1. ATS Application Programming: Java Programming
Method Calling
2.6 Method Calling
What is a Method?
Declaring a Method
Method Calling
Method Call Stack
Parameter Passing
Pass by Value
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Course Code #Z16325

2. ATS Application Programming: Java Programming
Objectives
2.6 Method Calling
Define a method
Demonstrate how to properly declare a method.
Demonstrate how methods call each other
Demonstrate how methods are executed in call stack
Demonstrate parameter passing
Understand that Java passes arguments by value.
© Accenture 2005 All Rights Reserved
Course Code #Z16325

3. ATS Application Programming: Java Programming
What is a Method?
2.6 Method Calling
A method refers to a piece of code referring to behaviors associated either with an object or its class
A code found in a class for responding to a message
The executable code that implements the logic of a particular message for a class
An operation or function that is associated with an object and is allowed to manipulate the object's data
Procedure vs methods
The difference between a procedure (usually miscalled a "function") and a method is that the latter, being associated with a particular object, may access or modify the data private to that object in a way consistent with the intended behavior of the object.
Consequently, rather than thinking "a method is just a sequence of commands", an OO programmer will consider a method to be "an object's way of providing a service" (its "method of doing the job", hence the name). A method call is thus considered to be a request to an object to perform some task.
© Accenture 2005 All Rights Reserved
Course Code #Z16325

4. ATS Application Programming: Java Programming
Creating a Method
2.6 Method Calling
Steps in declaring a method
Set the return type
Provide method name
Declare formal parameters
class Number {
int multiply(int i, int j) {
return i*j; }
int divide(int i, int j) {
return i/j;
double getPi() {
return ;
void printSum(int i, int j) {
System.out.println(i+j);
method signature
consists of the method name and its parameters
must be unique for each method in a class
return statement
allows the method to return a value to its caller
also means to stop the execution of the current method and return to its caller
implicit return at the end of the method
A method with empty parameters
A method that does not return a value must specify void as its return type
© Accenture 2005 All Rights Reserved
Course Code #Z16325

5. ATS Application Programming: Java Programming
Method Calling
2.6 Method Calling
How to call a method
Method name should match
Number of parameters should match
Type of parameters should match
Ways of calling a method
Calling a method through its object name
Calling a method within the same class
Calling a static method through its class name
© Accenture 2005 All Rights Reserved
Course Code #Z16325

6. ATS Application Programming: Java Programming
Method Calling
2.6 Method Calling
public class JavaMain {
public static void main(String[] args) {
// create a Person object
Person you = new Person();
you.talk();
you.jump(3);
System.out.println(you.tellAge());
JavaMain.talkOnly(you);
// create object of main program
JavaMain me = new JavaMain();
me.jumpOnly(you); }
static void talkOnly(Person p) {
p.talk();
void jumpOnly(Person p) {
p.jump(2);
class Person {
void talk() {
System.out.println("blah, blah..."); }
void jump(int times) {
for (int i=0; i<times; i++) {
System.out.println("whoop!");
String tellAge() {
return "I'm " + getAge();
int getAge() {
return 10;
blah, blah...
whoop!
I'm 10
© Accenture 2005 All Rights Reserved
Course Code #Z16325

7. ATS Application Programming: Java Programming
Method Call Stack
2.6 Method Calling
The Method Call Stack refers to all the methods currently active and being processed by a Java application
invoke print()
invoke compute()
invoke check()
main() begins
print()
compute()
check()
main()
execute print()
execute compute()
execute check()
main() ends
© Accenture 2005 All Rights Reserved
Course Code #Z16325

8. ATS Application Programming: Java Programming
Passing Parameters
2.6 Method Calling
Passing parameters in Java is always Pass by Value!
When passing a parameter of primitive type:
A copy of the value of the variable is passed
The passed variable cannot be changed in the called method as the method only possesses a copy of that variable.
When passing a parameter of reference type:
A copy of the reference (address) of the object is passed
The object reference cannot be changed in the called method (i.e., the object cannot be reassigned to another object)
The object state can be changed in the called method (i.e., attributes can be modified)
© Accenture 2005 All Rights Reserved
Course Code #Z16325

9. ATS Application Programming: Java Programming
Passing Parameters
2.6 Method Calling
public class MySelf {
public static void main(String[] args) {
int age=18;
int[] stats = {30,22,33};
String name = "Mary Jane";
MySelf me = new MySelf();
System.out.println("I am just " + age);
me.changeAge(age);
System.out.println("Nope, I'd rather be " + age);
System.out.println("I hate my body at " + me.getStats(stats));
me.changeStats(stats);
System.out.println("Wow! I'm now " + me.getStats(stats));
System.out.println("My name is so naive, " + name);
me.changeName(name);
System.out.println("Naah, I still like " + name); }
void changeAge(int age) {
age += 5;
System.out.println("I imagine me being " + age); }
void changeStats(int[] stats) {
stats[0]=36; stats[1]=24; stats[2]=36;
System.out.println("I wish I'm sexy at " + getStats(stats));
String getStats(int[] stats) {
String s="";
for (int i=0; i<stats.length; i++) s += stats[i] + " ";
return s;
void changeName(String name) {
String newName = "MJ";
name = newName;
System.out.println("What about " + name + "?");
Same age
Changed stats
I am just 18
I imagine me being 23
Nope, I'd rather be 18
I hate my body at
I wish I'm sexy at
Wow! I'm now
My name is so naive, Mary Jane
What about MJ?
Naah, I still like Mary Jane
Same name
© Accenture 2005 All Rights Reserved
Course Code #Z16325

10. ATS Application Programming: Java Programming
Key Points
2.6 Method Calling
A method refers to what an object or class can do
A method must have a return type, a name, and optional parameters
The method signature refers to a method name and its parameters
Return statement returns a value to its caller or returns control to its caller
A method that does not return a value must specify void as a return type
Calling a method should match its method signature
When calling a method in the same class, use only the method name
When calling a method in a different class, use the object reference
When calling a static method, use the class name
Methods are invoked sequentially in the call stack and executed in reverse order
Passing parameters in Java is always pass by value
© Accenture 2005 All Rights Reserved
Course Code #Z16325