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Lecture 10
SYS-1S22 / MTH-1A66
Methods
Stuart Gibson
[email protected]
S01.09A
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DecimalFormat Again
• DecimalFormat rounds the input when formatting to a set
number of decimal places.
import java.text.*;
Specify Two
Decimal Places
public class DFTest
{
public static void main (String[] args)
{
DecimalFormat fmt = new DecimalFormat(“0.00”);
double x = 3.215, y = 3.2149;
System.out.println(“x: ” + fmt.format(x));
System.out.println(“y: ” + fmt.format(y));
}
}
x: 3.22
y: 3.21
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Data Types : Strings
• So far we have only been using primitive data types:
int, double, char, boolean.
• Another popular data type are Strings. (note the capital S!)
• Strings are objects that are included in the Java Library.
• So far we have seen and used String literals e.g:
System.out.println(“Hello World”);
String literal
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Data Types : Strings
• Even though they are not primitive types, Strings are so
fundamental and so often used that Java defines String
literals delimited by double quotation marks as we have seen.
• This is a shortcut notation, whenever a String literal
appears a String object is created.
• With this shortcut notation we can declare and assign
Strings just as we did with chars e.g.
char singleChar = ‘H’;
String manyChar = “Hello”;
• For String assignment we use double quotes “
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Data Types : Strings
• A String is an array of characters:
H e l l o
• We can declare String variables and read them in from the
keyboard.
String input;
input = Keyboard.readString();
• So it seems that we can use Strings just as we do all other
primitives. However if we use the same operators for
comparison, we can get some strange results:
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Data Types : Strings
public class StringTest
{
public static void main (String[] args)
{
String name = "Stuart";
String input;
System.out.println("Enter your name: ");
input = Keyboard.readString();
if(input == name)
{
System.out.println("Your name is "+name);
}
else
{
System.out.println("Your name is NOT "+name);
}
}
}
Are we feeling brave: Live demo!
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Data Types : Strings
• Even if the String entered is the same, the answer when
compared using the == evaluates to false and so we print
out that they are not the same.
• This behaviour is because Strings are objects and not
primitives, so to compare objects we have to do something
different, more next week.
• So for now we shall just use them for simple tasks such as
printing them out.
String name = “Fred”;
System.out.println(“Hello ”+ name);
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So Far….
• We can repeat statements, num times using a for loop
for (int i=1; i <= num; i++)
{
System.out.println(i +“. Hello ”+name);
}
1. Hello Stuart
2. Hello Stuart
3. Hello Stuart
4. Hello Stuart
What if more than one variable changes ?
What about using this for loop again later in the program ?
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Code Repetition
• So far the only way would be to re-type the for loop.
for (int i=1; i <= num; i++)
{
System.out.println(i +“. Hello ”+name);
}
System.out.println(“After all this……”);
System.out.print(“I would like Hello “+name);
System.out.print(“ on lines 1 to ”+num+“ again!”);
for (int i=1; i <= num; i++)
{
System.out.println(i +“. Hello ”+name);
}
• Problem: Most people dislike unnecessary work.
• Solution: Write code that is reusable.
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Simple Problem
Write a program that displays the lyrics of the
children’s song Old Macdonald, which go as follows:
Old Macdonald had a farm, E-I-E-I-O,
And on this farm he had a cow, E-I-E-I-O,
With a moo-moo here and a moo-moo there,
Here a moo there a moo everywhere a moo-moo
Old Macdonald had a farm, E-I-E-I-O.
Old Macdonald had a farm, E-I-E-I-O,
And on this farm he had a sheep, E-I-E-I-O,
With a baa-baa here and a baa-baa there,
Here a baa there a baa everywhere a baa-baa
Old Macdonald had a farm, E-I-E-I-O.
………. Many more verses
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Simple Implementation
Initial Approach
public class SimpleOldMacdonald
{
public static void main(String[] args)
{
System.out.print("Old Macdonald had a farm, E-I-E-I-O,\n"
+ "And on this farm he had a cow, E-I-E-I-O, \n"
+ "With a moo-moo here and a moo-moo there, \n"
+ "Here a moo there a moo everywhere a moo-moo, \n"
+ "Old Macdonald had a farm, E-I-E-I-O.\n\n");
System.out.print("Old Macdonald had a farm, E-I-E-I-O,\n"
+ "And on this farm he had a sheep, E-I-E-I-O, \n"
+ "With a baa-baa here and a baa-baa there, \n"
+ "Here a baa there a moo everywhere a baa-baa, \n"
+ "Old Macdonald had a farm, E-I-E-I-O.\n\n");
//Many More Verses
}
}
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Problem Analysis
• Out of the 40 words in a verse, 32 are repeated in every verse!
