Simple Java Programs Covering Many Basic Concepts :
1.JavaPractise.java
package JavaBasics;
public class JavaPractise {
public static void main(String[] args)
{
// Switch and Do While
int a=20;
switch(a)
{
case 19: System.out.println("This is 19");
break;
case 20: case 21: System.out.println("This is 20 ");
break;
default : System.out.println("This is default");
}
do {
System.out.println("welcome to do while");
a++;
} while(a<23);
// Arrays Examples
int[] arr={23,34,45,55};
System.out.println(arr[3]);
String[] str={"india","welcome","java"};
System.out.println(str[0]);
int arr1[]=new int[4];
arr1[2]=22;
System.out.println(arr1[2]+ " " + arr1[0]);
String[] s=new String[6];
s[1]="java";
System.out.println(s[1]+" "+s[0]);
}
}
Output:
This is 20
welcome to do while
welcome to do while
welcome to do while
55
india
22 0
java null
2.StringClass.java
package JavaBasics;
public class StringClass {
public static void main(String[] args)
{
char ch='a'; char ch1='c';
int index,index1,index2;
String str="Welcome";
String str2="me";
index=str.indexOf('a');
index1=str.indexOf('c');
index2=str.indexOf(str2);
System.out.println(str.toUpperCase());
System.out.println("the not found index is "+index);
System.out.println("the char index is "+index1);
System.out.println("the string index is "+index2);
String dot=".com";
String email="meme@me.com";
if(email.endsWith(dot))
{
System.out.println("Valid email address");
}
String sub=email.substring(5,7);
System.out.println(sub);
// int x=23; int y=34;
String s1="abcd";
String s2="Abcd";
if(s1.equals(s2))
{
System.out.println("both are equal");
}
if(s1.contains("ab"))
{
System.out.println("the given sub string is present");
}
char c1=s1.charAt(0);
System.out.println(c1);
String trim=" Welcome to Java ";
System.out.println(trim);
String aftertrim=trim.trim();
System.out.println(aftertrim);
String replace=trim.replace("Java","C++");
System.out.println(replace);
}
}
Output:
WELCOME
the not found index is -1
the char index is 3
the string index is 5
Valid email address
me
the given sub string is present
a
Welcome to Java
Welcome to Java
Welcome to C++
3.StaticPublic.java
package JavaBasics;
public class StaticPublic {
static int a=5;
int b;
int c=5;
StaticPublic()
{
a++;
System.out.println(a);
}
static {
System.out.println("This will execute before the main method");
}
static int myMethod()
{
return ++a;
}
public static void main(String[] args)
{
StaticPublic obj=new StaticPublic();
// System.out.println(b); Static method can not use non static members directly
System.out.println("does this execute "+a+" and "+obj.b+obj.c);
System.out.println("test"+a+obj.c+"test");
System.out.println(a+obj.c);
StaticPublic obj1=new StaticPublic();
StaticPublic obj2=new StaticPublic();
int r= StaticPublic.myMethod();
System.out.println("Static method return value "+r);
}
}
Output:
This will execute before the main method
6
does this execute 6 and 05
test65test
11
7
8
Static method return value 9
4.Broom.java
package JavaBasics;
class Cover
{
static class InnerCover
{
void go()
{
System.out.println("I am the first Static Inner Class");
}
}
}
class Broom
{
static class InnerBroom
{
void go1()
{
System.out.println("I am second Static Inner Class");
}
}
public static void main(String[] args)
{
// You have to use the names of both the classes
Cover.InnerCover demo = new Cover.InnerCover();
demo.go();
// Here we are accessing the enclosed class
InnerBroom innerBroom = new InnerBroom ();
innerBroom .go1();
}
}
Output:
I am the first Static Inner Class
I am second Static Inner Class
5.ArrayListCollections.java
package JavaBasics;
import java.util.*;
public class ArrayListCollections {
// use an ArrayList when you're not sure how many elements are going to be in a list of items.
public static void main(String[] args)
{
ArrayList ls=new ArrayList();
ls.add("book");
ls.add("pen");
ls.add("pc");
// To go through each item in your ArrayList you can set up something called an Iterator.
