Java Tutorial 13 – Visibility and Scope

Visibility is how accessible methods and variables to other class is. We’ve used methods from other class in tutorial Multiple Class. Which was possible because all the method we used from other class was public. There are mainly three types of access specifier.

1. Public
   Every Public variable and method is accessible to all other class.
2. Protected
   Protected elements are inheritable, means each child class has all the protected method that parent has and it only accessible between all it’s inheritance tree.
3. Private
   Private elements are accessible only for class itself.

Lets have a cup of Example:

we will create 2 class parent class will have 1 public, 1 protected and 1 private int, and we will try to access these ints in its child class.

Parent:

public class Parent {

    static public int a;

    static protected int b;

    static private int c;

}

Child:

public class Child extends Parent {

    public static void main(String[] args) {

        a = 0;

        b = 0;

        c = 0;  // this line give you error

    }

}

We do not need to run it, just type it and when you mansion c=0 it will give you error because child can not inherit private elements from parent and so Child class doesn’t know the int c exist.

Scope:

Scope means where the method or variable is accessible in the same class.

for example if u define an int inside a for loop, this int is not accessible out side the for loop and Same thing is true for any method also.

if you have any variable inside a method than its only accessible inside it.

In short we can say elements are accessible between {…} inside which they are declared.

So, if you declare variable or method inside class ( outside other methods ) it’s accessible between {..} of class means inside whole class and so it’s called Global declaration.

Some Examples:

1) Global declaration

public class yash{

int a;           // accessible inside all methods of class

int b;           // accessible inside all methods of class

method1()

…..

…..

method10()

}

2) inside Method which is not accessible to other method

public class yash{

method1(){

int a;           // accessible only for method1

int b;           // accessible only for method1

}

…..

…..

method10()

}

I hope you cleared if not merriment declaring and using variables different places in class.

Doubts are invited.

have a nice day.

Peace.

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Java Tutorial 12 – Constructors

constructors are a special type of method which we can use to initialize Objects when we create it. If u do not use constructor, objects contains all the variables and methods, But Using constructor you can initialize values of variables. For example you can initialize some variables to zero or Strings to default values and etc. Lets Clear this with an example. Create two classes Tom and Jerry.

Tom:

public class Tom {

public static void main(String[] args){
Jerry object = new Jerry();
object.print();
}
}

Jerry:

public class Jerry {
int a;
String b;

public void print(){
System.out.println(“int a = “+a);
System.out.println(“String b = “+b);
}
}

Now if u run the Tom.java you will see the output like:

int a = 0
String b = null

Now if you want to initialize String b to some text say “Hi” and int a to value 5 whenever the new object is created, you can do it by creating constructor. There are some rules for it:

• Name of the constructor must be same as name of class.
• there is no other rule 😛

Now lets create constructor for Jerry class and initialize variables.

Jerry:

public class Jerry {
int a;
String b;

Jerry(){
a=5;
b = “Hi”;
}
public void print(){
System.out.println(“int a = “+a);
System.out.println(“String b = “+b);
}
}

 

!!!!! yap writing return type and access modifiers is optional. We can define Constructor by just Jerry(){ … }.

Now if you run the Tom.java you will see the output like:

int a = 5
String b = Hi

Here constructor initialized both variables when we created an object of Jerry.

catch you again later…

doubts are invited.

have a nice day.

Peace.

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