1. Introduction to Programming with Java, for Beginners
String Object
Primitive Types and References
The Stack and the Heap

2. Strings A sequence of characters
A String is a built-in Java object type
Java provides this type because it’s used so frequently
Examples of String creation:
> 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"

3. String Concatenation
+ usually means “add,” but if either operand (thing involved in the operation) is a String, then + means concatenation
If you concatenate anything with a String, that thing is first turned into a String
+ as the concatenation operator is an exception to the rule: Primitives have operations, Objects have methods
A String is immutable (more on this on slide 16)
Once you create it, there are no methods to change it
But you can easily make new Strings:
myName = "Dave"; myName = "Dr. " + myName;

4. String Object Methods Listed in the Java API
Part of the java language: java.lang
Many useful Methods
length()
toUpperCase()
charAt(int index)
substring(int beginIndex, int endIndex) …

5. System.out.println(String)
Method that which part of the print program in class System (java.lang.System)
System is stateless class like Math class
Has static variable called “out” of reference type PrintStream
Class PrintStream contains println(String x)
Input takes in string literal or variable
If something else the gets converted to string automatically
System.out.println(“hello, world”);
System.out.print(“x is “ + x);
System.out.println(“, answer is “ + foo());

6. Memory: Stack and Heap
When we run a standalone Java programs, memory is allocated for variables and objects
Understanding how this memory is managed helps us understand how Java works
The JVM uses two kinds of memory: stack and heap
The stack is used to store variables of primitive type:
When created in the DrJava interactions pane
During method calls
The heap is used to store objects

7. How the Stack Grows DrJava Interactions Stack > int x; > x = 5;
> double min = 0.5;
> boolean done = false;

8. Reference Types Examples of reference variables:
String name;
Counter c1;
In Java, no variable can ever hold an object
One variable can only contain one thing
Object consists of multiple of data/state and hence stored on heap
A variable can only hold a reference to an object
a reference to an object is the address of the memory location where the object is stored on the heap

9. Value of a Reference Variable
The value of are reference variable is either null or a “heap address”
Null means currently not pointing at any location
Example:
> Counter c1;
> c1
null
> c1 = new Counter();
e05ad6 is location in memory where c1 resides
e05ad6 hexadecimal (base 16) number
We don’t have to (and can’t) deal with these hex numbers directly

10. How the Heap Grows DrJava Interactions Stack and Heap > int x = 99;
> Counter c1;
> c1
null
> c1 = new Counter();
> c1.incrementCount();
> Counter c2 = new Counter();
> c2

11. Freshman DormRoom Example

12. DormRoom Code and UML > 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;

13. A DormRoom on the Heap > DormRoom room = new DormRoom(208, "Hill");
> room.getLocation()
"208 Hill"

14. Freshman Code and UML > DormRoom room = new DormRoom(208, "Hill");
> Freshman f = new Freshman("jo", room);
> f.getName()
"jo"
> f.getRoom().getLocation()
"208 Hill"
public class Freshman{
private String name;
private DormRoom room;
public Freshman(String n, DormRoom r){
name = n;
room = r; }
public String getName(){ return name;}
public DormRoom getRoom(){ return room;}

15. A Freshman on the Heap > DormRoom room = new DormRoom(208, "Hill");
> Freshman f = new Freshman("jo", room);
> f.getName()
"jo"
> f.getRoom().getLocation()
"208 Hill"

16. String Immutability
String s1 = new String("Hello");
String s2 = new String("There");
System.out.println(s1);
s1 = s2;
If Strings cannot be changed then s1 should still print out Hello
But it prints “There”
The immutability really refers to what the String reference is pointing to
When s2 is assigned to s1, the String containing "Hello" is no longer referenced and s1 now points to the same string as s2
The fact that the "Hello" string has not actually been modified is fairly theoretical as you can no longer "get at it".