Transcript Methods - NYU Computer Science Department

Introduction to Computers and Programming in
JAVA: V22.0002
Math class methods
&
User defined methods
Math class methods
Math.sqrt(4.0)
Math.random()
• java.lang is the library/package that provides Math
class methods such as
– Math.random() to generate random numbers
• Java.lang is accessed by all java programs by
default.
– YOU do not have to include it at the beginning of the
program as you do with javax package.
3
Random-Number Generation
• Often we want our programs to generate random
numbers.
– games of chance
– testing without user interaction
• Java random-number generators
– Math.random()
• Returns a double value with a positive sign, greater than
or equal to 0.0 and less than 1.0.
– What if we want to generate random integers?
 2000 Prentice Hall, Inc. All rights reserved. Modified for use with this class.
4
Random-Number Generation
– Math.random()
• Produces double
• from
0.0 to 1.0 (excluding 1)
• ( int ) ( Math.random() * 6 )
• Produces integers from
0–5
Scaling
– 1 + ( int ) ( Math.random() * 6 )
• Produces integers from
Shifting
1–6
 2000 Prentice Hall, Inc. All rights reserved. Modified for use with this class.
Benefits of Methods

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Experience has shown that the best way to develop
and maintain large programs is to build it from
smaller components or modules.
In Java, one such module is the method.
Each module is generally simpler and more
manageable than the entire program.
This concept is known as Divide and Conquer,
also with Abstraction in the mix.
6
User defined Method returning a value
• General format of method declaration:
Method header
modifiers return-value-type method-name( parameter1, …, parameterN )
{
Method body
Scope of local
variables
declarations
and statements
}
• Method can return one value:
return expression;
• Or, it can returns nothing using keyword void
in the header.
 2000 Prentice Hall, Inc. All rights reserved. Modified for use with this class.
Return Value Types
• You can only return one value from a method.
• Returning void
– void: means nothing
– A method that returns void therefore returns nothing.
– Hence, there is no need for the optional return statement. But
using one can force early exit from the method.
– Example:
public static void printIntro (int n);
Parameter Data Types
• Unlike return values, you can pass as many
parameters as you like.
• To pass more than one parameter, you need to
separate the parameters with commas.
public static int maximum (int x, int y)
{
/*body*/
}
Warning
• Unlike declaring variables, you must specifically
state the type for multiple variables
– For example
takeInTwoFloats( float x, y )
is incorrect
– Instead you must write
takeInTwoFloats(float x, float y)
No parameters
• You can also have a method that accepts no
parameters. In such case, you would just have an
empty parameter list.
E.g.
public static int rollDie ()
public static void printIntro ()
11
6.5
Argument Promotion
• Coercion of arguments
– Forcing arguments to appropriate type to pass to method
• e.g., System.out.println( Math.sqrt( 4 ) );
– Evaluates Math.sqrt( 4 )
– Then evaluates System.out.println()
• Promotion rules
– Specify how to convert types without data loss
 2000 Prentice Hall, Inc. All rights reserved. Modified for use with this class.
12
Type
double
float
long
int
char
short
byte
boolean
Valid promotions
None
double
float or double
long, float or double
int, long, float or double
int, long, float or double
short, int, long, float or double
None (boolean values are not considered to be
numbers in Java)
Fig. 6.5 Allowed promotions for primitive types.
 2000 Prentice Hall, Inc. All rights reserved. Modified for use with this class.
Understanding Scope
14
Local Variables & Scope
A local variable: a variable defined inside a
method.
Scope: the part of the program where the
variable can be referenced.
The scope of a local variable starts from its
declaration and continues to the end of the
block that contains the variable. A local
variable must be declared before it can be
used.
 2000 Prentice Hall, Inc. All rights reserved. Modified for use with this class.
15
Local Variables & Scope, continued
You can declare a local variable with the same
name multiple times in different nonnesting blocks in a method, but you cannot
declare a local variable twice in nested
blocks.
 2000 Prentice Hall, Inc. All rights reserved. Modified for use with this class.
Overloading methods
17
6.15 Method Overloading
• Method overloading
– Several methods of the same name
– Different parameter set for each method
• Number of parameters
• Parameter types
– The Java compiler determines which method to use based on
the parameters.
– Can also be used in conjunction with argument coercion.
• The combination can lead to ambiguous invocation which is an
error
 2000 Prentice Hall, Inc. All rights reserved. Modified for use with this class.
18
Ambiguous Invocation
Sometimes there may be two or more possible
matches for an invocation of a method, but
the compiler cannot determine the most
specific match. This is referred to as
ambiguous invocation. Ambiguous
invocation is a compilation error.
 2000 Prentice Hall, Inc. All rights reserved. Modified for use with this class.
19
Ambiguous Invocation
public class AmbiguousOverloading {
public static void main(String[] args) {
System.out.println(max(1, 2));
}
public static double max(int num1, double num2) {
if (num1 > num2)
return num1;
else
return num2;
}
public static double max(double num1, int num2) {
if (num1 > num2)
return num1;
else
return num2;
}
}
 2000 Prentice Hall, Inc. All rights reserved. Modified for use with this class.
review
• What is a method?
• What information can you learn about a method
from its header?
• What does it mean to invoke a method?
• What is call by value?
• What is scope of local variables within a method?
• Why don’t we have to import the Math class?
• What is abstraction in computer science?