Lesson14

Lesson Fourteen

Table of Contents

The Lesson
The Program
The Assignment

The Lesson

• In this lesson we are going to learn about some things you can do with Arrays and Strings.
• The Arrays class:
  • The Arrays class is a utility class that is very similar to the Collections class in use except it works with arrays.
  • There are methods for searching and filling as well as copying into new arrays.
  • Some of the special methods that exist are:
    • asList(T[]) which returns a List of type from the array.
      • The type of the list returned is a custom type defined for the Arrays.
      • If you would like a certain type you can usually just construct it and pass in the list returned by this method as a parameter.
      • It will usually copy it in.
    • toString(T[]) which returns a string representation of the array which the normal toString method for the array doesn't do correctly.
  • Just as Arrays can turn an array into a list a list can be turned into an array.
    • Just use the toArray method within a list object.
      • This is not affected by whether it is an ArrayList, LinkedList, or any other type of list.
    • You can pass in nothing in which case an Object[] array is given or you can pass in an array of the correct type to be filled.
      • If the passed in array is not large enough or is null then a new array is created and returned so I usually do this.
    • If I am making many arrays I will create a constant for my class that is null.
      • If it is only once in a while I will just do: list.toArray(new T[0]) where T is the correct type [since arrays can't be generic!].
• varargs:
  • WHAT IS THIS CRAZY WORD?
  • This word represents the idea of passing in multiple parameters in as one array.
  • The syntax for using Varargs is ... after the variable type (String... vars).
  • It can only be done for the final variable in a parameter list as it allows the user to pass in an unlimited amount of variables into the function.
    • If it was allowed to be in the middle parsing the method would be really complicated for the compiler.
  • This is very useful in cases such as printf where it needs to take in as many arguments as you need.
    • So generally it is useful when you want to be able to accept as many things as possible.
  • The variable will be treated as if it was an array and infact an array can be passed in, instead of a bunch of variables.
• Methods on Strings:
  • Strings also have a few important methods that are useful for us to know how to use.
  • .charAt(int)
    • This method returns the character at the selected position. Remember all positions are usually from 0.
  • .contains(String)
    • This method returns true if the string being searched has the specified string within it.
  • .indexOf(String)
    • This method finds the index of the first letter in the specified string within the string being searched.
      • In ababab, .indexOf("bab") would return 1.
  • .endsWith(String)
    • This method returns true if the string being searched ends with the string specified.
      • There is also a .startsWith method that is self explanatory.
  • .lastIndexOf(String)
    • This method works similar to indexOf but instead it finds the final string if there were multiples.
    • It still counts from the left even though it is like it is searching from the right for the first match.
      • In ababab, .lastIndexOf("bab") would return 3 and not 1.
  • .getBytes()
    • This method returns an array of bytes representing each character within the string.
    • It is useful for writing all bytes to a file and I use it for a way of encrypting strings that I can show you.
  • Through the use of a StringBuilder you can also reverse strings as I do in the example program.
  • The String class also has a static method called format which is quite useful.
    • It works like printf except it doesn't print it automatically.

The Program

• In this program I show you how all of the Array and String methods work through examples.
• It is important to note that the arrays created by List.toArray and the lists created by Arrays.toList are not linked to the original which I show.

The Assignment

• Create a method that accepts an unknown amount of Strings and finds out how many start with a vowel and how many start with consonants.
• Then take all of these strings and put them into a list and remove all of the strings that have a consonant as the 4th character (only if it is large enough check).
  • This will require one more method that you may not know from the String class so think about it.
• Then turn the list back into an array and print out the array.