Lecture From Chapter 6 & 7

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Method of Classes

Review

• Overloading Methods• Using objects as parameters• Argument passing• Returning objects• Recursion• Access control• Understanding static• Nested and inner classes• String class

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Overloading Methods• Overloading means more

than two methods with the same class that share the same name.• This method is called

method overloading.• This is one of the ways that

Java implements polymorphism.

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Class overload {void test() {//no argument}void test(int a) {//one argument}void test(int a, int b) {// two arguments

}

Example of four methods

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Overloading Constructors• Constructor is to initialize the values

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Example of three constructors

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Using Objects as parameters• We could also pass

objects to methods

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Example

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Argument passing

• In java, there are two ways of passing arguments, call-by-value and call-by-reference• Call-by-value will pass the value to the method or

subroutine. It will not modify the original data.• Call-by-reference will pass the object to the

method or subroutine. It might modify the original data depending on the method.

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Call-by-value, there is no change in a and b in the lecture65{}, but change in test{}

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Call by object

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Returning the objects• A method can return any type of data.• The return type can be class types or values

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Example of returning an object, tmp

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Recursion

• Recursion is the process of defining something in terms of itself.• It is a method to call itself.• For example,

1, 1, 2, 3, 5, 8, 13 etc.Here, 2 = 1 + 1 n(2) = n(1) + n(0) 3 = 2 + 1 n(3) = n(2) + n(1) 5 = 3 + 2 n(0) =1, n(1) =1

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Example of recursion

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Recursion - factorial

Fact(n) means n*(n -1)*(n – 2)*(n-3)…3*2*1

For example, n = 3, fact(3) = 3*2*1 = 6 n = 4, fact(4) = 4*3*2*1 = 24

The relationship isfact(1) = 1fact(n) = n*fact(n-1)

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Example of recursion, page 169

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Access control

• We can specify which part of program can be accessed. This will prevent misuse. • There are three access specifiers that are used in

Java. They are public, private and protected.• public: that member can be accessed by any other

code• private: member can only be accessed by other

members of its class

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Example – difference between public and private

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Explanation

• As a is defined as default, we can access it.• b is also defined as public, we can also access it.• c is defined as private and we cannot access it, we

have to access it through a method.

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Understand static

• Sometimes, we want to define a class member that will be used independently of any object of that class. We used the definition of static (initial value).• When a member is declared static, it can be

accessed before any objects of its class are created. • main() is declared as static. • We can also declare a static block which gets

executed first (before main()).

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Example

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Explanation

• While loading the program, this program will set i = 1 and j = 2, • it executes the static block to compute the value of

i (4 = 3 + 1) and j (6 = 2 + 4).• It then executes main() and call String show() to

display the value.

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Example – without static

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Example – more

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Nested Classes• We can define a class within

another class.• This class is called nested class.• The scope of a nested class is

bounded by the enclosing class.• If class B is defined within class

A, then B is known to A, but not outside A.

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A

B

More about nested class

• There are two types of nested classes: static and non-static.• A static nested class is one which has the static

modifier applied.• Because it is static, it must access the members of

its enclosing class through an object.• The most important type of nested class is the

inner class.

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Example of inner class

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Explanation

• The program will execute static void main()….• It then create a new object called outer().• Outer() then defines the value outer_val and call

inner class.• The inner class is to display the value of outer_val.

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Example of generating error

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String class

• String is the most commonly used class in Java’s class library.• Every string that we created is an object of type

String.• System.out.println(“I am MSc student.”);• “I am a MSc student” is a String. It can be rewritten

as:• String name =“ I am MSc student.”• System.out.println(name);

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Example

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Summary

• Overloading Methods – two or more methods with the same class• Using objects as parameters – pass object• Argument passing – value and object• Returning objects – return any data type• Recursion – calling itself• Access control – public, private and protected• Understanding static- initial value• Nested and inner classes – a class within a class• String class – each string is an object

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