Chapter 7:

Strings and Characters

2

Objectives

• The String Class

• String Processing

• The StringBuffer Class

• Program Design and Development: Program Performance Measures and Object-Oriented Technology (OOT)

• Common Programming Errors

3

The String Class• A string literal is a sequence of characters

enclosed in double quotation marks

• A string value is created as an object of:

– String

– StringBuffer

• The main difference between the two classes:

– Strings created as String objects cannot be modified

– StringBuffer objects can be modified

4

Creating a String• Syntax:

String identifier = new String(string-value);

e.g. String s1 = new String(“Yikes!”);

String identifier;

identifier = new String(string-value);

e.g. String s1;

s1 = new String(“Wassup?”);

String identifier = string-value;

e.g. String s1 = “I just love this Java stuff.”;

5

Creating a String (continued)

• Storage for a string is created in memory when a string is created

– Location of where the string is stored is placed in a variable

• Called a reference variable

6

Creating a String (continued)

• When a data value such as a string is created from a class:

– A variable is declared

– An object of the correct data type is created

– The location of the object created in step 2 is stored in the reference variable declared in step 1

7

Constructors

• Instantiating an object is the process

of using the new operator to create an object

• The name of the constructor method must be

the same name as the class

• The String class provides nine different constructors for creating String objects – two of these methods have been depricated.

8

9

String Input and Output

• Output:

– print()

– println()

• Input:

– read()

10

Immutability

• String objects of the String class are always created as immutable objects – i.e. one whose stored values cannot be altered.

• This means that any characters comprising the string are specified when the string is created, and these characters cannot be changed afterwards.

• OK – then what about string concatenation?

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Example

• // create a string and declare its value• String message = “old fart”;• // oops!• String message = message + “hing”;

• The concatenation operator actually creates a new string and changes the location info stored in the reference variable to access the string.

12

Task

• Pages 350 • # 1 & 2