Some past notes on JAVA

JAVA doc

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/** 
     * @Title Method main 
     * @Description This method handles the main process, demonstrates go() and initialize GUI. 
     * @Param args Unused. 
     * @Return Nothing. 
     * @update 2021-04-10      
     */ 

Multi-threading

• Putting a Thread to Sleep
  • Can be used to make threas take turns
  • sleep will force thread to leave running state and allow another thread to run
  • Sleeping thread will always go back to runnable state after wake up

Concurrency problem

• May occur when 2 or more threas have access to same object on the heap
  • race condition -> Possibly Data corruption

Synchronization

• To solve concurrency problem
  • Making method “atomic”
  • Use keyword “synchronized” to modify a method so that only 1 thread at a time can access it

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private synchronized void methodA(){
    // ...
}

• To protect data, synchronize the methods that act on that data

Life cycle of object 

• Stack
  • Methods to be run 
  • LIFO 
  • Local Variable lives in Stack 
• Heap 
  • All alive objects 
  • Objects live in Heap 
  • Instance Variable lives in Heap 

Alive Vs. In scope 

• Alive = Not destroyed 
• In scope = Within current running method 

Lifetime: 

Instance variable 

• Live and die with object  Object 
• Alive when live references exist 
• Destoryed when last live reference disappears 
• Destroyed when containing method completed & out of stack 
• Destroyed when Object = null;  Reference variable 
• Destroyed when Reference = null; 
• Call to super() must be first statement in constructor 
• A constructor can have either super() or this() 

GUI

Frame settings

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frame.setDefaultCloseOperation(JFrame. EXIT_ON_CLOSE); 
frame.setTitle("A Simple Card Game"); 

Event-Handling

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class ReplaceListener implements ActionListener { 
public void actionPerformed(ActionEvent event) {}} 
btn_rpcard1.addActionListener(new ReplaceListener()); // Listener for replace button

Interface

Creation

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public interface Pet { 
    public abstract void play(); // Interface methods are implicityly public & abstract 
} 

Usage

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public class Dog extends Canine implements Pet {} 

Modules

GUI

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Import javax.swing.*; 
Import java.awt.*; 
Import java.awt.event.*; 

Arraylist

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Import java.util.ArrayList; 

Serialize

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Import java.io.*; 

Networking

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Import java.io.*; 
Import java.net.*; 

Exception(s) handling

Finally will always run even if try or catch has return() 

Ducking exception

• Repeats until some method in the calling chain has try{} catch{} handling 

Catching multiple exceptions

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try {

} catch (Exception1 ex1){

} catch (Exception2 ex2){
    
} catch (Exception3 ex3){
    
} catch (Exception4 ex4){
    
}

Other notes & Concepts

• Superclass of sub class should have constructor defined 
• Static variable = same value for all instance of same class   
• Defind constant = static final double…   
• super.roam() <- roam() in superclass is called   
• IS-A vs HAS-A   
• Abstract class -> To prevent a class from ever being instantiated  
• Abstract method -> Method has to be overridden (Class with abstract method must be abstract class) 
• Abstract class can choose to implement (abstract) methods in any ways, but concrete class must implement all    
• Polymorphism -> To allow reference type as superclass of the actual new object type 
• Override -> To have specialized operations for same method 
• Overload -> To have methods of same name but different arguments   
• Final variable, method, class = uneditable, cannot override, cannot extend 
• Objects O override 
• Method X override 
• Method O Polymorphism 
• Variable X Polymorphism