1. Introduction to Java Objects CSIS 3701: Advanced Object Oriented Programming

2. Object-Oriented Programming “Client programmer”: Programmer who uses class in their own code Methods to access member variables Constructors to set initial value of member variables Member variables representing current state of object Object

3. Basic Object Syntax Declaring objects: classname objectname; Constructing objects: objectname = new classname(params); Calling object methods: returntype = objectname.methodname(params);

4. Basic Object Syntax Example Stack s; s = new Stack(); s.push(“Larry”); s.push(“Curley”); if (!s.empty()) { System.out.println(s.pop()); } object in Stack class constructed (no parameters) method calls on this stack object

5. Objects and Memory Objects represented as a reference to memory –Must be dynamically allocated Stack s; s 37A4 Memory for s ’s attributes (stack contents, etc.) 37A4

6. Arrays in Java Arrays are treated as reference types Must be allocated with new int[] A; A = new int[5]; Arrays have object-like attributes –Example: length int total = 0; for (int i = 0; i < A.length; i++) total += A[i]; A refers to null A refers to newly allocated array of 5 ints (160 bits) Evaluates to 5

7. Memory Allocation Initially s points to null –Will get NullPointerException if attempt to use Must allocate memory with new s = new Stack(); –Allocates memory for object (size of state variables) –Invokes constructor (initializes state variables) –Returns memory location for storage in reference s

8. Memory Management Avoid using assignment with objects: Stack s1 = new Stack(); Stack s2 = new Stack(); s2 = s1; Many classes have clone method that create copy of any state variables –s2 = s1.clone(); Now refer to same location in memory s1 37A4 4E15 s2 37A4

9. References and Comparison Example: Stack s1 = new Stack(); Stack s2 = new Stack(); if (s1 == s2) {…} Better to use built-in equals method: if (s1.equals(s2)) {…} –Default version works like == –Often designed to do “expected” comparison for class –String version does character-by-character comparison Evaluates to false since s1 and s2 are different objects s1 s2

10. Memory Deallocation Manual deallocation ( delete in C++): –Potentially dangerous delete t2; t1.setText(“Barney”); “Garbage collection” in Java –JVM periodically scans sandbox for unreachable memory and reallocates back to OS –Safer, but takes time  tradeoff t1 37A44E15 t2 Memory at location 4E15 inaccessible – should be reallocated for future use