CS112 2012S 02 Creating Classes 02 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 method 02 1 Java Programs Simpliest Java Program Hello World 3 ways 02 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 Employee 02 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 ends 02 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 stack 02 6 Implementation I locals Example MethodTest Code Memory contents on whiteboard 1 CS112 2012S 02 Creating Classes 02 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 MethodTest2 02 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 new 02 9 Objects HeapData HeapDriver1 02 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 method 02 11 Objects HeapData HeapDriver2 MethodTest3 2
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