This are various refined descriptions those may help you while learning Java . Moreover the discussion is largely related to the practical approach...be in touch to learn more friends........
Class: keyword to create a template that describes the kinds of state and behavior that objects of its type support.
New: it is a keyword that creates the object of the desired class by taking memory from the heap.
Object: the object is the instance of the class, that object will have its own state, and access to all of the behaviors defined by its class.
State: it is the set of instance variable that is described by the class. the value assigned to these instance variables make up the object’s state.
Behavior: it is the class logic where algorithms are executed and the data get manipulated.
Inheritance (extends): defining the behavior of super class in the sub class.
Interface (implements): A 100 % abstract super class that defines a method a subclass must support . Its main purpose is to unattached the method definition from the hierarchy .
Source files declaration rules:
1. There can be only one public class.
2. If there is a public class then the name of the file(.java ) must match the name of the public class.
3. If the class is the part of the package then then package statement must be the first line in a source code file.
4. The file can have more than one non public class.
5. Files with no public class can have a name that does not match any match any of the classes in the file.
Modifiers:
Access modifiers: public protected and private.
Access modifiers cannot be applied the local variables.(compile but no use) A class cannot be protected or private. there are four access controls but there are only three access modifiers. The forth is default(package access) which we get if we does not use any.Non access modifiers: strictfp, final, abstract, transient, synchronized, native, static.(s3atn).
local variable can be final..
Access means:
§ Whether method code in one class can access a member of another class.
1. Generally using dot operator.(class.method());{achually Constracter.method (this,super can also be used)}.
§ Whether a subclass can inherit a member of its superclass .
1. When a subclass wants to access a member of the superclass . { here method can also be understood as this.method(),instead of simplely method()
Class access:
If class A has access to another class B then it means:-
Class A can:
1.create an instance of class B .
2. Extend class B.
3. Access certain methods and variables in the class B.depending on the Access controls of these methods and variables.
Default Access: default access is a package level access.
That is a class in a package can:
· Create instance
· Declare variable
· Return type
Of the another class in a same package with default Access control.
PUBLIC ACCESS: A class declared with public keyword can be accessed by the any class in the java universe (ju).
Þ Class must be declared public so that it is visible to the java universe
Þ Method to be invoked must be declared public .
Þ Constructor should also be declared public if you want to make a instance of the class in some other class.
Þ Always remember to import the class firstly.
Protected Access: protected member can be accessed (through inheritance) by a subclass even if subclass is in a different peckage.Protected means the variable can be accessible to all other classes inside the same package as well as inheritable by any subclass outside the package.
PROTECTED=DEFAULT+INHERITANCE
¤ import the class .
¤ make the imported class as extended class(inherited )
¤ any subclass can access it through inheritance.
¤ We cannot access the protected member through a reference to an instance of the class.
¤ The protected member can be accessed by only the subclass and for any other class(like subclass of the actual subclass it act like the private package.
Private Access: private means access in the declaring class only.
o When a member is declared private a subclass cannot inherit it (hence use it ).
o Private method cannot be overwridden.
o Method with same name can be declared in the subclass but it is not the overridden method.
(nonAccess) modifiers:
Strictfp: keyword to modify class and method.
Final: (keyword) it tells that the class cannot be subclassed.that is no other class can ever extend a final class.
Abstract: it is just creating a imaginary class whose methods are defined
Ø If even a single method is abstract then whole class must be declared abstract.
Ø An abstract class can contain a concrete method.
Ø An abstract class can never be instantiated.
Ø Its sole purpose is to be extended(subclassed).
Ø It can be compiled unless you don’t make an instance.
Ø It can be executed unless you don’t make an instance.
Ø The method marked abstract ends in the semicolon rather than curly braces.
Ø A class cannot be both abstract and final.
Final:
Ø Prevent a method for being overwridden in a subclass.
Ø Formal parameter can also be declared as final to prevent it to be modified by the method.
Interface:
it tells what a class must do.it is just used to unattached the methods from hierarchy .
that is suppose that there are some behaviours /methods (like actionPerformed) which can be in various different classes. The definations of such methods is declared in a 100% abstract class .
v All interface method must be implemented,
v All interface method must be marked public(when overridden in subclass).
v Interface can have only abstract method.
v All interface methods are implicitly public and abstract(we don’t need to type public and abstract while declaration).
v All interface variables are implicitly public ,static and final( we don’t need to type public static and final while decreration).
v Interface can declare only constants,non instance variable( public,static final).
v An interface can extends(not implements) one or more other interfaces.
v Constants in the interface (public static final) is same as #define in c..
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