There are four principle concepts upon which object oriented design and programming rest. They are: Abstraction Polymorphism Inheritance Encapsulation (i.e. easily remembered as A-PIE).
Abstraction refers to the act of representing essential features without including the background details or explanations.
Encapsulation is a technique used for hiding the properties and behaviors of an object and allowing outside access only as appropriate. It prevents other objects from directly altering or accessing the properties or methods of the encapsulated object.
Abstraction focuses on the outside view of an object (i.e. the interface) Encapsulation (information hiding) prevents clients from seeing it’s inside view, where the behavior of the abstraction is implemented. Abstraction solves the problem in the design side while Encapsulation is the Implementation. Encapsulation is the deliverables of Abstraction. Encapsulation barely talks about grouping up your abstraction to suit the developer needs.
Inheritance is the process by which objects of one class acquire the properties of objects of another class. A class that is inherited is called a superclass. The class that does the inheriting is called a subclass. Inheritance is done by using the keyword extends. The two most common reasons to use inheritance are: To promote code reuse To use polymorphism
Polymorphism is briefly described as "one interface, many implementations." Polymorphism is a characteristic of being able to assign a different meaning or usage to something in different contexts - specifically, to allow an entity such as a variable, a function, or an object to have more than one form.
(Inheritance, Overloading and Overriding are used to achieve Polymorphism in java). Polymorphism manifests itself in Java in the form of multiple methods having the same name. In some cases, multiple methods have the same name, but different formal argument lists (overloaded methods). In other cases, multiple methods have the same name, same return type, and same formal argument list (overridden methods).
There are two types of polymorphism one is Compile time polymorphism and the other is run time polymorphism. Compile time polymorphism is method overloading. Runtime time polymorphism is done using inheritance and interface. Note: From a practical programming viewpoint, polymorphism manifests itself in three distinct forms in Java: Method overloading Method overriding through inheritance Method overriding through the Java interface
In Java, runtime polymorphism or dynamic method dispatch is a process in which a call to an overridden method is resolved at runtime rather than at compile-time. In this process, an overridden method is called through the reference variable of a super class. The determination of the method to be called is based on the object being referred to by the reference variable.
Binding refers to the linking of a procedure call to the code to be executed in response to the call. Dynamic binding (also known as late binding) means that the code associated with a given procedure call is not known until the time of the call at run-time. It is associated with polymorphism and inheritance.
Method Overloading means to have two or more methods with same name in the same class with different arguments. The benefit of method overloading is that it allows you to implement methods that support the same semantic operation but differ by argument number or type. Note: Overloaded methods MUST change the argument list Overloaded methods CAN change the return type Overloaded methods CAN change the access modifier Overloaded methods CAN declare new or broader checked exceptions A method can be overloaded in the same class or in a subclass
Method overriding occurs when sub class declares a method that has the same type arguments as a method declared by one of its superclass. The key benefit of overriding is the ability to define behavior that’s specific to a particular subclass type. Note: The overriding method cannot have a more restrictive access modifier than the method being overridden (Ex: You can’t override a method marked public and make it protected). You cannot override a method marked final You cannot override a method marked static
| Overloaded Method | Overridden Method | |
|---|---|---|
Arguments |
Must change |
Must not change |
Return type |
Can change |
Can’t change except for covariant returns |
Exceptions |
Can change |
Can reduce or eliminate. Must not throw new or broader checked exceptions |
Access |
Can change |
Must not make more restrictive (can be less restrictive) |
Invocation |
Reference type determines which overloaded version is selected. Happens at compile time. |
Object type determines which method is selected. Happens at runtime. |
Yes, derived classes still can override the overloaded methods. Polymorphism can still happen. Compiler will not binding the method calls since it is overloaded, because it might be overridden now or in the future.
NO, because main is a static method. A static method can't be overridden in Java.
In java, string objects are immutable because String objects are cached in String pool. Immutable simply means unmodifiable or unchangeable means once string object is created its data or state can’t be changed but a new string object is created. JVM has string pool which shared between multiple clients so there is always risk of modification by one client can affect all other clients.
String Pooling requirement: Suppose one client has a string object with value “java” if this client changes the value of String “java” to “JAVA” then all other clients will also see that changed value. As we know JVM provide String pool system to cache the string value for performance reason this risk was avoided by making String class Immutable.
Hashing Requirement i.e. caching HashCode: One more thing about string class, it is final class so that no one can override or inherit of String class e.g. Immutability, Caching, hashcode calculation etc by extending and overriding behaviors. Another reason of why String class is immutable could die due to HashMap. In the HashMap, we can use string as Key of map, even it is very popular to use as key of HashMap. So it is one more reason of string immutablity because we can retrieve the value object which was stored in HashMap via key any where. Since HashMap works in the principle of hashing, which requires same has value to function properly. If suppose string is a Mutable class then String would produce two different hashcodes at the time of insertion and retrieval if contents of String was modified after insertion, potentially losing the value object in the map.
String use in Security: There lots of cases where we use String class widely as a parameter for many java classes, e.g. database connection setting, network connection, opening files, etc. Were String not immutable, a connection or file would be changed and lead to a serious security threat. The method thought it was connecting to one machine, but was not. Mutable strings could cause a security problem in Reflection too, as the parameters are strings.
