Agenda CS121/IS223. Reminder. Object Declaration, Creation, Assignment. What is Going On? Variables in Java

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1 CS121/IS223 Object Reference Variables Dr Olly Gotel Having problems? -- Come see me or call me in my office hours -- Use the CSIS programming tutors Agenda Make sure you have completed lab 1 and the shopping questions in lab 2 Objects & object reference variables - what are these object things anyway? Understanding parameter passing passing objects is not the same as passing primitives Finish Lab 2 after this week CS121/IS223 Week 12, Slide 1 CS121/IS223 Week 12, Slide 2 Reminder Use visibility to enforce encapsulation Object Declaration, Creation, Assignment class Car{ Primary class private String colour, licence; private double speed, fuel; public void honk(){ System.out.println( Beep Beep! ); Driver class with main() method class CarTester{ public static void main(string[] args){ Car mycar = new Car(); mycar.honk(); Instance variables private data members - means can only be changed inside the class (by its methods) Instance method public means that it can be called from outside the class on objects of Car class Make a new car object & call it mycar Get mycar to honk its horn CS121/IS223 Week 12, Slide 3 (Called an object reference variable) 1 - Declare a reference variable, of class type Car, called mycar (3) (2) Car mycar = new Car(); (1) 2 - Create a new Car object 3 - Link the new object to the named reference variable What is going on here? 3 distinct things Need to step back a moment CS121/IS223 Week 12, Slide 4 What is Going On? Variables in Java Java is STRONGLY typed 1. Tells the JVM to allocate space for a reference variable on the stack; reference variable is called mycar; it is of type Car Stack smallish area of memory; holds an address in memory (where to find an object); we know how much memory we need to hold an address 2. Tells the JVM to allocate space for a new Car object on the heap Heap - big area of memory; holds the object because we do not know how big it will be 3. Assigns the new Car object to the reference variable mycar; think of the reference variable as a locator for the object Variables are particular shaped containers (hold data) Variables can hold a value (so long as it is of the right shape) The shapes are called types Value 1 Type Value 2 CS121/IS223 Week 12, Slide 5 CS121/IS223 Week 12, Slide 6 1

2 Variables & Values in Java Variables implemented as memory locations Values implemented as 0 s & 1 s Values placed in memory locations The Java compiler needs to know the type of every variable - to know the values it can take and permissible operations In Java, every variable must be declared before it can be used - so it can allocate space in memory CS121/IS223 Week 12, Slide 7 Types in Java Primitive types: directly represented by typical processors the most efficient begin with lowercase letter Class types: a type for objects begin with UPPERCASE letter 8 primitive types in Java, others are represented using objects sometimes called object reference types CS121/IS223 Week 12, Slide 8 A Recap on Types A type defines: the set of values belonging to the type the set of operations that can be applied to the values (e.g. the + operator can be applied to an integer, but not to a boolean) Your Java program won t compile if you try to apply invalid operators to values Primitive Types in Java byte 8-bit integer short 16-bit integer int 32-bit integer long 64-bit integer float 32-bit floating point double 64-bit floating point Integer data types no fractions Floating point data types fractions char 16-bit unsigned Unicode character code boolean 1 bit true or false Values are actual integers, floats, characters, etc CS121/IS223 Week 12, Slide 9 CS121/IS223 Week 12, Slide 10 A Way to Think of Primitives We can predict these! Think Predict/Primitive Perhaps Class Types Look Like This? We can t predict these! Think Chaotic/Class CS121/IS223 Week 12, Slide 11 CS121/IS223 Week 12, Slide 12 2

3 Class Types in Java This is a string - strings are literal values Variables in Java Non primitive types exist & are important Class types are abstractions created from primitive types (like a bank account, an address, etc) A class type that is widely used in java is String to represent a sequence of characters (char) There is a class String that you use to implement strings, so strings are actually objects, which requires a bit of extra code to use They are user-defined types CS121/IS223 Week 12, Slide 13 A container of sorts Must have a type & a name (identifier) Can hold primitives or class types When they hold primitive types they hold the values directly in the container When they hold class types, they hold references to objects in the container (these are called object reference variables), the objects are elsewhere CS121/IS223 Week 12, Slide 14 Object Reference Variables in Java Pointers In Java, we are VERY concerned with references An object reference variable holds bits that represent a way to access an object, it does not hold the object itself (it actually holds something that works a bit like a pointer to the address at which the object can be found) Your object reference variable actually points to the object! Objects do not live in variables on the stack, they live on the heap Object reference variables provide a way to get hold of the object on the heap When you create a new object, you put a locating device in the variable container CS121/IS223 Week 12, Slide 15 CS121/IS223 Week 12, Slide 16 Example from [Sierra & Bates 2003] - best ever! Object Declaration, Creation & Assignment Buy Why Indirection? Index cards Dog = new Dog(); We don t know how big will be yet - so can t pre-allocate memory! 1. Declare a reference variable: Dog = new Dog(); 3. Create an object: Dog = new Dog(); 5. Link the object & the reference: Dog = new Dog(); CS121/IS223 Week 12, Slide 17 CS121/IS223 Week 12, Slide 18 3

