[Java Basics] Stack, Heap, Constructor, I/O, Immutable, ClassLoader

Good about Java:

friendly syntax, memory management[GC can collect unreferenced memory resources], object-oriented features, portability.

Stack

Stores method invocations, local variables(include object reference, but the object itself is still stored in heap).

If you look at the stack trace, the top method is the one being called, and once it is finished, it will be removed out of the stack.

Heap

Stores all objects(include instance variables in the class).

.hashcode() returns the memory address of the object, so no same hashcode for two objects.

But when the Class is value object, you could override .hashcode() and .equals().

Constructor

Default parameterless constructor: only generated by compiler when there is no your constructor.

Parent constructor: if parent class has default constructor, compiler will automatically call it without .super(). But if not, or you want to call the parent constructor with parameters, you need to call .super() explicitly.

Overloaded constructor: use .this(...) at the first line. 

File I/O

There are two streams: Character stream & Binary stream

Character stream: to read/write human readable files, like .txt .xml .html

Class: FileReader/Writer[extends InputStreamReader, 缺点：使用系统默认字符字节编码方式], more advanced: BufferedReader/Writer

Binary stream: to read/write machine readable files, like .exe .class .obj

Class: FileInput/OutputStream, more advanced for Object: ObjectInput/OutpoutStream(.writeObject() method).

FileInputStream是最基本的，只能读出来Byte，而Wrapper class DataInputStream高级一些，可以读出来各种primitive types。ObjectInputStream另有readObject()。如果你想读出来String, 那么使用InputStreamReader[优点：可以指定字符字节编码方式].InputStreamReader (and its father Reader) are used specifically to deal with characters (so strings) so they handle charset encodings (utf8, iso-8859-1, and so on) gracefully.

Example:

// 读取字节转换成字符
 String charset = "US-ASCII";
 FileInputStream inputStream = new FileInputStream(file); 
 InputStreamReader reader = new InputStreamReader(inputStream, charset); 
 StringBuffer buffer = new StringBuffer(); 
 char[] buf = new char[64]; 
 int count = 0; 
 try { 
    while ((count = reader.read(buf)) != -1) { 
        buffer.append(buffer, 0, count); 
    } 
 } finally { 
    reader.close(); 
 }

读写file要注意的: new FileReader(filename)会throw FileNotFoundException(extends IOException)，bufferedReader.readLine()会throw IOException，最后别忘记bufferedReader.close()。读写file更好的方法是用InputStreamReader(new FileInputStream(filename))，因为FileReader does not allow you to specify an encoding and instead uses the plaform default encoding, which makes it pretty much useless as using it will result in corrupted data when the code is run on systems with different platform default encodings. 

Immutable Class

Instance could not be modified. 这就保证了当前线程用到的immutable object不会被其它线程修改。

How to: private final fields, final class, no setter methods, ensure exlusive access to mutable components.

String

StringBuilder不是thread-safe的，而StringBuffer由于有synchronized methods所以是thread-safe的。

ClassLoader

JVM有Bootstrap(核心类库), Extensions(扩展类库), System Class Loaders，App classloader(CLASSPATH类)，会load很多classes before loading包含main()的class。

程序的开始是main(String[] args), 由此开始由Class loader来load用到的class。

一个classloader只能load同一个class一次，而同一个class可以由不同的classloaders来load。

A software library is a collection of related object code. In the Java language, libraries are typically packaged in JAR files. Libraries can contain objects of different types. The most important type of object contained in a Jar file is a Java class. A class can be thought of as a named unit of code. The class loader is responsible for locating libraries, reading their contents, and loading the classes contained within the libraries. This loading is typically done "on demand", in that it does not occur until the class is actually used by the program. A class with a given name can only be loaded once by a given classloader.

Even building a JAR file from the class files doesn't change this -- the JAR is just a container for the class files.

The initial set of class loads are all triggered by the attempt to load the 起始class. These are the core classes that are used by every Java program, no matter how small. 然后开始load 当前class用到的class, recursively.

A lot happens inside the JVM when a class is loaded and initialized, including decoding the binary class format, checking compatibility with other classes, verifying the sequence of bytecode operations, and finally constructing a java.lang.Class instance to represent the new class.

Sockets

In UDP, as you have read above, every time you send a datagram, you have to send the local descriptor and the socket address of the receiving socket along with it. Since TCP is a connection-oriented protocol, on the other hand, a connection must be established before communications between the pair of sockets start. So there is a connection setup time in TCP.

Client:

Socket MyClient;
    try {
           MyClient = new Socket("Machine name", PortNumber);
    }
    catch (IOException e) {
        System.out.println(e);
    }

Server:

ServerSocket MyService;
    try {
       MyServerice = new ServerSocket(PortNumber);
        }
        catch (IOException e) {
           System.out.println(e);
        }

Socket clientSocket = null;
    try {
       serviceSocket = MyService.accept();
        }
    catch (IOException e) {
       System.out.println(e);
    }

Input at Client side:

DataInputStream input;
    try {
       input = new DataInputStream(MyClient.getInputStream());
    }
    catch (IOException e) {
       System.out.println(e);
    }

Input at Server side:

DataInputStream input;
    try {
       input = new DataInputStream(serviceSocket.getInputStream());
    }
    catch (IOException e) {
       System.out.println(e);
    }

Output at Client side:

PrintStream output;
    try {
       output = new PrintStream(MyClient.getOutputStream());
    }
    catch (IOException e) {
       System.out.println(e);
    }

Passing parameter

Java is always passing by value. Value means copy of primitive variable, or copy of object reference[pointing to the same object].

So the original variable is not changed, in the method there will be a copy, but if you set the object, it will change the object content.

Why arguments passed to an anonymous inner class have to be final?

如果你pass local variables to anonymous class, 那么当当前method退出的时候这些local var会被stack清除，而你的anonymous class object如果再想access这些fields就会报错。所以，如果你把这些local var定义为final，那么JVM compiler就把它们变成了constants，之后再access这些fields就不会有问题。

 Boxing & Unboxing

Why do we need wrappers for primitive types? 

So that a null value is possible, to include in a collection...

Boxing: auto from primitive to Wrapper class. Unboxing: vice versa, but can throw NullPointerException, if Integer i = null; int x = i;

 Public methods of java.land.Object class

.toString(), .hashCode(), .equals(), .clone(), .wait(), .notify(), .notifyAll(), .getClass()

What problems can be encountered when only overriding .equals() and not .hashcode()?

x.equals(y) does not mean they have same hashcode[default implementation], then it is not possible to use this type as a key in hashmap: because hashmap will first call .hashcode() and then .equals() when determining if the key is present in the map. 

所以实际中你override了.equals()以后，一定要override .hashcode()，用同一套fields来calculate both methods.