• Solution: Eliminate redundant code.
What is the difference between the verses?
Old Macdonald had a farm, E-I-E-I-O,
And on this farm he had a cow, E-I-E-I-O,
With a moo-moo here and a moo-moo there,
Here a moo there a moo everywhere a moo-moo
Old Macdonald had a farm, E-I-E-I-O.
Old Macdonald had a farm, E-I-E-I-O,
And on this farm he had a sheep, E-I-E-I-O,
With a baa-baa here and a baa-baa there,
Here a baa there a baa everywhere a baa-baa
Old Macdonald had a farm, E-I-E-I-O.
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Problem Analysis
• Out of the 40 words in a verse, 32 are repeated in every verse!
• Solution: Eliminate redundant code.
What is the difference between the verses?
Old Macdonald had a farm, E-I-E-I-O,
And on this farm he had a cow, E-I-E-I-O,
With a moo-moo here and a moo-moo there,
Here a moo there a moo everywhere a moo-moo
Old Macdonald had a farm, E-I-E-I-O.
Old Macdonald had a farm, E-I-E-I-O,
And on this farm he had a sheep, E-I-E-I-O,
With a baa-baa here and a baa-baa there,
Here a baa there a baa everywhere a baa-baa
Old Macdonald had a farm, E-I-E-I-O.
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Solution Design
Can we make a generic verse?
Old Macdonald had a farm, E-I-E-I-O,
And on this farm he had a animal, E-I-E-I-O,
With a sound-sound here and a sound-sound there,
Here a sound there a sound everywhere a sound-sound
Old Macdonald had a farm, E-I-E-I-O.
• The items that differ between the verses are known
as the parameters of the verse.
• We require variables to represent the parameters
String animal;
String sound;
How do we print each verse using these variables?
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Method Example
public class OldMacdonald
{
// First Attempt at a method
public static void buildVerse(String animal, String sound)
{
System.out.print("Old Macdonald had a farm, E-I-E-I-O,\n"
+ "And on this farm he had a "+animal+", E-I-E-I-O, \n"
+ "With a "+sound+"-"+sound+" here and a "+sound+"-"+sound
+ "there, \n"
+ "Here a "+sound+" there a "+sound+" everywhere a “
+ sound+"-"+sound+", \n"
+ "Old Macdonald had a farm, E-I-E-I-O.\n\n");
}
public static void main(String[] args)
{
buildVerse("cow","moo");
// Calls Method for cow
buildVerse("sheep","baa"); // Calls Method for sheep
}
}
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Method Declaration
public static void buildVerse(String animal, String sound)
{
// statements to print out a verse
// The part between the curly braces is the method body
}
• This is the method declaration.
• The method name is buildVerse.
We could call it almost anything we like!
Although there are a few exceptions such as keywords etc.
• More about public and static later…
• The method concatenates the “hard-wired” Strings such as
“Have a ” with the values stored in the String variables
animal and sound.
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Invoking the Method
• In the main method we see the statement:
buildVerse(“cow”,“moo”);
Method Identifier (Name)
Arguments
• The main progam calls the buildVerse method to invoke it.
• The buildVerse method takes two arguments.
• The arguments are both String types.
• In the example above, the main program passes the “hardwired” values of “cow” and “moo” to the buildVerse method.
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Method Initialisation
• The main program calls the buildVerse method:
buildVerse(“cow”,“moo”);
• The method declaration:
passes
buildVerse(String animal,String sound);
• The method declares two parameters: animal and sound
that are both String types.
• When the method is called the variables animal and sound
are initialised to the values passed from the main method.
• In our example:
animal is initialised to “cow”
sound is initialised to “moo”
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Method Execution Flow
The main method
The buildVerse method
buildVerse(“cow”,“moo”);
• Calls method
• The two arguments are
set to “cow” and “moo”
and passed to the
buildVerse method.
animal
sound
“cow”
“moo”
• Concatenate Strings
• Output to Screen
• Return to main method
buildVerse(“sheep”,“baa”);
Flow of execution
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Method Execution Flow
• main method pauses when it invokes buildVerse
method.