// This class can also be found in the java.util library
Iterator ir=ls.iterator();
while(ir.hasNext())
{
System.out.println(ir.next());
}
System.out.println("whole list is: "+ls);
ls.remove(1);
System.out.println("whole list is: "+ls);
ls.remove("book");
System.out.println("whole list is: "+ls);
System.out.println("Single item is: "+ls.get(0));
}
}
Output:
book
pen
pc
whole list is: [book, pen, pc]
whole list is: [book, pc]
whole list is: [pc]
Single item is: pc
6.InheritanceOverriding.java
package JavaBasics;
class A
{
int x;
public void method(int a)
{
System.out.println("This is in class A"+a);
}
public void sample()
{
System.out.println("This is sample method");
}
}
class B extends A
{
public void method(int b)
{
x=6;
System.out.println("This is in class B "+b+ "and x is "+x);
}
}
public class InheritanceOverriding
{
public static void main(String[] args)
{
B obj=new B();
obj.method(8);
obj.sample();
}
}
Output:
This is in class B 8and x is 6
This is sample method
7.ReadTextFile.java
package JavaBasics;
import java.io.IOException;
import java.io.FileReader;
import java.io.BufferedReader;
import java.io.*;
public class ReadTextFile
{
public int nooflines() throws IOException
{
FileReader fr=new FileReader("D:\\sample.txt");
BufferedReader br=new BufferedReader(fr);
String line1;
int i=0;
while((line1=br.readLine())!=null)
{
i++;
}
br.close();
return i;
}
public void writeFile() throws IOException
{
try {
FileWriter fw=new FileWriter("D:\\file2.txt");
BufferedWriter bw=new BufferedWriter(fw);
fw.write("I am writing this line to file");
fw.append("This is for appending further");
fw.close();
}
catch(Exception e)
{
System.out.println("Error Message"+e.getMessage());
}
}
public static void main(String[] args) throws IOException
{
FileReader fr=new FileReader("D:\\sample.txt");
BufferedReader br=new BufferedReader(fr);
String line1=br.readLine();
System.out.println(line1);
String str[]=new String[10];
for(int i=0;i<4;i++)
{
str[i]=br.readLine();
}
System.out.println(str.length);
for(int j=0;j<str.length;j++)
{
System.out.println(str[j]);
}
br.close();
ReadTextFile n=new ReadTextFile();
int number=n.nooflines();
System.out.println("the number of lines in the text file is "+number);
n.writeFile();
}
}
Output:
abcd
10
bcde
cdef
null
null
null
null
null
null
null
null
the number of lines in the text file is 3
8.ThisJavaProgram.java
package JavaBasics;
/*
1.this keyword is used to refer current class instance variables
2.Used to call current class constructor and this() should put at the beginning
*/
public class ThisJavaProgram
{
int a;
double b;
ThisJavaProgram(int a,double b)
{
this();
this.b=b;
}
ThisJavaProgram()
{
System.out.println("default constructor");
}
public void display()
{
System.out.println(a+ " "+b);
}
public void test()
{
display();
}
public static void main(String[] args)
{
ThisJavaProgram obj=new ThisJavaProgram(20,30);
obj.display();
obj.test();
}
}
Output:
default constructor
0 30.0
0 30.0
9.SuperClass.java
package JavaBasics;
/*
1.super is used to refer to the parent class instance variable
2.super() is used to invoke parent class constructors
3.super is used to invoke immediate parent class methods
*/
class ParentClass
{
int a=10;
ParentClass(int x)
{
System.out.println("This is ParentClass Constructor "+x);
}
public void display()
{
System.out.println("This is ParentClass Method");
}
}
class BaseClass extends ParentClass
{
int a=20;
BaseClass()
{
super(5);
System.out.println("This is Child Class Constuctor");
System.out.println("The value of a in ParentClass is "+super.a);
super.display();
}
}
public class SuperClass
{
public static void main(String[] args)
{
BaseClass obj=new BaseClass();
}
}
Output:
This is ParentClass Constructor 5
This is Child Class Constuctor
The value of a in ParentClass is 10
This is ParentClass Method
10. (a) OnlyClass.java
package JavaBasics;
public class OnlyClass
{
public int a;
public String str;
public void myMethod()
{
a=20;
str="india";
System.out.println(a+" "+str);
}
}
(b) AnotherClass.java
package JavaBasics;
public class AnotherClass {
public String myMethod2()
{
return "java";
}
public static void main(String[] args)
{
OnlyClass obj=new OnlyClass();
// Since it is in the same package, import is not required
obj.myMethod();
AnotherClass obj2=new AnotherClass();
System.out.println(obj2.myMethod2());
}
}
Output:
20 india
java
