notes.md

Notes (September 16, 2022)

Topics

• Package & Import

General Notes

• Naming Collision conflict is a serious issue in Java i.e happens when multiple people are working with the same classes, etc on a project. This can be overcome using Packages.
• Base class in one package and derived class in the different package is possible.
• Avoid * while importing packages as it will increase the compile time. But it can be used when we don't know the package.
• Protected is not allowed for the class.

Commands

• When using javac -d . {{file.java}} this command the PWD is searched for the source file and then it is compiled and is present under a new p1 folder. Here . search for the source file in all directories in the physical or network location.
• This command dir *.class lists all .class files.

Package & Import

• Problem -> Naming collision conflict ; Solved using -> Packages.
• If a package isn't used then the .class file is created in PWD. When we use Packages .class file is created under a subdirectory.

Syntax:

package <packagenme>;

Example:

package pack1.pack2;

Access Specifier

Table: Access Specifiers in Java

Access Specifier	Same class	Sub class (in pack)	Non-Sub class (in pack)	Subclass (in different pack)	Non-Subclass (in different pack)
Private	Yes	No	No	No	No
Default	Yes	Yes	Yes	No	No
Protected	Yes	Yes	Yes	Yes	No
Public	Yes	Yes	Yes	Yes	Yes

Program 1a: Basic Program using Packages

/* A.java : Java Program using package & import*/
package p1;
public class A{
    int a;
    public A(){
        a=10;
    }
    public String toString(){
        return "a="+a;
    }
}

Program 1b: Basic Program using Packages

/*Test.java*/
import p1.A;
class Test{
        public static void main(String args[]){
                A objA= new A();
                System.out.println(objA.getClass()+"..."+objA);
        }
}

Output:
class p1.A...a=10

Program 2: Basic Program using Packages (Multiple Programs)

Commands:
javac -d . A.java 
javac -d . B.java 

/*A.java: Under p1 folder after executing command*/
package p1;
public class A{
    protected int a;
    protected A(){
        a=10;
    }
    public String toString(){
        return "a="+a;
    }
}

/*B.java: Under p1 folder after executing command*/
package p2;
import p1.A;
public class B extends A{
    int b;
    public B(){
        super();
        b=20;
    }
    public String toString(){
        return "a="+a+"b="+b;
    }
}

/*Test.java: Inside main directory*/
import p2.B;
class Test{
        public static void main(String args[]){
                B objB= new B();
                System.out.println(objB.getClass()+"..."+objB);
        }
}

Output:
class p2.B...a=10b=20