vs. Primitive Reference Types
Introduction to Programming
with J ith Java, f B i for Beginners
We've seen Java's 4 primitive types: int, double, boolean, char Types other than the primitive types are known as reference types Used for objects Examples of reference variables: Student s1; Counter c1; String name; Note: String is an object not primitive type
Primitive vs. References Type Stack vs. Heap Null Pointer Exception Keyword this K d hi Strings Has a Relationship
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Stack vs. Heap
Stack is section of computer's memory used to store temporary information
E.g. Variable created inside a method g Information ceases to exist after method finishes execution
How the Stack Works
DrJava Interaction (same applies for methods)
> int x = 0;
Stack
> x = 5;
Objects (i.e. object's data) are stored in section of memory called heap
Information exists as long as the program is not finished In Dr Java's interaction pane storage information exists as long as RESET button is not clicked
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double min = 0 5; 0.5; boolean done = false;
Note: Variables are added in the order they are declared
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Reference Type
The term reference is used because it refers to a memory location where the object lives
The variable of reference type is used to access the object
Null Pointer Exception
null is a legal value for any kind of object
i.e. Person p, Counter c; Player mario
null can be assigned tested, and printed assigned, tested But if you try to use a field or method of null, you get a nullPointerException i.e. you try to access some object that has not been created
p.getName() mario.getStrength()
The value of variable of reference type is either "null" or a "heap address"
null means currently not pointing at any location
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Value of a Reference Variable
Example: > Counter c1; > c1 null > c1 = new Counter(); > c1 Counter@e05ad6
> int x = 99; > Counter c1; > c1 null
How the Heap Works
DrJava Interactions
Stack and Heap
e05ad6 is location in memory where object that c1 is pointing resides
e05ad6 hexadecimal (base 16) number This location will diff Thi l ti ill different on your computer t t
> c1 = new Counter(); > c1 Counter@2f996f
We don't have to (and can't) deal with these hex numbers directly
Convenience of using variables
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> c1.incrementCount(); > Counter c2 = new Counter(); > c2 Counter@4a0ac5
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Aliases
Two or more references can point to the same object
These references are then known as aliases
Scope Issues with variables
public class Dot{ private int x; private int y;
Local variable x & y shadows the instance variable x & y
Example (In Dr Java Interactions Pane)
> Student s1= new Student("Lisa", 5); > s1 Student@83d8be > Student s2 = s1; > s2 Student@83d8be > s1.getAge() s1 getAge() 5 > s2.getAge() 5
p public Dot(int x, int y){ ( , x = x; // problem!! y = y; } }
public class Dot{ private int x; private int y; public Dot(int x, int y){ this.x = x; // fixed!! this.y = y; y } }
Solution
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Keyword this
this is a reference (a variable) to the current object
The object whose method or constructor is being j g called The value of "this" is an object's heap address
Can be passed as argument or input to a method Can be returned as value Cannot be modified (i.e. it is final)
String
A sequence of characters A String is a built-in Java object type Java provides this type because it's so used frequently Examples of String creation: fS
> String s1 = new String("hello"); > String s2 = "hello"; // commonly used shortcut > s2 + " you!" "hello you!" > s2 = "The result is " + 100; > System.out.println(s2); "The result is 100"
Used to Differentiate objects & avoid scope issued with instance variable & local variable names
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String (contd..)
DrJava Interactions
> String c1 = new String("Hill");
Comparing Strings
If the == operator is used
Java compares the addresses where the String objects are stored, not the letters in the String For example:
Stack and Heap
> String a = "hi"; hi ; > String b = "hi"; > a == b >false > String c1 = new String ("Hill"); > c1 "Hill" Hill Note: We do not get heap address of String reference Later, it will be clear why this case..
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Use the String class' equals method to compare two Strings for equality > a.equals(b) Just like the Math true class, String class > b.equalsIgnoreCase("HI") is part of Java true Language & hence directly used
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"Has a" Relationship
An object of type A has an instance variable which is an object whose type is B. (A "has a" B.) E.g: A Freshman object whose room is of reference type DormRoom
DormRoom Code
> DormRoom room = new DormRoom(208, "Hill"); > room.getLocation() "208 Hill"
public class DormRoom{ private int num; private String bldgName; public DormRoom(int n, String b){ num = n; bldgName = b; } public String getLocation(){ return num + " " + bldgName; }
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The UML diagrams below show instance variables and methods of Freshman and DormRoom object:
UML(Universal Modeling Lanaguage) industry standard used to describe classes in OOP }
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A DormRoom on the Heap
Freshman Code
> DormRoom room = new DormRoom(208, "Hill"); > Freshman f = new Freshman("jo", room); > f.getName() "jo" >...