A summary of string usage in Java _java

The examples in this article summarize the use of strings in Java. Share to everyone for your reference. The specific analysis is as follows:

The essence of a string is an array of char types, but in Java, all strings declared with double quotes "" are objects of a string class. This is also a manifestation of Java's fully object-oriented language features.

String class

1. A String class object represents a constant string. It is immutable in length. that is, once an instance of a string class is created, the string represented by this instance cannot be changed. Similar to

str = str + "Hello";

This action essentially combines the two string objects of str with "Hello" to create a new string object and then assigns a reference to the new string object to Str. Extensive use of this operation can result in performance flaws. If you need to change the contents of a string frequently, you should use the StringBuffer class or the StringBuilder class. We can use a small program to see how much the performance difference between String and StringBuffer classes is.

Class Stringtest 
{public 
 static void Main (string[] args) 
 { 
  /** 
  * Executes 10 with String objects and StringBuffer objects, respectively) , 000 additional operations
  * Test Execution Time 
  * 2013.3.18 
  /String conststr = ""; 
  Long ltime = System.currenttimemillis (); 
  for (int i = 0; I < 10000 ++i) 
   conststr + = i; 
  System.out.println ("Const String:" + (System.currenttimemillis ()-ltime));
  StringBuilder strbuf = new StringBuilder ("");
  Ltime = System.currenttimemillis (); 
  for (int i = 0; I < 10000 ++i) 
   strbuf.append (string.valueof (i)); 
  System.out.println ("Buffered String:" + (System.currenttimemillis ()-ltime));
 } 

Execution results:

As you can see, the StringBuilder seconds are over, and the string class takes 300ms of time, and there is a huge difference in performance.

Both StringBuilder and stringbuffered represent a variable-length (mutable) string object, and the difference is that the Stringbuffered class does some security work on thread synchronization. And StringBuilder is vice versa. So if you're just programming on a single line, StringBuilder is a little bit more efficient than stringbuffered (the difference is really not big)

2, "Hello World" is a string object
we can use "Hello world" as an object directly, such as:

if ("Hello". Equals ("Hello")) 
   System.out.println ("Yes");

The output result is yes.

3, the string object comparison should use Equals () method, but cannot simply use = = to judge. the Equals () method compares the equality of strings in two objects, because = = compares the two reference names to the same instance.

A common question is how many string objects are generated by the following code fragment altogether?

String str1 = new String ("Hello");
String str2 = new String ("Hello");

Probably most people would answer, 2. But in fact there were 3 string objects. In addition to STR1 and str2, don't forget that "Hello" is a string object, too.

4. String pools (pool)

A string pool is maintained during Java execution. When a string is declared in double quotes, the JVM first checks for the existence of the same string object (the same character) in memory and, if it exists, returns a reference to an object that already exists in memory, and creates a new string object if it does not exist. This can save memory. As in the following code, STR1 and str2 are actually pointing to the same string object.

String str1 = "Hello";
String str2 = "Hello";

5. Receive command line parameters

When we declare the main method, we declare a formal parameter of the string[type. The array of this string object holds the command-line arguments that the user passes in when the program is executed. Note that unlike the C language, command-line arguments in Java start with the first variable after the name of the program. In other words, the string[] data does not include the application name. For example:

Class Strcmd 
{public 
 static void Main (string[] args)
 { 
  if (args.length > 0) 
  {for 
   (String Str:args) 
    System.out.println (str);}} 

The output for executing Java strcmd Hello world! is:

6, the separation of strings

Similar to the C language, the Strtok,string class also has a split method that separates a string in the specified format. The Split method returns an array of String objects, representing each segment of the string that is detached. Such as:

Class Strsplit 
{public 
 static void Main (string[] args)
 { 
  String str = "hello\tworld\ti\tlove\tyou!"; 
  System.out.println ("Original String:" + str);
   
  string[] Strarr = Str.split ("T");
  for (String S:strarr)
   System.out.println (s);
 } 
}

Where the parameter of the split () method can be a regular expression. You can use the static method matches () of the Matcher class in the Java.util package to determine whether a string conforms to a regular expression

There are more powerful features in the string class, and we should learn to read API documentation. The importance of English is embodied here,

I hope this article will help you with your Java programming.