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  Java Learning: elegant string
     
  Add Date : 2018-11-21      
         
         
         
  Foreword

Whether we believe that the use of Java language to develop projects or for data analysis and processing, to be applied to the string and associated processing methods. The society has everywhere and string associated shadow: logs, documents, books and so on. Since we can not do without a string, then to get to know about it.

How to construct a string?

In the Java language, there are two ways to create a string object: First, the amount of direct string

1 String message = new String ( "Welcome to Java");
2 String message2 = "Welcome to Java";
Second, through an array of characters.

1 char [] array = { 'G', 'o', 'o', 'd', '', 'D', 'a', 'y'};
2 String message3 = new String (array);
Note: String variables are stored is a reference to a String object, the value of the String object is stored in the string. So strictly speaking, the term String variable, String objects and string values are different, we usually just for simplicity, are collectively referred to as a string only.

Immutable? So pull the string!

String objects are immutable, its contents can not be changed. For example, the following two lines of code:

1 String s = "Java";
2 s = "html";
The first statement creates a String object contents as "Java" and will assign its reference to s. The second statement creates a new String object content "html", and will assign it to s. So, after the implementation of these two statements, two String objects have not changed, and the contents of which have not changed, just the last point to the variable s new String object.

String comparison

Once you see the comparison of the word, we think of "==." However, the operator == can only detect strings String1 and String2 point to the same object, but it does not tell us whether they are the same content. So Java determine which strings are the same, we should use the method equals or compareTo methods.

1 String s1 = new String ( "Welcome to Java");
2 String s2 = "Welcome to Java";
3 String s3 = "Welcome to C ++";
4 String s4 = "welcome TO java";
5 System.out.println (s1.equals (s2));
6 System.out.println (s1.equals (s3));
7 System.out.println (s1.compareTo (s2));


NOTE: When comparing two strings are equal, when the method can be ignored equalsIgnoreCase and compareToIgnoreCase capitalization and compare.

System.out.println (s1.equalsIgnoreCase (s4));
System.out.println (s2.compareToIgnoreCase (s4));


String length, characters, and a combination of strings

length is a method of the String class, but it is an attribute array object, so we need to get the number of characters in the string s, it is necessary to use s.length (). Methods s.charAt (index) can be used to extract a particular character string s, where the subscript index in the range of 0 to s.length () - between 1.

In addition, we can use the concat method to connect two strings. As shown in the following code, the strings s1 and s2 connected to form a s3.

1 String ss = "BaiYi";
2 String ss2 = "ShaoNian";
3 String ss3 = ss.concat (ss2);
4 System.out.println (ss3);


Of course, like we usually prefer a more simple way, that is the direct use of the plus sign (+) to connect two or more strings to form a new string.

How to get a substring of it?

As already mentioned we can get through any string method charAt a single character, but it's a substring should get how to do it? We use the substring method.

1 System.out.println ( "Welcome to Java" .substring (0,11) + "C ++");
String conversion, replacement and separation methods, learn ing ...

toLowerCase: Returns all of the characters are converted to lowercase after the new string

toUpperCase: Returns all characters are converted to uppercase after a new string

Returns a new string to remove whitespace characters at both ends after: trim

replace: Back with a new substring replace this string and it matches all characters the new string

replaceFirst: Returns the substring with a new string to replace the new string after string and it matches the first child

replaceAll: Back with a new substring replaced by the new string after string this string and it matches all sub

split: Returns an array of strings separated by a delimiter substring posed

 1 System.out.println ( "Welcome TO" .toLowerCase ());
 2 System.out.println ( "Welcome to" .toUpperCase ());
 3 System.out.println ( "Welcome to" .trim ());
 4 System.out.println ( "Welcome to" .replace ( 'e', 'A'));
 5 System.out.println ( "Welcome to" .replaceFirst ( "e", "ABC"));
 6 System.out.println ( "Welcome to" .replaceAll ( "e", "AB"));
 7
 . 8 String [] str = "Java # HTML # C ++" split ( "#", 0);
 9 for (int i = 0; i < str.length; i ++) {
10 System.out.println (str [i] + "");
11}
. 12 String [] str2 = "Java # HTML & C ++" split ( "[# &]", 0);
13 for (int i = 0; i < str2.length; i ++) {
14 System.out.println (str2 [i] + "");
15}
     
         
         
         
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