4 Type Safety Assigning Ref Variables to Ref Variables Can t do: Dog = new Cat(); can only point to a! Ouch! Dog = new Dog(); Dog = ;.equals() 1 object can be referenced by lots of variables But we could do: Animal = new Dog(); If Dog is defined as a type of Animal. Later! CS121/IS223 Week 12, Slide 19 Stack - area of memory for Heap area of storing memory where variables you can request (small boxes of big variable-sized known size) boxes CS121/IS223 Week 12, Slide 20 An alias Remember Getting Rid of a Value in a Ref Variable The object reference variable (ORV) and the object are = null; A reference variable that doesn t point to an object is called a null reference 2 distinct things! x Heap CS121/IS223 Week 12, Slide 21 Check for a null reference before you follow it! CS121/IS223 Week 12, Slide 22 Getting Rid of a Value in a Ref Variable What if you try to Follow a Null Reference? = null; When an object is not referenced it is a candidate for garbage collection we get to reuse the memory x x Heap NullPointerException Farewell poor dog we knew him well CS121/IS223 Week 12, Slide 23 CS121/IS223 Week 12, Slide 24 4

5 References to Different Objects BUT of Same Class Recap So Far Dog = new Dog( snowy ); Dog = new Dog( spot ); class Dog{ private String name= ; name= snowy name= spot Heap public Dog(String dogname){ name=dogname; CS121/IS223 Week 12, Slide 25 When you declare a variable of class type, you get a container that holds an object reference Object reference variables hold bits that represent a way to access an object doesn t hold the object itself JVM uses the reference to access the object Equality (==) compares values of reference variables and not the objects they reference - need to use equals() - and so now you know that is why we use this with String types! When objects are passed as method arguments, the reference to the object is copied into the method s formal parameter & this last point is where the fun begins! CS121/IS223 Week 12, Slide 26 Placeholder polymorphic variables Important! Parameter Passing Trace the code in listings: & fig 5.3 (for text ed. 3) & fig 6.5 (for text ed. 4+) Passing an object reference as a parameter to a method void mymethod(int number){ Pass/call by value (default in Java): Local copy stored in method s parameter copy Changes are only made to the local copy Old value remains in calling method when returns CS121/IS223 Week 12, Slide 27 void mymethod(dog ){ Pass/call by reference (objects): Passes a reference to the object (i.e. its address) Local copy is therefore an alias for the object ref Change the object, change remains once returned from the method, as actual parameter references the same object Change the object reference & make it point to a new object, the actual parameter still refers to the original object Do the remaining questions in Lab 2 CS121/IS223 Week 12, Slide 28 Special Things! Static Modifiers Try to initialise a variable as close to the point of declaration as possible The null reference: a reference that doesn t point to a valid object (watch for NullPointerException) The this reference: a reference to the current executing object class Example{ private int i; The final reserved word used to indicate that a variable holds a constant value (i.e. one that cannot be changed), often used when the variable is static private static final int MAX = 100; public int foo(int i){ this.i=i; final can also be used with classes & methods discussed when we get to inheritance CS121/IS223 Week 12, Slide 29 Variables: instance variables (each object of a class gets its own copy) local variables (used temporarily inside a method) static variables (single copy shared by all instances of a class; they are class variables) private int instancevariable; private static int classvariable; Methods: instance methods must be called for an object of the class static methods are class methods & do not need to be called for an object if private, any method in the class can call them; if public, any other method in any class can call them Examine the code in listings 5.4 & 5.5 (3 rd ed.) or 6.1 & 6.2 (4 th + ed.) CS121/IS223 Week 12, Slide 30 5

6 Wrappers NOTE for Java autoboxing & auto-unboxing Outer & Inner Classes Wrapper classes a way to represent primitive types so they too can be treated like objects primitive int value passed to the wrapper constructor int i = 42; Integer myint = new Integer(i); class type for treating ints as objects so object myint has 42 as a private instance variable int unwrap = myint.intvalue() Wrapper classes have methods to convert objects back to primitives Wrapper classes for each primitive type Helps for polymorphism - another cool thing for later! CS121/IS223 Week 12, Slide 31 int Integer Outer & inner classes safe nesting We will return to this when we look at GUIs, event -driven programming & listeners in the next course Examine the code in listings 5.7 & 5.8 (3 rd ed.) or 4.10 & 4.11 (4 th + ed.) CS121/IS223 Week 12, Slide 32 Key Points Java has primitive types & class types: variable of primitive type bits represent the value of the variable variable of class type (called object reference variable) bits represent how to access the object Be aware of what you are actually doing when you pass objects as parameters to methods - it is not the same as passing primitives To Do Complete reading Chapter 5 of (ed 3) or Chapter 6 of (ed 4+) & use the book website for downloads of code to experiment with. Prepare for next class by reading Chapter 6.0 & 6.1 (ed 3) or Chapter (ed 4+) Complete programming project 5.2 (ed 3) or 6.4 (ed 4+) Try PP (ed 3) or (ed 4+) if you have lots of spare time Complete LAB2 get as far as you can seek help if you need it Go through the lecture slides again & ask me if you have any questions don t forget your projects! CS121/IS223 Week 12, Slide 33 Extra task find out about autoboxing & auto-unboxing CS121/IS223 Week 12, Slide 34 Coming Up Next PRACTICE AND REVISION More practice with OO program design - a bit more UML and putting it all together Project clinics where needed if you are working as a pair, you must tell me this week! Sample exam questions CS121/IS223 Week 12, Slide 35 6