Pass by Value 2
COSC 112, Spring 2019
Lecture 09
Friday, March 1 2019
Scott Alfeld
Announcements
Recall: Midterm will be Friday, March 8
In class -- usual time, usual room.
I will be out of town.
If you have accommodations you’d like to use:
Email me Today!
Today:
Using object references
Consider the following code:
int myArray[] = new int[100];
for (int i = 0; i < myArray.length; i ++){
myArray[i] = getIntFromUser();
}
for (int i = myArray.length - 1; i >= 0; i --){
System.out.println(myArray[i]);
}
A thought experiment
Suppose Java didn’t have arrays.
Specifically:
Instead of:
● I want to be able to iterate over
int myArray = {2, 4, 8};
all the ints.
I could:
● I want to be able to hold
int i1 = 2; int i2 = 4; int i3=8;
arbitrary ints (not, e.g., just
But what if I want 100 ints?
powers of 2)
How can I achieve that
with < 100 lines of code?
Your task: Without using arrays, ArrayLists, Vectors, etc.:
Create a class, IntHolder, which stores ints. It lets me:
● add a new int
● print all the ints I’ve ever added (in reverse order of how I added them)
After that:
● Make it a SphereHolder, holding Spheres instead of ints
● Instead of printing, it can call mySphere.update() on each
● Add the ability to remove an element
A solution:
class Node{
Sphere s;//int num
Node next;
public Node(Sphere s){
this.s = s;
}
}
public class SphereHolder{
Node end;
public SphereHolder(){
end = null;
}
}
public void append(Sphere toAppend){
Node toAdd = new Node(toAppend);
toAdd.next = end;
end = toAdd;
}
public Sphere peek(){
return end.s;
}
public Sphere pop(){
Sphere toReturn = end.s;
end = end.next;
return toReturn;
}
public void update(){
Node n = end;
while (n != null){
n.s.update();
n = n.next;
}
}
public String toString(){
String toReturn = “”;
Node n = end;
while (n != null){
toReturn = toReturn + n.s;
//toReturn = n.s + “ ” + toReturn; //to print order added
n = n.next;
}
return toReturn;
}
public static void main(String[] args){
SphereHolder d = new
SphereHolder();
d.append(new Sphere());
d.append(new Sphere());
d.append(new Sphere());
d.pop();
}
Recall:
public void append(Sphere toAppend){
Node toAdd = new Node(toAppend);
toAdd.next = end;
end = toAdd;
}
Local
Scope
append
pop
toAdd
Node
3121
4487
2122
d
SphereHolder
1299
Memory
Addr:
1299
end:
Node
3121
4487
2122
null
Addr:
3121
num: 1
next: null
Addr:
4487
num: 2
next: 3121
Addr:
2122
num: 3
next: 4487
null
Node
s1
Node
s2
Node
s3
Node
s4
Node
s6
Node
s5
For ints:
class Node{
int num;
Node next;
public Node(int num){
this.num = num;
}
}
(The everything is basically the same)
null
Node
num = 3
Node
num = 1
Node
num = 4
Node
num = 1
Node
num = 9
Node
num = 5