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Chapter 6 Java Methods

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Method Syntax

[access specifier][qualifier] return type method name(argument list)

Access specifierpublic – method is accessible from anywhereprivate – restrict access to the class in which the method is

declaredprotected – used with inheritanceDefault – (the access specifier is left off) accessible only from

classes in the same package

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Method Syntax – continued

QualifierStatic makes the method usable by any code that has

access to the method and the class regardless of whether any objects of the class have been created

ArgumentsC++ has two ways to pass arguments – call by value

and call by referenceAll arguments in Java are call by value.

– – methods ALWAYS receive a copy of the argument

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Method Syntax - continued

Arguments● Objects are not passed to methods; references

to objects are passed.● If the argument is an object reference, a copy of

a reference is passed to a method. Therefore, the method can manipulate the object directly.

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3 ways to call a method

●Refer to the method name– int x = square (y);

●Call using an object– System.out.println(“Hello”); //System.out is the

object and println is the method

●Call using the class name (means the qualifier is static)– Integer.parseInt(numberString);

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Scope of Declarations

●Scope of a parameter declaration is the body of the method in which the declaration appears.

●Scope of local variable is the block in which the local variable is declared.

●The scope of a local-variable declaration that appears in a for initialization is the body of the for

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Scope Declarations - continued

●The scope of a method or field of a class is the entire body of the class. (except for static methods)

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Scope Example

public class Scope{

int x=1; public void init()…………..public void local1(){

int x=25;}public void local2(){

x*=10;}

}

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Method Overloading

Method overloading – concept that more than 1 method in a class has the same name but different signatures.

Signature – combination of method name and number and types of arguments

Example of Method Overloadingpublic double square (double value)public int square (int value)

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Method Overloading

Syntax Error Example

public double square (int value)

{ ………}

public int square (int x)

{………}

These two methods have the same signature – results in a syntax error

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Recursive Methods

Recursive method – a method which continues to call itself directly or through another method

Concept: a recursive method is capable of solving only the simplest case of a problem (base case). If the problem is complex, the method divides the problem into two smaller pieces: a base case and a smaller problem

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Recursion Example

long y = factorial(5);

Base case :1! = 1

5! 5 * 4! 4 * 3! 3 * 2! 2 * 1! 1

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Recursion Example

public long factorial (long number)

{if (number <=1)

return 1;

else

return (number * factorial (number – 1));

}

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Recursion

●Avoid recursion in situations requiring performance. Recursive calls take up processor time and memory space.

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Static Methods

●Also called class method– Method that doesn’t depend on the contents of an

object

●Java’s Math class contains a collection of static methods that enable one to perform common mathematical calculations.

●When a method is static, it only can activate static fields and static methods.

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Methods is Math class

●abs(x)●max(x,y)●min(x,y)●pow (x,y) xy

●sqrt (x)●ceil (x) smallest integer not less than x

–ceil (9.2) = 10–ceil (-9.8) = -9

●floor(x) largest integer not greater than x–floor (9.2) = 9–floor (-9.8) = -10

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Methods in Math class

●Math.random()–Returns a double with a value from 0.0 to 1.0–(not including 1.0)

●How to randomly pick an integer from 1 to 6–(int)(Math.random() * 6) + 1

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Random-Number Generation

●Class Random (in java.util)– Can produce random boolean, byte, float, double,

int, long and Gaussian values

●Example:– Random randomNumbers = new Random();– int randomValue = randomNumbers.nextInt(2);– returns 0 or 1

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Fields in Math class

●PI (public final static)●E (public final static)●Static fields are called class variables

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Calling methods in Math

int z = Math.max (x,y);

float d = Math.PI * y;

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●g.setColor(Color.BLUE);●public void fillRect (int x, int y, int width, int height)●public void drawOval(int x, int y, int width, int height)●public void fillOval(int x, int y, int width, int height)

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How to create GUI classes

●Classified into 3 groups–Container classes–Component classes–Helper classes

●Container classes–JFrame, JPanel, JApplet

●Component classes–JButton, JRadioButton, Jmenu, JTextField, etc

●Helper classes–Graphics, Color, Font, LayoutManager

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How to create GUI classes

●Classified into 3 groups–Container classes–Component classes–Helper classes

●Container classes–JFrame, JPanel, JApplet

●Component classes–JButton, JRadioButton, Jmenu, JTextField, etc

●Helper classes–Graphics, Color, Font, LayoutManager

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EndChapter 6 - Methods Presentation