Advanced Java Programming CS 537 Data Structures and

Advanced Java Programming CS 537 – Data Structures and Algorithms

The Stack • The stack is the place where all local variables are stored – a local variable is declared in some scope – Example int x; // creates the variable x on the stack • As soon as the scope ends, all local variables declared in that scope end – the variable name and its space are gone – this happens implicitly – the user has no control over it

The Heap • The heap is an area of memory that the user handles explicitly – user requests memory through new operator – java does garbage collection to reclaim unused memory • A user maintains a handle on memory allocated in the heap with a reference variable

Creating Objects • All objects are created on the heap • A reference to object is stored on the stack – simply declaring an object does not create it • automatically set to null – new operator allocates space on the heap

Creating Objects • Example Object obj 1 = new Object(); Object obj 2; func 1 obj 1 = 200 obj 2 = null Object 200 main x=3 Stack Heap

Assigning Object References • Reference can be assigned through new operator • obj 2 = new Object(); • Reference can assigned to another reference • obj 2 = obj 1; • WARNING – when assigning to another reference, both references now refer to the same object

Assigning Object References • Example • Object obj 1 = new Object(); • Object obj 2 = obj 1; func 1 obj 1 = 200 obj 2 = 200 Object 200 main x=3 Stack Heap

Simple Class class Foo implements Cloneable { private int num; public void Foo(int num) { this. num = num; } public void set. Num(int num) { this. num = num; } public int get. Num() { return num; } }

Copying an Object • Want to create and modify copy of object – remember, simple assignment not enough Foo f 1 = new Foo(5); Foo f 2 = f 1; // still only one object – 2 references f 2. set. Num(10); System. out. println(“f 1’s num = “ + f 1. get. Num()); // prints 10 System. out. println(“f 2’s num = “ + f 2. get. Num()); // prints 10 – need to use the clone() method

clone() Method • To use clone() must implement Cloneable • Object. clone() is automatically inherited by every class – by default, it creates a new object and copies all fields • Example Foo f 1 = new Foo(5); Foo f 2 = f 1. clone(); f 2. set. Num(10); System. out. println(“f 1’s num = “ + f 1. get. Num()); // prints 5 System. out. println(“f 2’s num = “ + f 2. get. Num()); // prints 10

Shallow Clone • Only copies the fields – does not copy what the fields reference • Doesn’t work well for sophisticated objects • Example: Class Foo { private int [] nums; public void Foo(int size) { nums = new int[size]; } … } Foo f 1 = new Foo(5); Foo f 2 = f 1. clone();

Shallow Clone Foo f 1 = new Foo(5); Foo f 2 = f 1. clone(); func 1 f 1 = 200 f 2 = 100 nums = 50 200 Array nums = 50 100 Stack Heap 50

Deep Clone • Copies fields and what they refer to • Must reimplement the clone() method class Foo { … public Object clone() { try { Foo fobj = (Foo)super. clone(); // copies fields fobj. nums = (int)nums. clone(); // arrays implement clone return fobj; } catch(Clone. Not. Supported. Exception e) { } } }

Inheritance • lets one class inherit fields and methods from another class • use keyword extends to explicitly inherit another classes public and protected fields/methods • can only explicitly extend from one class • all classes implicitly extend the Object class

Inheritance • overriding a method – must have the same signature as original – declaring a method final means future derived classes cannot override the method • overloading a method – method has same name but different signature – they are actually different methods

Inheritance • abstract classes and methods – declaring a class abstract • must have an abstract method • class cannot be directly used to create an object • class must be inherited to be used – declaring a method abstract • method must be defined in derived class

Abstract Class abstract class Pixel {. . . public abstract void refresh(); } class Color. Pixel extends Pixel {. . . public void refresh() { do some work } } • Note: signature of method in derived class must be identical to parent declaration of the method

Interface • basically an abstract class where all methods are abstract • cannot use an interface to create an object • class that uses an interface must implement all of the interfaces methods • use the implements keyword • a class can implement more than one interface

Interface • simple example class Tester implements Foo, Bar {. . . } • Foo and Bar are interfaces • Tester must define all methods declared in Foo and Bar

Array Review • • • Consecutive blocks of memory Creation: int [] grades = new int[25]; __. length: holds size of array __. clone(): makes copy of array data out-of-bounds exception: trying to access data outside of array bounds generates an exception • Array size is fixed at creation

Vector Class • Very similar to an array • Major difference: vectors can grow beyond original size – if a certain capacity is exceeded, a new, larger memory region is allocated for vector – copy all data to the new area • See Java documentation on-line for complete details