java day 7 examples

Author: javaiskool

java/day7/day7.txt

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+* relationships between classes
+real life scenario we have to handle multiple classes that exists seperately/individually and are related to each other.
+it implements reusability 
+
+classes communicate with each other through their objects.
+
+3 types of relationship between the classes
+** agreegation - object reference variable  of one class is member variable of another class
+"has a" relationship
+
+order - food
+emp - gadget
+student - study_material
+library - books
+dept - emp
+
+
+** association
+Association
+in association the classes communicate through methods. i.e. for functionality of one class it can use object of another class
+
+"uses a" relationship
+
+** aggregation VS association
+emp has gadget          aggregation
+salary uses employee    association
+
+order has food  aggregation
+bill uses order association
+
+
+** inheritance => one of the pillars of OOP
+"is type of" relationship
+parent - child relationship
+child class inherits non-private properties of parent class
+parent class => base class/ super class
+child class => derived class/ sub class
+In java inheriting properties from only one parent class is supported.
+all parts of parent class that are non-private properties including member varibles, constructors, methods of parent class are directly accessible in child class as if these are its own properties.
+
+* to implement inheritance betwwen classes extends keyword is used
+
+* non-private members are directly accessible in child class. private members are accessible through non-private methods of parent class
+
+* a child class can inherit properties of only one class 
+
+* parent class do not have access to child class
+
+* when an object of child class is created , by default, parameterless constructor of parent class is called followed by execution of child class constructor.
+	to call parameterized constructor of parent class 
+		use super keyword with appropriate parameters.
+		call to parameterized constructor of parent class should be the first statement in the child class constructor.
+
+* if static block is present in parent and/or child class then the sequence of execution at the time of child class object creation is
+	static block of parent class
+	static block of child class
+	constructor of parent class
+	constructor of child class
+
+ * convention
+according to OOP when inheritance is used, then object of parent class is very rarely used. properties and functionalities of parent class are used through object of child class.
+
+ 
+* types of inheritance
+# single P - C
+one super class one sub class
+Emp(eno,ename,sal) - Programmer (skill)
+
+# hierarchical 
+P - C1, P - C2 => P - C1,C2
+one superclass, 2 or more sub classes
+Emp(eno,ename,sal) - Programmer (skill), Tester (type)
+
+# multilevel 
+P-C-G
+one superclass one subclass , subclass - another subclass
+Emp(eno,ename,sal) - Programmer (skill)-TeamLead (teanSize)
+
+#multiple inheritance
+P1,P2 - C not supported in JAVA for classes
+
+
+
+
+

java/day7/inh1.java

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+class Parent
+{
+	private int pv;
+	protected int pt;
+	int pk;
+	public int pb;
+
+	public Parent()
+	{ 
+		this.pv=1;
+		this.pt=2;
+		this.pk=3;
+		this.pb=4;
+	}
+	public int getPV(){return pv;}
+	public void displayParent()
+	{
+		System.out.println("private pv = "+this.pv);
+		System.out.println("protected pt = "+this.pt);
+		System.out.println("default/package pk = "+this.pk);
+		System.out.println("public pb = "+this.pb);
+	}
+	
+}
+class Child extends Parent
+{
+	private int y;
+	public Child(){this.y=100;}
+	public void dispY(){System.out.println(" y = "+this.y);}
+	public void displayAll()
+	{
+		//System.out.println("private pv = "+this.pv); // only non-private properties of parent class are directly accessible
+		System.out.println("private pv = "+this.getPV()); // private properties of parent class are accessible through non-private methods of parent class
+		System.out.println("protected pt = "+this.pt);
+		System.out.println("default/package pk = "+this.pk);
+		System.out.println("public pb = "+this.pb);
+		System.out.println(" y = "+this.y);
+	}
+	
+}
+class Demo
+{
+	public static void main(String[] args)
+	{
+		Parent p1=new Parent();
+		p1.displayParent();
+		//p1.dispY(); // properties of child class are not accessible to parent class
+		Child ch1=new Child();
+		ch1.displayParent();
+		ch1.dispY();
+		ch1.displayAll();
+		
+
+	}
+}
+

java/day7/inh2.java

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+// inheritance demo  example 2 constructors 
+
+class Parent
+{
+	private int pv;
+	
+	public Parent()
+	{ 
+		System.out.println("parameterless constructor of parent class");
+		this.pv=1;
+		
+	}
+	public Parent(int pv)
+	{ 
+		System.out.println("parameterised constructor of parent class");
+		this.pv=pv;
+		
+	}
+
+	public int getPV(){return pv;}
+	public void setPV(int pv){this.pv=pv;}
+
+	public void displayParent()
+	{
+		System.out.println("private pv = "+this.pv);
+	}
+	
+}
+class Child extends Parent
+{
+	private int y;
+	public Child()
+	{
+		System.out.println("constructor of child class");
+		this.y=100;
+	}
+	public void dispY()
+	{
+		System.out.println(" y = "+this.y);
+	}
+}
+class Demo
+{
+	public static void main(String[] args)
+	{
+		Child ch1=new Child();
+		 
+	}
+}
+

java/day7/inh3.java

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+// inheritance demo  example 3 parameterized constructor of parent
+
+class Parent
+{
+	private int pv;
+	
+	public Parent()
+	{ 
+		System.out.println("parameterless constructor of parent class");
+		this.pv=1;
+		
+	}
+	public Parent(int pv)
+	{ 
+		System.out.println("parameterised constructor of parent class");
+		this.pv=pv;
+		
+	}
+
+	public int getPV(){return pv;}
+	public void setPV(int pv){this.pv=pv;}
+
+	public void displayParent()
+	{
+		System.out.println("private pv = "+this.pv);
+	}
+	
+}
+class Child extends Parent
+{
+	private int y;
+	public Child()
+	{
+		//super(555); // to call parameterized constructor of parent class 
+		System.out.println("constructor of child class");
+		this.y=100;
+		super(444);
+	}
+	public void display()
+	{
+		System.out.println(" pv = "+this.getPV());
+		System.out.println(" y = "+this.y);
+	}
+}
+class Demo
+{
+	public static void main(String[] args)
+	{
+		Child ch1=new Child();
+		ch1.display();
+		 
+	}
+}
+

java/day7/inh4.java

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+// inheritance demo  example 4 static block
+
+class Parent
+{
+	private int pv;
+	private static int cnt;
+	
+	static
+	{
+		System.out.println("static block of parent class");
+		cnt=10001;
+	}
+	public Parent()
+	{ 
+		System.out.println("parameterless constructor of parent class");
+		this.pv=1;
+		
+	}
+	public Parent(int pv)
+	{ 
+		System.out.println("parameterised constructor of parent class");
+		this.pv=pv;
+		
+	}
+
+	public int getPV(){return pv;}
+	public void setPV(int pv){this.pv=pv;}
+
+	public void displayParent()
+	{
+		System.out.println("private pv = "+this.pv);
+	}
+	
+}
+class Child extends Parent
+{
+	private int y;
+	
+	static
+	{
+		System.out.println("static block of child class");
+	}
+	public Child()
+	{
+		super(555); // to call parameterized constructor of parent class 
+		System.out.println("constructor of child class");
+		this.y=100;
+		// super(444); // call to super should be the first statement
+
+	}
+	public void display()
+	{
+		System.out.println(" pv = "+this.getPV());
+		System.out.println(" y = "+this.y);
+	}
+}
+class Demo
+{
+	static
+	{
+		System.out.println("static block of demo class");
+		 
+	}
+	public static void main(String[] args)
+	{
+		Child ch1=new Child();
+		ch1.display();
+		 
+	}
+}
+