replace character in string java

public static void main(String args[]){  
String s1="my name is khan my name is java";  
String replaceString=s1.replace("is","was");//replaces all occurrences of "is" to "was"  
System.out.println(replaceString);  
}}  

public class JavaExample{
   public static void main(String args[]){
	String str = new String("Site is BeginnersBook.com");

	System.out.print("String after replacing com with net :" );
	System.out.println(str.replaceFirst("com", "net"));

	System.out.print("String after replacing Site name:" );
	System.out.println(str.replaceFirst("Beginners(.*)", "XYZ.com"));
   }
}