Embed Size (px)
Citation preview
Methods & Activation Record
Recap: what’s a method?
http://codingbat.com/java
Method Headers
public static void main(String[] args) {}
Method Headers
public class Car {
…
public double getSpeed();
}
public class Math {
…
public static double abs(double a);
}
Car c = new Car();
double s = c.getSpeed();
double a = Math.abs(-4);
Scope
What happens in the curly braces,
Stays in the curly braces.
How does this work with methods?
Example Code: Does this work?
public static void main(String[] args) {
Scanner sc = new Scanner(System.in);
double a = sc.nextDouble();
double sum = addFive();
}
public static void addFive() {
double sum = a + 5;
}
main()
Scanner sc
double a
double sum = ?
public static void main(String[] args) {
Scanner sc = new Scanner(System.in);
double a = sc.nextDouble();
double sum = addFive(); // executing this line
}
String[] args
addFive()
main()
Scanner sc
double a
double sum = ?
public static void addFive() {
double sum = a + 5;
}
String[] args
?!?!?!?!?!?!?!?!
Let’s fix that…
public static void main(String[] args) {
Scanner sc = new Scanner(System.in);
double a = sc.nextDouble();
double sum = addFive(a);
}
public static void addFive(double a) {
double sum = a + 5;
}
main()
Scanner sc
double a
double sum = ?
public static void main(String[] args) {
Scanner sc = new Scanner(System.in);
double a = sc.nextDouble();
double sum = addFive(a); // executing this line
}
String[] args
addFive()
main()
Scanner sc
double a
double sum = ?
public static void addFive(double a) {
double sum = a + 5;
}
String[] argsdouble a
Note that main() still has the original variable a – addFive() just has a COPY. We call this “pass by value.”
double sum = a + 5
main()
Scanner sc
double a
double sum = ?
public static void main(String[] args) {
Scanner sc = new Scanner(System.in);
double a = sc.nextDouble();
double sum = addFive(a); // we’re back to this line
}
String[] args
Example Code
public static void main(String[] args) {
Scanner sc = new Scanner(System.in);
double a = sc.nextDouble();
double sum = addFive(a);
}
public static double addFive(double a) {
return a + 5;
}
main()
Scanner sc
double a
double sum =
public static void main(String[] args) {
Scanner sc = new Scanner(System.in);
double a = sc.nextDouble();
double sum = addFive(a); // executing this line
}
String[] args
addFive()
main()
Scanner sc
double a
double sum =
public static int addFive(double a) {
return a + 5;
}
String[] argsdouble a
a + 5
main()
Scanner sc
double a
double sum =
public static void main(String[] args) {
Scanner sc = new Scanner(System.in);
double a = sc.nextDouble();
double sum = addFive(a); // we’re back to this line
}
String[] argsa + 5
Let’s draw the stack frame a bit cleaner.
Local Variables
Return addressParameters
Now get out a sheet of paper…
public static void main(String[] args) {
int product = multiply(5, 10);
}
public static int multiply(int a, int b) {
int product = 0;
for (int i = 0; i < b; ++i) // at i = 0
product += a;
return product;
}
int i = 0int product = 0
[address of main()]int b = 10int a = 5
[address in system]String[] argsM
AIN
MU
LTIP
LY
What about now?
public static void main(String[] args) {
int product = multiply(5, 10);
}
public static int multiply(int a, int b) {
int product = 0;
for (int i = 0; i < b; ++i)
product += a;
return product;
}
int product = 50[address of main()]
int b = 10int a = 5
[address in system]String[] argsM
AIN
MU
LTIP
LY
Another examplepublic static void main(String[] args) {
double r = 54.56;
int num = round(r);
}
public static int round(double d) {
double dec = d – (int) d;
int ret;
if (dec < 0.5)
ret = Math.floor(d);
else
ret = Math.ceil(d);
return ret;
}
int ret;double dec = 0.56;[address of main()]double d = 54.56double r = 54.56
[address in system]String[] argsM
AIN
()R
OU
ND
()
Another examplepublic static void main(String[] args) {
int num = A(10);
}
public static int A(int i) {
return B(i + 2);
}
public static int B(int i) {
int r = i;
int p = r + i;
return i + p + r;
}
int p = 24
int r = 12[address of A()]
int i = 12[address of main()]
int i = 10[address in system]
String[] argsMA
IN()
A()
B()
To Sum Up
• Scope – What happens in the curly braces, stays in the curly braces
• This concept translates into chunks of memory
• These chunks get pushed onto a stack and get popped off when there’s a “return” or the method ends
• Note that we’ve only been passing primitive types
• Something different happens with complex types called pass by reference
