CS110D: PROGRAMMING LANGUAGE I · PDF fileto such Java entities. The scope of a declaration is...

CS110D: PROGRAMMING

LANGUAGE I

Lecture 7&8: Methods Computer Science

department

Lecture Contents

dr. Amal Khalifa,Fall14

What is a method?

Static methods

Declaring and using methods

Parameters

Scope of declaration

Overloading

Introduction


Best way to develop and maintain a large program

is to construct it from small, simple pieces, or

modules.

divide and conquer.

Modular Programming!!


Methods help you modularize a program by separating its tasks into self-contained units.

Written only once

Hidden from other methods

Can be reused : Use existing methods as

building blocks to create new

programs.

Methods

A method:

groups a sequence of statement

takes input, performs actions, and produces output

In Java, each method is defined within specific class

Tips!!


Java API


Java programs combine new methods and classes that you write with predefined methods and classes available in the Java Application Programming Interface and in other class libraries.

Class Math provides a collection of static methods

enable you to perform common mathematical calculations.

Method arguments may be constants, variables or expressions

Defines common mathematical constants

Math.PI (3.141592653589793)

Math.E (2.718281828459045)

The Math methods

Example 1

Solve quadratic equation ax2 + bx + c = 0.

public class Quadratic {

public static void main(String[] args) {

double a,b,c,d;

// input coefficient values..

// calculate roots

d = Math.sqrt(b*b - 4.0*a*c);

double root1 = (-b + d) / (2.0*a);

double root2 = (-b - d) / (2.0*a);

// print them out

System.out.println(root1);

System.out.println(root2);

}

}

Dr. Amal Khalifa, 2012

Example2: Simulation- Flipping a coin

can produce only double values in the

range of 0.0 and 1.0.

public class Flip {

public static void main(String[] args) {

for (int i=0; i<1000; i++)

if (Math.random() < 0.5)

System.out.println("Heads");

else

System.out.println("Tails");

}

} % java Flip

Heads

% java Flip

Heads

% java Flip

Tails

Dr. Amal Khalifa, 2014

10

The Math Class


Tip!!


User defined methods


declare

Call

test

Reuse

Method Declaration: Header

A method declaration begins with a method header

method

name

return

type

parameter list

The parameter list specifies the type and name of each parameter The name of a parameter in the method

declaration is called a formal argument

public class MyClass

{

static int min ( int num1, int num2 ) …

properties

Method Declaration: Body

The header is followed by the method body:

static int min(int num1, int num2)

{

int minValue = (num1 < num2) ? num1 : num2;

return minValue;

}

class MyClass

{ …

…

}

Computer Science Department

The return Statement

The return type of a method indicates the type of

value that the method sends back to the calling

location

The return statement specifies the value that will be

returned

Its expression must conform to the return type

A method can return at most 1 value

A method that does not return a value has a void

return type


Be careful!!


Calling a Method

Each time a method is called, the values of the actual arguments in the invocation are assigned to the formal arguments

static int min (int num1, int num2)

{

int minValue = (num1 < num2 ? num1 : num2);

return minValue;

}

int num = min(2, 3);


Example

Method call

Example

Method

declaration

static

return

Example: void methods

22

Write a function

that outputs the

multiplication

table of a given

number.

Method calling


A method can call another method, who can call

another method, …

min(num1, num2, num3)

println()

…println(…)

min(1, 2, 3);

main

Method Call Stack


Stack:

data structure

Analogous to a pile of dishes

A dish is placed on the pile at the top (referred to as pushing the dish onto the stack).

A dish is removed from the pile from the top (referred to as popping the dish off the stack).

Last-in, first-out (LIFO) data structures

The last item pushed (inserted) on the stack is the first item popped (removed) from the stack.

LIFO behavior:

• the caller

pushes the

return address

onto the stack,

• when it

finishes, the

called function

pops the return

address off

the call stack

• Control is

transfers to

that address.

• memory is

finite -->

stack

overflow may

occurs

25

Argument Promotion and Casting

Argument promotion

Converting an argument’s value, if possible, to the type that the

method expects to receive in its corresponding parameter.

Conversions may lead to compilation errors if Java’s

promotion rules are not satisfied.

Promotion rules

specify which conversions are allowed.

apply to expressions containing values of two or more primitive

types and to primitive-type values passed as arguments to methods.

Each value is promoted to the “highest” type in the

expression.

Argument casting

In cases where information may be lost due to conversion, the Java compiler

requires you to use a cast operator to explicitly force the conversion to occur—

otherwise a compilation error occurs.

Be careful!!

Scope of Declarations


Declarations introduce names that can be used to refer

to such Java entities.

The scope of a declaration is the portion of the program

that can refer to the declared entity by its name.

Such an entity is said to be “in scope” for that portion of the

program.

If a local variable or parameter in a method has the

same name as a field of the class, the field is “hidden”

until the block terminates execution—this is called

shadowing.

Scope Rules…

Dr. Hamad, Dr. Khalifa

30

Global: Declared "outside" function body

final double TAXRATE = 0.05;

final double PI = 3.14159;

Local: Declared “inside" a function or block

* Declared inside body of given function or block of code

* Available only within that function

* Can we have variables with same names declared in

different method??

Example

Method Overloading

Methods of the same name declared in the same class

Must have different sets of parameters (signatures).

the compiler differentiates signatures by :

the number of parameters,

the types of the parameters and

the order of the parameter types in each signature.

Method calls cannot be distinguished by return type.

Overloaded methods can have different return types if the methods have different parameter lists.

Overloaded methods need not have the same number of parameters.

Example

Same name

different

tasks

matching by

signature

Literal

floating-point

values are

treated as type

double

Be careful!!

[1] 6.1 6.7, 6.11, 6.12

That’s all …


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