An exception thrown when an object variable is cast with an incompatible class.

safe downcasting

use instance of to check if variable instance of that class

multiple inheritance

A derived class inheriting class members from two or more base classes

**not supported by java

Inheritance vs Composition

Inheritance defines what objects are and hierarchial nature of that strucutre IS A Composition defines how objects interact HAS A

\*\*favor composition if you dont need polymorphism, composition has much more flexible design

final

1) final variable

2) final in front of object variable declaration (variable cant change but object can change)

3. final class (dont want any class to extend)

4. final method (cannot override / modify)

public visibility

can be referenced anywhere

protected visibility

Any class in the same package or any class derived can reference it (public would violate encapsulation). Provides the best possible encapsulation that permits inheritance.

package visibility

only classes in same classes can access

private visibility

can be used anywhere inside the class definition but cannot be referenced externally

Shadowing

override variable thats inherited...dont do it

abstract class

a class that defines attributes and methods for subclasses but is never instantiated

can be extended , instance variables, abstract methods

unchecked exception

- errors and run time exceptions
- result of programming errors
AssertionError, VirtualMachineError, ArithmeticException, NullPointerException, IndexOutOfBoundsException

checked exception

-must "catch or declare"
1. handle with catch block
2. declare that method throws it

Comparator Interface

Allows for multiple and different comparisons for ordering of sorting. has one method compare(a,b)

use by saying

Collections.sort(list, new ClassThatImplementsComparator());

implementing a comparator

Collections.sort(list, new ClassThatImplementsComparator());

static nested class

-like having 2 classes
-both can access private features of the other
-usually nested private and helps the outer class

inner class

\-privacy not respected -each inner class must be "associated with" a particular outer class object -NOT STATIC -B object knows everything abt A object, A doesnt usually know B exists (cannot edit B) -Parasite(B), Host(A) -B hooked onto "this" current object

instantiating inner class object

\** A is outer class and B is inner class

A outerObject \= new A();
A.B innerObject \= outerObject.new B();

inner class for iterator

outer class implements Iterable

Anonymous Inner Class

class with no name declared and instantiated in same spot inside a class must implement or extend -no constructors

Anonymous inner class syntax

new Foo(){...};

\** foo interface or class extending

lambda expression

-used to pass code to method
-javas attempt at being "Functional Language"
-used in a method with a parameter thats a functional interface type
-lambda expression implements sole method from a the functional interface

functional interface

any interface that contains only one abstract method

USE FOR LAMBDA EXPRESSION
ex. comparator, comparable, runnable, action listener

lambda exppression syntax

(a, B) -\>{ ...};
-NO TYPES ON VARIABLES
-NAME OF METHOD INFERRED FROM FUNCTIONAL INTERFACE

if you have an argument for a method that is a functional interface

use lambda expression!!!!!!!!!!!!!!!!!!!!!!!

why lambda expression better than anonymous class

-slightly less memory
-runs slightly faster
-sometimes fewer objects at runtime
-reduces need for garbage collector

shallow copy

Copying only the reference to an object.

deep copy

a copy that completely duplicates the contents of the source object to a destination object

clone method

inherited from object class

provides very fast shallow copy of entire object
has a checked exception --\> CloneNotSupported

clone method for an immutable address object return

return (Address)super.clone();

**CATCH CHECKED EXCEPTION
CloneNotSupportedException

clone() method with mutable component

save copy of object, make clone of individual mutable component

how to make a class cloneable

1. only possible if suprtclass is Object or implements cloneable
2. include "implements cloneable"
3.carefully override the clone() method

static initialization block

Code is automatically executed when the class is loaded (once at the very beginning of program execution) to initialize all static variables

Instance initialization block

executed everytime object initiated before constructor

gui

Graphical user interface;
interface between user and software that includes both input and output

main thread

executes main method and then dies

sends request to EDT to render and initiate GUI, SHE DEAD

Event Dispatch Thread (EDT)

handles user interaction,

responds to events,

certain events cause EDT to run code in listeners

Event-driven programming

some program statements run when triggered by an event, like a mouse click or a key press

container

rectangular region contains widgets