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public class StringReplaceEmp{
   public static void main(String args[]){
      String str="Hello World";
      System.out.println( str.replace( 'H','W' ) );
      System.out.println( str.replaceFirst("He", "Wa") );
      System.out.println( str.replaceAll("He", "Ha") );
   }
}

Wello World
Wallo World
Hallo World