Java Basics

Java

A programming language that is built around object-oriented paradigm. Everything is an object in Java. Objects can be instiated from a class using the unary operator: new. All classes are an extension of the Object class.

JVM

Short for Java Virtual Memory. The virtual machine, on top of the operating system, which runs the compiled Java code.

JDK

Short for Java Development Kit. The set of tools to compile the Java programs.

Memory is allocated implicitly (variable declaration or new operator) and deallocated implicitly (by the runtime).

Terminology

Class

A template for a object, giving information about state and actions. State of an object is represented by variables and behavior is represented by methods.

The class definition contains a constructor method which is a special static method used to instantiate objects. Inside methods this keyword is used to refer to the current object instance.

Interface

Defines the structure of a class. Can include state variable definitions but not values. Can include method signatures. After Java 8, can also include method body.

Child class

Aka. sub class, derived class. A class extending another class (all public and protected members), which is called its super class. Members can be overridden by defining a member again in the sub class. New members can be declared. super keyword can be used to access methods of super class.

Abstract class

A special type of class, which cannot be used to instantiate objects from. Defined by abstract keyword. Can only be extended from.

Abstract methods

Can only be defined inside abstract classes. Defined by abstract keyword. Any child class must either override the abstract method or declare itself abstract

Access modifiers

In Java, access modifiers control visibility and accessibility of classes, methods, and variables:

For classes:

• public: Class is visible everywhere
• default (no modifier): Class is only visible within same package

For methods:

• public: Accessible from any other class
• protected: Accessible within same package and by subclasses
• default: Only accessible within same package
• private: Only accessible within declaring class

For attributes:

• public: Accessible from any class
• protected: Accessible in same package and subclasses
• default: Only accessible in same package
• private: Only accessible within declaring class

Non-access modifiers

For classes:

• final: Class cannot be inherited
• abstract: Class cannot be instantiated and may have abstract methods
• strictfp: Class uses strict floating-point calculations

For methods:

• final: Method cannot be overridden
• abstract: Method has no implementation and must be implemented by subclasses
• synchronized: Method can only be accessed by one thread at a time
• native: Method is implemented in platform-dependent code
• strictfp: Method uses strict floating-point calculations

For attributes:

• final: Attribute cannot be changed after initialization
• static: Attribute belongs to class rather than instance
• transient: Attribute will not be serialized
• volatile: Attribute value may be changed by multiple threads

Static members

Aka. class members. Used to store data on a class instead of an instance. There can be static variables and static methods. Static methods cannot access non-static variables and cannot call non-static methods. this keyword is not available inside static methods.

Type Casting

The process of converting one data type to another. Can be implicit or explicit.

If a wrong cast operation is performed on a primitive type, it will cause a compile error or loss of accuracy. If a wrong cast is done on a reference type, it will cause a runtime error ClassCastException.

Upcasting

Casting a subclass to a superclass. Implicit and safe. Used in polymorphism to call overridden methods.

Example:

Animal animal = new Dog(); // Upcasting
animal.makeSound(); // Calls Dog's implementation

Downcasting

Casting a superclass to a subclass. Explicit and requires a cast operator. Used to access subclass-specific methods or properties.

Example:

Animal animal = new Dog();
Dog dog = (Dog) animal; // Downcasting
dog.fetch(); // Access subclass-specific method

Upcasting is common in method arguments and collections. Downcasting is used when subclass-specific behavior is needed.

Object overhead

In Java, objects have additional memory overhead due to metadata stored alongside the actual data.

For example, in a 32-bit system, a primitive int requires 4 bytes of memory; An Integer object, requires 16 bytes. The extra memory is used to store object metadata:

• class pointer: A reference to the class metadata that defines the object’s structure and behavior.
• flags: Indicators used to store object-specific state information, such as garbage collection status.
• synchronization locks: Mechanisms to manage thread-safe access to the object. More on this here.
• object size information: Data about the memory footprint of the object.

These metadata are essential for the Java Virtual Machine (JVM) to manage objects effectively.

Object Type	Memory Overhead (Bytes)	Notes
String	44	Includes length, hash code, and character array overhead.
HashMap	48	Includes hash table structure and bucket overhead.
Array	16	Includes array length and element type information.
LinkedList	24	Additionally, each item has 24 bytes overhead.
ArrayList	32	Includes capacity overhead for resizing and internal array storage.

Collections framework

A Java framework that includes production-grade implementations for commonly used data structures. The implementation is done using interfaces, abstract classes and (regular) classes.

• Iterable interface represents an object that can be used in a forEach statement.
• Collection interface adds common functionliaty such as add, remove to Iterable interface. Provides a common interface for all the implementations of different data structures.
• List, Queue, Set interfaces extends Collection to specific data structures
• ArrayList is a dynamic array implementation
• PriorityQueue is an implementation of Queue
• HashSet is an implementation of Set
• Map is a similar interface to Collection. Allows access to items by reference to contents that are used as a key
• HashMap is an implementation of Map

All these interfaces and classes are implemented around generics. Generics allow them to be created with compile-time types.

Pre-Collection

Originally, JDK provided the ability to group objects through:

• Array which is a native language feature
• Vector which can grow in size but not shrink
• Hashtable which is an array with 2 parts: a hashcode and the actual data. Hashcode is generated from the content of the data. Data is accessed using the hashcode.

But they didn’t have a common interface.