Summary of Java classic usage (II)

Next, the previous article will introduce to you the common methods of Java applications and input and output for your reference. The specific content is as follows

1. Application

1. Use StringBuilder or StringBuffer

 // join(["a", "b", "c"]) -> "a and b and c"String join(List<String> strs) { StringBuilder sb = new StringBuilder(); boolean first = true; for (String s : strs) { if (first) first = false; else sb.append(" and "); sb.append(s); } return sb.toString();}

• Don't use duplicate string concatenation like this: s += item , because its time efficiency is O(n^2).
• When using StringBuilder or StringBuffer, you can use the append() method to add text and use the toString() method to get the connected entire text.
• Priority is given to StringBuilder as it is faster. All methods of StringBuffer are synchronized, and you usually don't need a synchronized method.

2. Generate random integers in a range

 Random rand = new Random(); // Between 1 and 6, includingint diceRoll() { return rand.nextInt(6) + 1;}

• Always use the Java API method to generate a random number in the range of integers.
• Don't try to use Math.abs(rand.nextInt()) %n for these uncertain uses, because its results are biased. In addition, its result value may be negative, such as when rand.nextInt() == Integer.MIN_VALUE.

3. Use Iterator.remove()

 void filter(List<String> list) { for (Iterator<String> iter = list.iterator(); iter.hasNext(); ) { String item = iter.next(); if (...) iter.remove(); }}

The remove() method acts on the recently returned entry of the next() method. Each entry can only use the remove() method once.

4. Return string

 String reverse(String s) { return new StringBuilder(s).reverse().toString();}

This method should probably be added to the Java standard library.

5. Start a thread <br />The following three examples use different methods to accomplish the same thing.

How to implement Runnnable:

 void startAThread0() { new Thread(new MyRunnable()).start();} class MyRunnable implements Runnable { public void run() { ... }}

How to inherit Thread:

 void startAThread1() { new MyThread().start();} class MyThread extends Thread { public void run() { ... }}

How to inherit Thread anonymously:

 void startAThread2() { new Thread() { public void run() { ... } }.start();}

Do not call the run() method directly. The Thread.start() method is always called, which creates a new thread and causes the newly created thread to call run().

6. Use try-finally
I/O stream example:

 void writeStuff() throws IOException { OutputStream out = new FileOutputStream(...); try { out.write(...); } finally { out.close(); }}

Lock example:

 void doWithLock(Lock lock) { lock.acquire(); try { ... } finally { lock.release(); }}

• If the statement before the try fails to run and an exception is thrown, then the finally statement block will not be executed. But no matter what, in this example, there is no need to worry about the release of resources.
• If the statement in the try statement block throws an exception, the program's running will jump to the finally statement block to execute as many statements as possible, and then jump out of this method (unless this method has another peripheral finally statement block).

2. Input/Output

1. Read byte data from the input stream

 InputStream in = (...);try { while (true) { int b = in.read(); if (b == -1) break; (... process b ...) }} finally { in.close();}

The read() method either returns the next number of bytes read from the stream (0 to 255, including 0 and 255), or returns -1 when the end of the stream is reached.

2. Read block data from the input stream

 InputStream in = (...);try { byte[] buf = new byte[100]; while (true) { int n = in.read(buf); if (n == -1) break; (... process buf with offset=0 and length=n ...) }} finally { in.close();}

Remember that the read() method does not necessarily fill the entire buf, so you have to consider the length of the return in the processing logic.

3. Read text from the file

 BufferedReader in = new BufferedReader( new InputStreamReader(new FileInputStream(...), "UTF-8"));try { while (true) { String line = in.readLine(); if (line == null) break; (... process line ...) }} finally { in.close();}

• The creation of the BufferedReader object appears to be very verbose. This is because Java treats bytes and characters as two different concepts (this is different from C).
• You can use any type of InputStream instead of FileInputStream, such as socket.
• BufferedReader.readLine() returns null when the end of the stream is reached.
• To read one character at a time, use the Reader.read() method.
• You can use other character encodings without UTF-8, but it's better not to do so.

4. Write text to the file

 PrintWriter out = new PrintWriter( new OutputStreamWriter(new FileOutputStream(...), "UTF-8"));try { out.print("Hello "); out.print(42); out.println("world!");} finally { out.close();}

• The creation of Printwriter objects appears to be very verbose. This is because Java treats bytes and characters as two different concepts (this is different from C).
• Just like System.out, you can use print() and println() to print multiple types of values.
• You can use other character encodings without UTF-8, but it's better not to do so.

The above is all about this article, I hope it will be helpful to everyone's learning.