import java.util.Scanner;

public class StringDemo 
{
	public static void main(String[] args) 
	{
		String x = "abc";
		String y = x.concat("Cef").replace('a', 'z');
		System.out.println(y);
	}
}

zbcCef

import java.util.Scanner;

public class StringDemo 
{
	public static void main(String[] args) 
	{
		System.out.println("enter the string = ");
		String a=new Scanner(System.in).nextLine();
		System.out.println(a+" in upper case = "+a.toUpperCase());
		System.out.println(a+" in lower case = "+a.toLowerCase());
	}
}

enter the string =
jtechies
jtechies in upper case = jtechies
jtechies in lower case = jtechies

import java.util.Scanner;

public class StringDemo 
{
	public static void main(String[] args) 
	{
		System.out.print("enter the string = ");
		String a=new Scanner(System.in).nextLine();
		System.out.print(a+" in proper case = ");
		String temp=a;
		String arr[]=a.split(" ");
		for(String z:arr)
		{
			System.out.print(String.valueOf
				(z.charAt(0)).toUpperCase()
				+z.substring(1).toLowerCase());
		}
	}
}

enter the string = jtechies
jtechies in proper case = jtechies

import java.util.Scanner;

public class Test 
{
	public static void main(String[] args) 
	{
		System.out.print("enter the string = ");
		String a=new Scanner(System.in).nextLine();
		System.out.println(a+" split by comma(,)");
		String arr[]=a.split(",");
		for(String z: arr)
		{
			System.out.println(z);
		}
		
		String c=a+" hello";
		System.out.println(c.concat(" bye"));
	}
}

enter the string = jtechies,jtechies
jtechies,jtechies split by comma(,)
jtechies
jtechies
jtechies,jtechies hello bye