•
Control passes to buildVerse method.
• buildVerse method executes
•
When buildVerse terminates, it returns control back to
the main method
• main method continues.
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Method Output
buildVerse(“cow”,“moo”);
Old Macdonald had a farm, E-I-E-I-O,
And on this farm he had a cow, E-I-E-I-O,
With a moo-moo here and a moo-moo there,
Here a moo there a moo everywhere a moo-moo
Old Macdonald had a farm, E-I-E-I-O.
buildVerse(“sheep”,“baa”);
Old Macdonald had a farm, E-I-E-I-O,
And on this farm he had a sheep, E-I-E-I-O,
With a baa-baa here and a baa-baa there,
Here a baa there a baa everywhere a baa-baa
Old Macdonald had a farm, E-I-E-I-O.
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Method Variables - Scope
• The scope of a variable is from when it is declared to the
closing curly brace of the part of the code in which it was
declared.
• The variable is created when it is declared.
• The variable is destroyed after the closing brace of the block
of code it was declared in.
• The variables animal and sound are available in the
buildVerse method only.
• They are local to the buildVerse method.
• They do NOT exist in the main method.
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Scope Error
For example the following code would give a compilation error:
public static void main(String[] args)
{
buildVerse("cow","moo");
System.out.println(animal);
}
cannot resolve symbol
symbol: variable animal
location: class OldMacdonald
System.out.println(animal)
^
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Passing Variables to a Method
• In our previous example the values of the arguments passed
to the method were “hard-wired”.
• In practice we want to use variables to store the values that
are passed to the method, for example:
public static void main(String[] args)
{
String animalOne = “cow”;
String animalOne_snd = “moo”;
buildVerse( animalOne, animalOne_snd );
}
public static void buildVerse(String animal,
String sound)
• Why do the variables have different names in the main
method and the buildVerse method ?
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Passing Variables to a Method
• animalOne and animalOne_snd are local to the main
method.
• The variable name is NOT passed to the method.
• The compiler evaluates what the value of the variable is and
then passes that to the method.
• In our example, the compiler evaluates: animalOne and
animalOne_snd to “cow” and “moo” respectively and then
sends these to the buildVerse method.
• In the buildVerse method:
The value of animal is set to “cow”
The value of sound is set to “moo”
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Passing Variables to a Method
Argument:
animalOne
animalOne_snd
“cow”
“moo”
“cow”
Parameter:
“moo”
“cow”
“moo”
animal
sound
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Returning Variables from a Method
What happens if you are asked to repeat the song lyrics?
public static void main(String[] args)
{
// Calls Method for cow
buildVerse("cow","moo");
// Calls Method for sheep
buildVerse("sheep","baa");
// build remaining verses of the song
// repeat song
buildVerse("cow","moo");
buildVerse("sheep","baa");
// build remaining verses of the song
}
Is it necessary to call the buildVerse method twice
(or possibly more) times per animal?
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Returning Variables from a Method
• At present buildVerse has a return type of void .
public static void buildVerse(String animal,
String sound)
• void means that the buildVerse method does not pass
anything back to the point where it was called from.
• However we can return a variable back to where it was
invoked, when the method terminates.
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Returning Variables from a Method
public static String buildVerse(String animal,
String sound)
{
String verse =
"Old Macdonald had a farm, E-I-E-I-O,\n“+
"And on this farm he had a "+
animal+", E-I-E-I-O, \n"+
// rest of verse
return verse;
}
• The type of the variable to be returned MUST be the same as
the type specified in the method declaration.
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Returning Variables from a Method
• If you specify a return type you MUST return the same type
else you will get a compilation error:
public static String buildVerse(String animal,
String sound)
{
// statements
return;
}
OldMacdonald.java:10: missing return value
return;
^
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Returning Variables from a Method
• If you specify a return type you MUST return the same type
else you will get a compilation error:
public static String buildVerse(String animal,
String sound)
{
// statements
}
OldMacdonald.java:10: missing return statement
{
^
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Returning Variables from a Method
public static void main(String[] args)
{
String cowVerse = buildVerse("cow","moo");
String sheepVerse = buildVerse("sheep","baa");
System.out.print(cowVerse);
System.out.print(sheepVerse);
//to repeat
System.out.print(cowVerse);
System.out.print(sheepVerse);
}
• Here buildVerse is only executed twice, but the results can
be used many times!
