CS112-2012S-02 Creating Classes 102-0: Java Programs• Java programs are a collection of classes• Each class is a Template, not an Object• Can’t use a class until we create an instance (call “new”)• One exception: Static Methods• In this course, we will only use one static method: main method02-1: Java Programs• Simpliest Java Program:• Hello World 3 ways02-2: Classes• Classes contain data and methods (like functions in python)• Methods can access data members (called instance variables) of a class• Special method, called “Constructor”, which is called when an object of the class is created using new.02-3: Classes• Second Example: Employee02-4: Implementation I: Locals• Variables declared inside functions are called local variables• Stored on the Call Stack• Created when function is called• Dissappear when function ends02-5: Implementation I: locals• When a method is called:• Allocate space on the call stack to store method parameters & variables local to the method• Activation record for the method• Copy values into the space allocated for the parameters• Execute the body of the method• Pop activation record off the stack02-6: Implementation I: locals• Example: MethodTest• Code• Memory contents (on whiteboard)CS112-2012S-02 Creating Classes 202-7: Pass-By-Value• Methods in Java are “Pass-By-Value”• Value is copied into the parameter• Changes to the parameter don’t change the value passed int• Example: MethodTest202-8: Objects• Stack is not the only place where data is stored• Objects (specifically, instance variables) are stored on the Heap• Different section of memory than the stack• Memory is allocated (or set aside) on the heap through a call to new02-9: Objects• HeapData, HeapDriver102-10: Objects• Every object in your program is a pointer to memory on the heap• When we pass an object as a parameter, copy the pointer• Changes to the pointer itsef are not reflected back to the calling method• Changes made to what the pointer points to are reflected back to the calling method02-11: Objects• HeapData, HeapDriver2,
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