1

Chapter 7 Strings

Oum Saokosal, Head of IT DepartmentNational Polytechnic Institute of Cambodia

Tel: (855)-12-417214E-mail: oum_saokosal@yahoo.com

2

Strings p.257

• Introduction• The String class• The Character class• The StringBuffer class

3

Introduction

• A string is sequence (series) of characters.• A string is NOT an array of characters.

E.g. in C/C++: char s[20];• In Java, A String is an object. • Java has 3 String classes:

String StringBuffer StringTokenizer

4

The String class

5

The String class (1)

• String is in java.lang package.• Since java.lang.* is always imported

automatically, we don’t need to import the String class.

• Declaration:String s1;

• Initialization:s1=“Information Technology”;

• Or, short-cut:String s1=“Information Technology”;

6

The String class (2)

• Because String is a class, then s1 is an object.

• So there should be constructors, methods, or properties.

• String constructors: String() String(String value) String(char[] value)Ex: String s1 = new String(“IT”);

• You just say:String s1 = “IT”;

7

The String class (3)

Summary of String class• Constructors

String()• Methods

charAt() compareTo() concat() endsWidth() equals() getChars() equalsIgnoreCase() getChars() indexOf()

lastIndexOf() regionMatches() length() replace() startsWith() subString() toCharArray() toLowerCase() toString() toUpperCase() trim() copyValueOf() valueOf()

8

The String class (4)

• Q. How do I get those methods to use?• A. You just declare a variable as String.

String s1=“npic”;Then, you call a method, say length(). System.out.println(s1.length());

• Let’s try this out:Public class TestString {public static void main(String[] args){

String s1=“npic”; System.out.println(s1.length());

}}

9

The String class (5)

Note:• The String class is immutable (has no setter

method).• The String class is final so we cannot inherit

from it. //Discuss it in chapter 8

• Lab time: Example 7.1 Checking Palindromes p.266

10

The Character class

11

The Character class

• To declare a variable as a character, use primitive data type: char.Ex: char ch1 = ‘a’;

• But Java provides also Character class. It is useful for Data Structure. Ex: Character ch2 = new Character(‘b’);

Character ch3 = ‘c’;• After define ch2,ch3 as Character, then

these can use methods from Character class. Please see methods on page 268.

• Lab Time: Example 7.2 on page 268

12

The StringBuffer class

13

The StringBuffer class (1)

• StringBuffer class is more flexible than String class. Why?

• Because after creating a variable from StringBuffer class, we can use append, delete, insert etc. very easily.

• Example:StringBuffer sb = new StringBuffer(“NPI”);

sb.append(“C”);

14

The StringBuffer class (2)

• StringBuffer()• append():StringBuffer• capacity():int• charAt():char• delete():StringBuffer• deleteCharAt():StringBuffer• insert():StringBuffer• length():int• replace():StringBuffer• reverse():StringBuffer• setCharAt():void• setLength():void• subString():String

Homework

15

The StringBuffer class (2)

• AppendStringBuffer st = new StringBuffer("H");

st.append('i');

st.append(5);

st.append(".");

st.append("com");

//output: Hi5.com

Please make some note on the code.• StringBuffer provided overloaded methods to

append boolean, char, char[], double, float, int, long, String.

16

Lab: Exercise 1

• Count the number of words in a given StringExample: Input: National Polytechnic Institute of CambodiaOutput: Word Count: 5 words

17

Lab: Exercise 2

• On page 285, Section 7.4 The StringBuffer Class

7.11 (Sorting character in a String) Write a method that returns a sorted string using the following header:

public static String sort(String s)

For example, sort(“bac”) return abc.