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Returning Method – Execution Flow
The main method
The buildVerse method
cowVerse = buildVerse(“cow”,“moo”);
• Calls method
• Passes two String arguments
• cowVerse is set to the value that is
returned from the method
cowVerse
animal
sound
“cow”
“moo”
• Concatenate strings and store
result in verse variable.
verse
“Old Macdonald had a farm, E-I-E-I-O,
And on that farm he had a cow, E-I-E-I-O
………………..”
“Old Macdonald had a farm, E-I-E-I-O,
And on that farm he had a cow, E-I-E-I-O
………………..”
sheepVerse = buildVerse(“sheep”,“baa”);
return verse;
Flow of execution
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Tracing the Execution - Debugging
public static void main(String[] args)
{
System.out.println(“BEFORE CALL \n”);
String cowVerse = buildVerse("cow","moo");
System.out.println(“AFTER CALL \n”);
System.out.print(“cowVerse:\n” + cowVerse);
}
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Tracing the Execution - Debugging
public static String buildVerse(String animal,
String sound)
{
System.out.println(“In buildVerse Method!”);
System.out.println(“animal: ”+animal);
System.out.println(“sound: ”+sound);
String verse =
"Old Macdonald had a farm, E-I-E-I-O,\n“+
"And on this farm he had a "+animal+
", E-I-E-I-O, \n"+
// rest of verse
System.out.println(“End of Method”);
System.out.println(“Verse:\n”+verse+”\n”);
return verse;
}
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Tracing the Execution - Output
BEFORE CALL
In buildVerse Method!
animal: cow
sound: moo
End of Method
Verse:
Old Macdonald had a farm, E-I-E-I-O,
And on this farm he had a cow, E-I-E-I-O,
With a moo-moo here and a moo-moo there,
Here a moo there a moo everywhere a moo-moo
Old Macdonald had a farm, E-I-E-I-O.
AFTER CALL
cowVerse:
Old Macdonald had a farm, E-I-E-I-O,
And on this farm he had a cow, E-I-E-I-O,
With a moo-moo here and a moo-moo there,
Here a moo there a moo everywhere a moo-moo
Old Macdonald had a farm, E-I-E-I-O.
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Formal Method Syntax
modifiers returnType methodName(parameterDeclarations)
{
statements
}
modifiers
Java keywords that describe the method: public, static, final, …
returnType
Type of value returned: double, int, String, …
A method that returns nothing has a return type of void
methodName
Identifier that names the method: getSize, calcTime, …
parameterDeclarations
List of parameter declarations, separated by commas: int a, String b,
statements
Sequence of statements that define the behaviour of the method.
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return statement
return expression
• return is a Java keyword.
• The expression MUST be the same type as the method’s
return type.
• This statement causes the method to terminate.
• Execution transfers back to the point where the method was
called.
A method with return type of void
• nothing is returned to where the method was called from.
• Method terminated by either:
• single return statement e.g. return;
• at the terminating curly bracket }
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Method Design
Create a method to raise a number by a power
Example: 23 = 8
Analysis
Parameters Type
Base
Exponent
int
int
Return: Result
Value of baseexponent as int
Algorithm Design
Result = base;
Result = result*base; (Repeat exponent - 1 times)
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Method Implementation
public static int power (int base, int exponent)
{
int result = base;
for (int i = 1; i< exponent; i++)
{
result = result*base;
}
return result;
}
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Testing the Method
public static void main (String[] args)
{
int base = 2;
int exp = 3;
int result = power(base, exp);
System.out.print(“Result of raising "+base);
System.out.print(“ to the power “);
System.out.print(exp+" is “);
System.out.print(result + “\n”);
}
Result of raising 2 to the power 3 is 8
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Food for thought!
• Is the power method correct ?
• Can we make assumptions about the parameters
passed to the method ?
• Can we have more than one method with the same name ?
• If the variable passed to the method from the main method
and the parameter declared in the method have the same
name, does the variable in the main method change?
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