Java@Ch9. String and Text I/O

2010.12.17

Outline

• The String Class• Immutable Strings and Interned Strings• String Comparisons• String Length, Characters, and Combining Strings• Obtaining Substrings• Converting, replaceing, and Splitting Strings• Finding a Character or a Substring in a String• The File Class

[Sample code]

TestFileClass.java

The String Class

A string is a sequence of characters.

The String class has 11 constructors and more than 40 methods.

Declaration:

1. String S = new String(“Hello Java”);

2. String S = “Hello Java”;

3. Char[] C = { ‘H’, ‘e’, ‘l’, ‘l’, ‘o’, ’ ’, ‘J’, ‘a’, ‘v’, ‘a’ };

String S = new String(C);

Immutable Strings and Interned Strings

A string object is immutable; its content cannot be changed.

String object for “Java”

:StringS

String object for “Java”

:StringS

String object for “html”

:String

S = “html”;String S = “Java”;

X

S1.equals(S2): boolean

S1.equalsIgnoreCase(S2): boolean

S1.compareTo(S2): int

S1.compareToIgnoreCase(S2): int...

Java.lang.String

推薦網站 : http://www.tutorialspoint.com/java/java_strings.htm

String Comparisons

( 課本 p.327)

String Length, Characters, and Combining Strings

H e l l o J a v a

0 1 2 3 4 5 6 7 8 9

S.charAt(0)

Java.lang.String

S.length(): intS.charAt(index: int): charS.concat(S2): String

S.length() = 10

S.charAt(9)

A String object is presented using an array internally. ( 課本 p.329)

程式練習 :

1. 宣告一個 String object S1 = “HELLO JAVA ”

2. 宣告一個 String object S2 = “i love Java”

3. 印出 S1 的長度4. 印出 S1 的第 7 個 char 的內容5. 用 equals 比較 S1 和 S2, 並印出 true 或

false

6. 用 concat 將 S1 和 S2 字串連接起來 , 並印出

Obtaining Substrings

Java.lang.String

S.substring(beginIndex: int): StringS.substring(beginIndex: int , endIndex: int): String

H e l l o J a v a

0 1 2 3 4 5 6 7 8 9

S.substring(0,5)

( 課本 p.330)

程式練習 :

使用 substring method,

印出 S1 中的第 0~4 個 characters

Converting, replaceing, and Splitting Strings

Java.lang.String

S.toLowerCase(): String

S.toUpperCase(): String

S.trim(): String

S.replace(oldchar, newchar): String

S.replaceFirst(oldString, newString): String

S.replaceAll(oldString, newString): String

S.split (delimiter: String): String[ ]( 課本 p.331)

程式練習 :

1. 將 S1 轉成 lowercase 印出2. 將 S2 轉成 uppercase 印出3. 將 S1 中的” JAVA” 子字串 換成” C++” 印出4. 嘗試字串分割

String[] names = S1.split(" ");

for(String name:names)

{

System.out.println(name);

}

Finding a Character or a Substring in a String

Java.lang.String

“JAVA”.indexOf(‘J’) return 0.

“JAVA”.indexOf(‘A’) return 1.

“JAVA”.indexOf(‘A’,2) return 3.

“JAVA”.indexOf(”VA”) return 2.

“JAVA”.indexOf(”ABC”,2) return -1.

“JAVA”.lastIndexOf(‘A’) return 3.... ( 課本 p.332)

程式練習 :

從 S1 找出子字串” JAVA” 印出回傳值

The File Class

Create File object.

EX:

java.io.File file = new java.io.File(“abc.txt”);

Writing Data Using PrintWriter.

EX:

PrintWriter output = new PrintWriter(file);

( 課本 p.349)

程式練習 :

練習 p.349 Listing 9.7

WriteData.java

程式範例 :

TestFileClass.java