Strings Are Important

• The String class is one of the most important supplied classes with Java.

• Besides int and doubles we use chars (single characters) and Strings (multiple characters) the most.

• Strings are an abstract data type in Java (cf. primitive data types)

java.lang

• They are part of the java.lang package.

• A String is a sequence of characters.

• Because it’s a sequence, it has an order…– that is, a beginning, a middle, and an end.

String sequences

• Each character in a string can be identified by its sequential position.

• This position is a number called the index.• The 1st character in a string has an index of

zero (0).• This is weird but universal in programming

languages.

• Imagine the string “billy”.

My Favourite String

character b i l l y

index 0 1 2 3 4

• Note that billy has 5 characters but that the highest index is 4.

• That’s because the index begins at 0.

• The index of the last character is the length -1.

• This is a big “gotcha”.

Spaces Are Characters Too

• The string “billy billy billy.” is 18 characters long.

• Why?

• Because the spaces count as characters.

• So does the period.

String Methods

• Java supplies numerous methods to deal with strings.

• See p138 in your text.• Lets focus on a couple of key String

methods.• Assume: String phrase = “billy”;• length()

– System.out.println(phrase.length()); prints 5.

String Methods

• toLowerCase() returns a copy of the string with all lower case letters.– System.out.println

(phrase.toLowerCase()); prints billy.

• toUpperCase() returns a copy of the string with all upper case letters.– System.out.println

(phrase.toUpperCase()); prints BILLY.

String Methods

• replaceFirst(String str, String str2) returns a string with the 1st occurrence of str replaced by str2.– System.out.println

(phrase.replaceFirst(“ill”, “obb”)); prints bobby.

– str and str2 do not have to be the same length.

• cf. replaceAll(String str, String str2)

String Methods

• You need to know all the methods on P138 for the test.

• N.B. when we say that a method returns something we mean that the thing which is returned completely replaces the program code that was there before.

• So if you assign the result of a method to a variable it must have the right data type.

You Do.

• P139: Review: AccountSetup

String Methods (comparison)

• Some of the most important methods have to do with comparing strings.

• We cannot compare strings using the standard relational operators (<, <=, ==, >, >=).

• We must use special String comparison methods instead.

• See P140 for a list of important String comparison methods.

• Let’s go over the list together.

You Do

• P141: Review: FormalGreeting