[PDF] Polymorphism in Java We can perform polymorphism in

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Object & Polymorphism

Check out Polymorphism from SVN

Inheritance, Associations, and

Dependencies

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Field association lines

are solid

Dependency lines are dashed

Use association lines only when an item

is stored as a field.

Two types of open arrowheads

`Generalization (superclass) `Specialization (subclass) -a Two types: solid line= inherits, dotted line = implements

The superest class in Java

Everyclass in Java inherits from Object

`Directly and explicitly: ŃSXNOLŃ ŃOMVV 6PULQJ H[PHQGV 2NÓHŃP S"` `Directly and implicitly:

Ńclass BankAccountS"`

`Indirectly:

Ńclass SavingsAccountextends BankAccountS"`

Q1 `String toString() `booleanequals(Object otherObject) `Class getClass() `Object clone()

Often overridden

Sometimes useful

Often dangerous!

Q2 `Return a concise, human-readable summary of the object state `9HU\ XVHIXO NHŃMXVH LP·V ŃMOOHG MXPRPMPLŃMOO\

ŃDuring string concatenation

ŃFor printing

ŃIn the debugger

`getClass().getName() OR getClass().getSimpleName() comes in

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Q3 `equals(Object foo) ²should return true when comparing two objects of same type

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`How?

ŃMust check types³use instanceofOR

ŃMust compare state³use cast

Q4

Recall casting a variable: Taking an

Object of one particular type and

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Review and Practice

`A subclass instance is asuperclassinstance

ŃPolymorphism still works!

BankAccountba= new SavingsAccount();

ba.deposit(100); `But not the other way around!

SavingsAccountsa= new BankAccount();

sa.addInterest(); `Why not? BOOM! `If Bextends or implements A, we can write

A x = new B();

Declared type tells which

methods x can access.

Compile-time error if try to

use method not in A.

The actual type tells which

method to use. `Can cast to recovermethods from B: ((B)x).foo()

Now we can access all of

it gives a run-time error (class cast exception) `Step 1: Identify the Declared/Casted Type ŃThis is the item to the left of the variable name when the variable was declared: xBankAccountsa= new SavingsAccount();

ŃDeclared Type may be changed due to a cast:

Ń((SavingsAccount)sa).addInterest();

ŃIf there is a casted type, record that, otherwise use the declared type.

Declared Type

Casted Type

`Step 2: Identify the Instantiation/Actual Type ŃThis is the type on the right hand side of the equal sign the last time the variable was assigned to: xBankAccountsa= new SavingsAccount();

ŃRecord the instantiation type

Instantiation Type

`Step 3: Check for Compilation Errors Calling a method that is not available based on the declared or casted type of the object

BankAccountsa= new SavingsAccount();

sa.addInterest(); Compiler Error: BankAccountdoes not have addInterest

Incompatible type assignment

SavingsAccountx = new BankAccount();

Compiler Error: BankAccountscan not be stored in

SavingAccounttyped variables

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BankAccountsa= new SavingsAccount();

((SafetyDepositBox)sa).depositItem();

SafetyDepositBoxis not below BankAccount.

Cannot instantiate interfaces or abstract classes! `Step 4: Check for Runtime Errors

Runtime errors are caused by invalid casting.

An item may only be cast to a type IF:

xThe instantiation type matches the casted type xThe casted type is between the declaration type and the instantiation type

BankAccountsa= new SavingsAccount();

((CheckingAccount)sa).deductFees(); Runtime Error: SavingsAccountis not a CheckingAccount

Account a = new CheckingAccount();

((BankAccount)a).deposit(); This is valid because a CheckingAccountis a BankAccount `Step 5: Find Method to Run

ŃFind the instantiation type in the hierarchy.

1.If that type implements the given method, then use

that implementation.

2.Otherwise, move up to the parent type and see if

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a.If there is an implementation, use that. b.Otherwise, repeat step 2 until an implementation is found. `Do questions 5 through 7 from Quiz. `Please hand them in when done and then start reading the BallWorlds specification on your schedule page. Q5-7, hand in when done, then start reading BallWorldsspec

Pulsar, Mover, etc.

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