Advanced Java Programming

1. Advanced Java Programming CS 537 – Data Structures and Algorithms

2. 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

3. 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

4. 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

5. Creating Objects • Example Object obj1 = new Object(); Object obj2; Object func1 obj1 = 200 obj2 = null 200 main x = 3 Heap Stack

6. Assigning Object References • Reference can be assigned through new operator • obj2 = new Object(); • Reference can assigned to another reference • obj2 = obj1; • WARNING • when assigning to another reference, both references now refer to the same object

7. Assigning Object References • Example • Object obj1 = new Object(); • Object obj2 = obj1; Object func1 obj1 = 200 obj2 = 200 200 main x = 3 Heap Stack

8. Simple Class class Foo implements Cloneable { private int num; public void Foo(int num) { this.num = num; } public void setNum(int num) { this.num = num; } public int getNum() { return num; } }

9. Copying an Object • Want to create and modify copy of object • remember, simple assignment not enough Foo f1 = new Foo(5); Foo f2 = f1; // still only one object – 2 references f2.setNum(10); System.out.println(“f1’s num = “ + f1.getNum()); // prints 10 System.out.println(“f2’s num = “ + f2.getNum()); // prints 10 • need to use the clone() method

10. 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 f1 = new Foo(5); Foo f2 = f1.clone(); f2.setNum(10); System.out.println(“f1’s num = “ + f1.getNum()); // prints 5 System.out.println(“f2’s num = “ + f2.getNum()); // prints 10

11. 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 f1 = new Foo(5); Foo f2 = f1.clone();

12. Shallow Clone Foo f1 = new Foo(5); Foo f2 = f1.clone(); nums = 50 func1 f1 = 200 f2 = 100 200 Array 50 nums = 50 100 Heap Stack

13. 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(CloneNotSupportedException e) { } } }

14. 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

15. 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

16. 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

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

18. 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

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

20. 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

21. 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