Basic grammars of java for beginners

Running Environment

# run with encoding
java -Dfile.encoding=UTF-8 … com.x.Main
java -Dfile.encoding=EUC-KR … com.x.Main

#########################
# run.sh
#########################
for i in ../lib/*.jar; do
    CLASSPATH="$CLASSPATH":"$i"
done

# Not using package
java -classpath $CLASSPATH:. TestSender

# Using package (class or jar)
java -Xdiag -classpath $CLASSPATH:./test.jar:. com.test.TestSender -f/app/inf/java/cfg/test.json -l/app/inf/java/lib

# Using charset
# java -Dsun.jnu.encoding=UTF-8 -Dfile.encoding=UTF-8 ...

• -Dfile.encoding=UTF-8 : change the default character set to UTF-8

https://www.programmersought.com/article/95251457972/

System.out.println("file.encoding:"+System.getProperty("file.encoding"));  
        System.out.println("sun.jnu.encoding:"+System.getProperty("sun.jnu.encoding")); 

sun.jnu.encoding refers to the default encoding of the operating system, file.encoding refers to the encoding of JAVA files (remember, not class files, all class files are encoded in UTF-8), so, in the same The JAVA application running on the operating system has the same sun.jnu.encoding, and the file.encoding can be encoded differently even in the same JAVA application.

Basic grammar

Expressions

// switch/case
switch(expression) {
  case x:
    // code block
    break;
  case y:
    // code block
    break;
  default:
    // code block
}

// for
String[] temp = { "aa", "bb", "cc" };
for (int i = 0; i < temp .length; i++) { 
    System.out.println(temp[i]);
}

// foreach
String[] temp = { "aa", "bb", "cc" };
for (String el : temp) {
    System.out.println(el);
}

Split, Tokenize

spaces are used to separate words

1. java.util.StringTokenizer : Not used these days
2. java.util.Scanner
3. String[] String.split

String phrase = "the music made   it   hard      to        concentrate";
String delims = "[ ]+";
String[] tokens = phrase.split(delims);

for (int i = 0; i < tokens.length; i++)
  System.out.println(tokens[i]);

Java Exception

try
{
  parseTask.execute(result); // Now we start the task to parse the xml
} catch(Exception ex) {
  Log.e("Exception Message: ", ex.getMessage());
}

URL functions

toString() : Full URL string
getProtocol() : Protocol(scheme) information such as http
getHost() : Host information
getPort() : Port
getPath() : Path (Starts with '/')
getQuery() : Parameter information
getFile() : path + query
getAuthority() : host + port
getRef()

File exist

// if the file does not exist, it occurs FileNotFoundException...
try {
  File file = new File("filename");
  if (file.exist()) {

  } else {
    // when ???
  }
} catch (FileNotFoundException e) {
  // File Not Found
} catch (Exception e) {

}

Delete File

try {
  File file = new File(C.MenuFilePath, C.MenuFileName);
  if(file.exists())
  {
    boolean result = file.delete();
    Log.i(TAG, "File is deleted " + C.MenuFilePath + "/" + C.MenuFileName + " " + result);
    finish();
  }
  else
  {
    Log.e(TAG, "Failed to delete file " + C.MenuFilePath + "/" + C.MenuFileName);
  }
} catch (Exception e) {
  Log.e(TAG, "ERROR:" + e.getMessage());
}

Get hostname

String hname = Runtime.getRuntime().exec("hostname");

// This can be failed. it depends on /etc/hosts
InetAddress.getLocalHost().getHostName();

Remove file path

String fname = "/tmp/test.txt";
int idx = fname.lastIndexOf(File.separator);
String _fileName = fname.substring(idx + 1); // Remove path

Run external programs

    // mv example
    Process process = new ProcessBuilder("mv", filename, targetfile).start();

    /**
     * Gets a string representing the pid of this program - Java VM
     */
    public static String getPid() throws IOException, InterruptedException {

        Vector<String> commands = new Vector<String>();
        commands.add("/bin/bash");
        commands.add("-c");
        commands.add("echo $PPID");
        ProcessBuilder pb = new ProcessBuilder(commands);

        Process pr = pb.start();
        pr.waitFor();
        if (pr.exitValue() == 0) {
            BufferedReader outReader = new BufferedReader(new InputStreamReader(pr.getInputStream()));
            return outReader.readLine().trim();
        } else {
            System.out.println("Error while getting PID");
            return "";
        }
    }