ITEC1620 Object-Based Programming. Lecture 13

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1 ITEC1620 Object-Based Programming Lecture 13 References

2 A Simple Class AnInt objecta = new AnInt(); AnInt objectb = new AnInt(); objecta objectb AnInt AnInt value value

3 Value Assignments objecta.value = 5; objectb.value = 7; objecta.value =objectb.value; objectb.value = 10; System.out.println(objectA.value); System.out.println(objectB.value);

4 Value Assignments II objecta =objectb; objectb.value = 12; System.out.println(objectA.value); System.out.println(objectB.value); How is objecta.value affected?

5 Value Assignments III Assignment copies a value from one memory location to another What are the memory locations, and what do they hold? Note the difference between copying Note the difference between copying values and copying references

6 Value Assignments IV AnInt objecta = new AnInt(); AnInt objectb = new AnInt(); objecta objectb AnInt AnInt value value

7 Value Assignments V objecta.value = 5; objectb.value = 7; objecta objectb AnInt AnInt value 5 value 7

8 Value Assignments VI objecta.value =objectb.value; objectb.value = 10; objecta objectb AnInt AnInt value 7 value 10

9 Value Assignments VII objecta =objectb; objectb.value = 12; objecta objectb AnInt AnInt value 7 value 12

10 Value Assignments VIII What does objecta = objectb do? Copies contents in objectb s memory location into objecta s memory location What is objectb.value? objectb follow the arrow What is objecta value? What is objecta.value? objecta follow the arrow

11 Value Assignments IX After objecta = objectb, both identifiers have references to the same object objecta and objectb are the same object Mathematical equating Any change to objecta is also a change to objectb Any change to objectb is also a change to objecta

12 Questions?

13 Creating and Copying AnInt objecta = new AnInt(); AnInt objectb = objecta; AnInt objectc = objectb; AnInt objectd = new AnInt(); AnInt objecte = null; AnInt objectf;

14 Creating and Copying II objecta objectc objecte objectb objectd objectf AnInt AnInt value value

15 Creating and Copying III How many objects created? How many identifiers declared? What is the difference between objecte What is the difference between objecte and objectf?

16 Null vs. Nothing objectf.value; Syntax error compiler sees nothing objecte.value; Runtime error OS tries to follow null Dot means follow the arrow no arrow if (objecte!= null && objecte.value > 0) sum += objecte.value;

17 Equals and Equality identifier1 == identifier2 What does the == relational operator do? Determines if the two identifiers have the Determines if the two identifiers have the same (or compatible) data types and the same contents in memory

18 Equals and Equality II Use previous objects objecta.value = 5; objectb.value = 7; objectd.value = 7; objecta == objectb objectb == objectd // true or false // true or false

19 Equals and Equality III objecta objectc objecte objectb objectd objectf AnInt AnInt value 7 value 7

20 Equals and Equality IV objecta and objectb refer to the same object Same contents in their memory locations == (equality) returns true same object objectb and objectd refer to different objects Different arrows in their memory locations == returns false different objects

21 Equals Equality asks if two objects are really one object Equals asks if two objects are clones Java doesn t know what is in our objects user defined datatype Must provide a method for each class public boolean equals () { } objectb.equals(objectd) true

22 Questions?

23 Review Whenever obja == objb is true, then obja.field == objb.field must also be true True or false? Whenever obja == objb is false, then obja.field == objb.field must also be false True or false?

24 Review II AnInt x = new AnInt(); AnInt y = new AnInt(); AnInt z = new AnInt(); x.value = 5; y.value = 7; z.value = x.value; x=y; y.value = 10;

25 Review III What are the final contents of x.value, y.value, and z.value?

26 Sample Final public class AnInt { public int data; } public AnInt (int data)

27 Sample Final II public class IndInt { public int data; public AnInt obj; } public IndInt (int data, AnInt obj)

28 Sample Final III public class MainClass { public static void main(string[] args) { AnInt x = new AnInt(3); AnInt y = new AnInt(5); AnInt z = new AnInt(7); IndInt a = new IndInt(2, x); // Part 1 draw the object diagrams at this time

29 // Part 1 draw the object diagrams at this time

30 Sample Final IV z.data = a.data; x = new AnInt(1); y = a.obj; a.obj.data = 8; } } // Part 2 draw the object diagrams at this time

31 // Part 2 draw the object diagrams at this time

32 Readings and Assignments Tutorial References

COE318 Lecture Notes Week 10 (Nov 7, 2011)

COE318 Software Systems Lecture Notes: Week 10 1 of 5 COE318 Lecture Notes Week 10 (Nov 7, 2011) Topics More about exceptions References Head First Java: Chapter 11 (Risky Behavior) The Java Tutorial: