String

Lesson plan

• Class & Object• String • Exercise for midterm

Class & Object

• A class definition provides a description of a typical object within that class.An individual object is an instance of a class.

• A class has its behavior (methods) and attributes (fields).

Midterm exam• Date: Monday, February 27, 2005, McGraw 115• Overview:

- Data type- Assignment statement- If, nested if- Switch- Do-while, While, and For

• Format:- Multiple choice questions- Given the code/expressions/statements.., detect the errors- Given the code, determine the results- Identify Class and Object from the code

Class & Object

• Access a dynamic method or field by:<object name>.<method name><object name>.<field name>Example:

df.format(loanAmount). Access a static method by<class name>.<method name>Example:JOptionPane. showInputDialog(null,"Loan

Amount (dollars.cents):");

String

• What is it?– A String is a sequence of

characters that is treated as a single value

– String class handles all operations related to String

– String class is defined in java.lang.

Create a String object

• Syntax:String <variable name>;<variable name>=new String(“constant”);

ORString <variable name>;<variable name> = “constant”

Create a String object

• Example:String strVar;strVar = new String(“CS 172 Course”);

ORString strVar;strVar = “CS 172 Course”;

Internal Representation of a String

Individual characters in a string object are indexed from 0

Compute Length of a string

• Method: length()Returns the length of a string• Example:

String strVar;strVar = new String(“CS 172 Course”);int len = strVar.length();

Substring • Method:

Extract a substring from a given string by specifying the beginning and ending positions

• Example:String strVar, strSubStr;strVar = new String(“CS 172 Course”);strSubStr = strVar.substring(0,6);

strSubStr =“CS 172”

Index position of a substring within another string

• Method: Find an index position of a substring within another string.

• Example:String strVar1 = “CS 172 Course”;String strVar2 = “Course”;int index;

index = strVar1.indexOf(strVar2);

index = 7

Index position of a substring within another string

• Example:String strVar1 = “CS 172 Course”;String strVar2 = “C”;int index;

index = strVar1.indexOf(strVar2);

index = 0

String concatenation

• Method: Create a new string from two strings by concatenating the two strings.

• Example:String strVar1 = “CS 172”;String strVar2 = “Course”;String sumStr;

sumStr = strVar1+strVar2;

Practice exercise

Determine values of some variables given codes

String comparison

Two methods:equalsequalsIgnoreCase

string1.equals(string2)Or

string1.equalsIgnoreCase(string2)

String comparison

Methods:equalsequalsIgnoreCasecompareTo

String string1 =“CS 172”;String string2 = “172”Boolean isEqual;isEqual = string1.equals(string2);

isEqual= false

String comparison

equalsIgnoreCase

Example:

String string1 =“COMPSCI”;String string2 = “compsci”Boolean isEqual;isEqual = string1.equals(string2);

isEqual= true

String comparison

compareTo

Example:

String string1 =“Adam”;String string2 = “Brian”int compareResult;compareResult =

string1.compareTo(string2);

compareResult < 0

String comparison

- string1.compareTo(string2)Compares two strings lexicographically

will return 0 if two strings are equalwill return negative value if string1 is less

than string 2will return positive value if string1 is

greater than string 2

The comparison is based on the Unicode value of each character in the strings

String comparison

The comparison is based on the Unicode value of each character in the strings

let k be the smallest index valid for both strings; compareTo returns the difference of the two character values at position k in the two string -- that is, the value: character at the position k of string 1 – character at the position k of string 2

Character at position k of a stringExample:String sample=“CS 172 Course”;char aChar;

aChar = sample.charAt(3)

aChar=‘1’

Character at position k of a stringExample:String sample=“CS 172 Course”;char aChar;

aChar = sample.charAt(3)

aChar=‘1’

Implicit type of conversion

Implicit type conversion is common:Example:

int num = 172; String s = “CS "+num;

s=“CS 172”

Practice

Determine results of the code

StringBuffer class• A String object is immutable.

Once it is created, we can’t add/delete/modify characters of a String object

• If we need to modify the content of a string directly, we must use StringBuffer class

Create a StringBuffer object

• StringBuffer word = new StringBuffer(“Java”);

word.setCharAt(0,’D’);word.setCharAt(1,’i’);

word = Diva

Delete a substring from a StringBuffer object• StringBuffer word = new

StringBuffer(“CS172 Course”);

word.delete(0,1);

word = “S172 Course”

Append a string

• StringBuffer word = new StringBuffer(“CS172”);

word.append(“ Course”);

word = “CS172 Course”

Insert a string

• StringBuffer word = new StringBuffer(“CS Course”);

word.insert(3,“172 ”);

word = “CS 172 Course”

Convert from StringBuffer to String

StringBuffer word = new StringBuffer(“Java”);

word.setCharAt(0,’D’);word.setCharAt(1,’i’);

System.out.println(word.toString());

Practice

Write a loop that prints out a string in reverse.

For example:input: “CS 172”output: “271 SC”

(This is just an example to show how the output should look like given a specific input value)