面試總結之 各種定義題 英文口語

Singleton

private static Singleton firstInstance = null;
    public static Singleton getInstance() {
        if(firstInstance == null) {
            
            // This is here to test what happens if threads try
            // to create instances of this class
            
            if(firstThread){
            
                firstThread = false;
                
                try {
                    Thread.currentThread();
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                
                    e.printStackTrace();
                }
            }
            
            // Here we just use synchronized when the first object
            // is created
            
            synchronized(Singleton.class){ 
            
                if(firstInstance == null) {
                    // If the instance isn't needed it isn't created
                    // This is known as lazy instantiation
            
                    firstInstance = new Singleton();
            
                    // Shuffle the letters in the list
                    Collections.shuffle(firstInstance.letterList);
                    
                }
            
            }
            
        }
        
        // Under either circumstance this returns the instance
        
        return firstInstance;
    }

In software engineering, the singleton pattern is a design pattern that restricts
 the instantiation of a class to one object. This is useful when exactly one object
  is needed to coordinate actions across the system. The concept is sometimes generalized
   to systems that operate more efficiently when only one object exists, or that restrict
    the instantiation to a certain number of objects. The term comes from the mathematical
     concept of a singleton.

 

 

Factory 

// This is a factory thats only job is creating ships
// By encapsulating ship creation, we only have one
// place to make modifications

public class ENEMYSHIPFACTORY{
    
    // This could be used as a static method if we
    // are willing to give up subclassing it
    
    public ENEMYSHIP makeEnemyShip(String newShipType){
        
        //ENEMYSHIP newShip = null;
        
        if (newShipType.equals("U")){
            
            return new UFOENEMYSHIP();
            
        } else 
        
        if (newShipType.equals("R")){
            
            return new ROCKETENEMYSHIP();
            
        } else 
        
        if (newShipType.equals("B")){
            
            return new BIGUFOENEMYSHIP();
            
        } else return null;
        
    }
    
}

When a method returns one of several possible classes that share a commom super class

the factory pattern allows you to create objects without specifying the exact class of 
object that will be created.

 

 

 a virtual function or virtual method is a function or method whose behavior can be overridden within an inheriting class by a function with the same signature. This concept is a very important part of the polymorphism portion of object-oriented programming (OOP).

 

"A pure virtual function or pure virtual method is a virtual function that is required to be implemented by a derived class that is not abstract" - Wikipedia

So, the virtual function can be overriden and the pure virtual must be implemented.

 

As the name inheritance suggests an object is able to inherit characteristics from another object. In more concrete terms, an object is able to pass on its state and behaviors to its children. For inheritance to work the objects need to have characteristics in common with each other.

 

static:

 

static variables are shared by the entire class, not a specific instance (unlike normal member variables)
static methods are also shared by the entire class

static can be used around a block of code in a class to specify code that runs when the virtual machine is first started up, before instances of the class are created.

 

 

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