Download Methods

AAA
Today’s lecture
• Review of Chapter 6
• Go over examples
Chapter 6 review
• What is a static method? (A static variable?)
• How does Java handle assignment (during method calls)?
• What is scope? Local? Parameter? Instance? Class?
• What is visibility? Public? Private?
Copying parameters Example
• What value gets printed?
public class Test {
public static void main(String[] args) {
int x=3;
changeVal(x,5);
System.out.println(x);
}
public static void changeVal(int p, int v) {
p = v;
}
}
("3" is printed.)
But what does this mean for objects?
• What gets printed?
public class Test {
public static void main(String[] args) {
Person p = new Person("Mason",21);
changeName(p,"Thomas");
System.out.println(p.getName());
}
public static void changeName(Person p, String n) {
p.setName(n);
}
}
Thomas (Java copies the reference)
How about now?
• What gets printed?
public class Test {
public static void main(String[] args) {
Person p = new Person("Mason",21);
changeName(p,"Thomas");
System.out.println(p.getName());
}
public static void changeName(Person p, String n) {
p = new Person(n,30);
}
}
Mason
(new object created/modified/discarded)
Does it matter that we have a local variable called p
in changeName? What if it were called q?
How about now? (v2)
• What gets printed?
public class Test {
public static void main(String[] args) {
Person p = new Person("Mason",21);
p = changeName(p,"Thomas");
System.out.println(p.getName());
}
public static Person changeName(Person q, String n) {
q = new Person(n,30);
return q;
}
}
"Thomas" (new object created/modified/returned, THEN p
is updated to point to it.)
Does it matter that we have a local variable called p
in changeName? What if it were called q?
Reference Assignment continued
• Why are the references copied, instead of the entire object?
• Objects can be big
• Wastes space
• Takes a long time to copy
• What can be a downside to copying just the reference?
• Now you have two things pointing to the same location in
memory
• What happens if you forget this mistakenly?
Creating lots of objects
We know how to create and allocate memory to programs with
new in Java
Could our program ever run out of memory?
◦ Not if it is super small
◦ What if you run something for weeks that is constantly creating objects it
only uses once?
Want a way to retrieve (recover) memory that is no longer being
used
◦ Turns out this is tricky! How do we know an object is no longer needed?
Garbage Collection
When an object no longer has any valid references
to it, it can no longer be accessed by the program
The object is useless, and therefore is called
garbage
Java performs automatic garbage collection
periodically, returning an object's memory to the
system for future use
In other languages, the programmer is responsible
for performing garbage collection
Coming up: String Methods
Method Overloading (Quick View)
We can have multiple methods with the same name in one class! They are
distinct methods with no actual relations other than the name – this is just a
mnemonic convenience!
To coexist, their method signatures must be uniquely identifiable by the
parameter types and method name alone.
• parameter names are ignored – arguments are nameless.
• return type is ignored – not explicit via method call, so can't help select
correct method.
Constructors are methods too – we can write multiple constructors for one
class, as long as they have different signatures.
• This is a valuable opportunity (something Python disallows).
Method Overloading - Examples
Example: these methods may all be in one class:
1. public int foo (int a, int b){…}
2. public int foo (char a, int b){…}
3. public int foo (int b, char a){…}
4. public int bar (int a, int b){…}
5. public String foo (int a, int b, int c){…}
The following could not be added to the class:
• public String foo (int a, int b) {…}
• public int foo (int other, int names){…}
• private int foo (int a, int b){…}
return type irrelevant
param names irrelevant
modifier irrelevant
Visibility Modifiers
public
Enforce
encapsulation
Variables
Methods
4-13
private
Provide services
to clients
© 2004 Pearson AddisonWesley. All rights reserved
Support other
methods in the
class
Let’s go over the examples
4-14
© 2004 Pearson AddisonWesley. All rights reserved
Questions?
4-15
© 2004 Pearson AddisonWesley. All rights reserved