13 Top Core Java Concepts All Java Programmers Need to Know

Whether you are a beginner or an experienced Java programmer, there are several core Java concepts that you should know. Learning programming languages like Java can be a challenge. Like other computer programming languages, Java has a lot of ins and outs that can seem overwhelming. 

How can you motivate yourself to learn? Well, keeping a positive perspective is key. For example, learning programming skills along with languages can change your life for the better. When you have strong programming skills, it becomes easier for you to expand your knowledge to more new languages and technologies.

Fully understanding how Java programming works offers many benefits. For example, it will make your life easier when you are coding in Java. It will also help ensure that you spend less time debugging your programming. In many cases, you can use what you know as a key advantage in an interview.

This article can help reinforce your understanding of Java programming. You can use this article as a quick reference guide to tools and information on the important core Java concepts. To help you focus on Java core concepts, this article does not include any programming code. This is because we recommend that you understand these concepts before you begin coding in Java.

Whether you aim to be a Java Developer, Java Designer, or other Java role, Java classes can be useful tools. You may have already learned Java and need a refresher. This could be your first attempt at learning how to program. Alternatively, you may be an experienced programmer who is new to the Java programming language. In any of these scenarios, this article will be of interest to you. 

Whether you are learning or reviewing the concepts that we discuss, you will ultimately rely on them when you code in Java. This article highlights some high-level Java core concepts and takes a deeper dive into other subtopics. Read on to learn about a number of Java concepts, including:

• Interfaces
• Executing in Java
• Object-oriented programming
• Variables
• Data Types
• Operators
• Classes and objects

1. Interfaces

In Java, you will program to an interface instead of an implementation. This is also known as programming to interfaces, which refers to a way to write classes based on an interface that defines the object’s behavior. This design concept may seem like more work initially because you have to start with these three steps:

• Create an interface.
• Define its methods.
• Write the class with the implementation.

However, this Java workflow ensures that you are:

• Using interface type on variables
• Return types of methods or argument type of methods

What’s great about these steps is that they make code more flexible and easier to test in Java.

2. Executing in Java

In Java, two basic units of execution are process and thread. Process and thread are defined in Java as follows:

• Process: A self-contained execution environment that can be either a program or application. When seen as a program, it contains multiple processes inside it. A Java runtime environment runs as a single process. Also, the process itself has different classes and programs as processes.
• Thread: Exists in the process. Requires fewer resources to create, and it shares the process resources. In this same vein, a thread is also a lightweight process. 

Using threads in Java offers some valuable benefits:

• As lightweight processes, threads take less time and resources to create.
• Threads share their parent process data and code.
• Context switching between threads is usually less expensive than between processes.
• Thread intercommunication is relatively easy in comparison to process communication.

Depending on your use case, Java also offers multithreading functionality. 

3. Object-Oriented Programming (OOP)

An important concept in Java is OOP, which stands for object-oriented programming. Java is an OOP language, which means that it emphasizes data more than it does functions. OOP programming languages revolve around objects. 

This object bundles up data and the methods that operate on this data in one unit. This protects data by having the methods inside the object. In OOP, the objects communicate with each other through messages. All OOP languages (including Java) support the following features:

• Classes
• Encapsulation
• Abstraction
• Inheritance
• Polymorphism

Polymorphism is an important feature of OOP. Polymorphism enables Java to allow objects or entities to assume multiple forms. A method with multiple implementations, for example, is polymorphic in Java. There are two types of polymorphism in Java:

• Overloading Polymorphism: The call to the polymorphic/overloaded method resolves at compile time. Overloading Polymorphism is also known as Compile Time Polymorphism.
• Overriding Polymorphism: In runtime polymorphism, the call to an overridden method in the Java program resolves at runtime. Overriding Polymorphism is also known as Runtime Polymorphism.

4. Variables

In Java, variables are containers for storing data values. Java supports the following three types of variables:

• Class or static variables: You can access this type of variable without an object.
• Non-static or instance variables: You can access these member variables using a class object.
• Local variables: These are local to a particular block of code and cease to exist outside of that block.

You need to specify a data type for every variable. You should specify the data type when you declare the variable. Every variable should also have a unique (or non-duplicate) name to identify it. Java requires that you apply the following rules for naming variables:

• No spaces in variable names
• No special symbols in variable names (for example: [email protected]#%^&*)
• Variable names can only contain alphabets, numbers, and the underscore ( _ ) symbol.
• Variable names cannot start with numbers (note that variable names can contain numbers)
• Variables are case-sensitive

It is also important to note that Java reserves certain words for its own use. These are known as Java keywords. Therefore, you can not use keywords as variable or identifier names. w3Schools contains a full list of Java keywords.

Java uses the following different data types of variables:

• String: Stores text and surrounds string values with double-quotes.
• int: Stores integers, which are whole numbers without decimals.
• float: Stores floating-point numbers with decimals.
• char: Stores single characters and surrounds char values with single quotes.
• Boolean: Stores values with two different states. For example, true or false.

Once you declare a variable, you can:

• Initialize it with some value. 
• Change it later to some other value.
• Use it for performing calculations or printing functionality.

5. Data Types

Java is a strongly typed language, which means every data or information has a set Data Type. Any variable that holds the value needs to hold a numeric or string or character. We call this the data type of the variable. Java uses Data Types to store different kinds of information. After you declare your Data Type in Java, you cannot change it. 

You can classify data Types into two categories: primitive data type and non-primitive data type.

6. Primitive Data Types

Primitive data types are not objects or references to objects. Java includes the following primitive data types:

• boolean: Stores true or false values.
• byte: Stores whole numbers from -128 to 127.
• short: Stores whole numbers from -32768 to 32767.
• char: Stores a single character/letter or ASCII values.
• int: Stores whole numbers from -2147483648 to 2147483647.
• long: Stores whole numbers from -9223372036854775808 to 9223372036854775807.
• float: Stores fractional numbers from 3.4e−038 to 3.4e+038. Note that you should end the value with an “f”.
• double: Stores fractional numbers from 1.7e−308 to 1.7e+308. Note that you should end the value with a “d”.

Primitive number types can divide into two groups:

• Integer types: Stores positive or negative whole numbers without decimals. Valid types include byte, short, int, and long.
• Floating-point types: Represents numbers with a fractional part that contains one or more decimals. Valid types include float and double.

7. Non-primitive Data Types

Non-primitive data types are reference types because they refer to objects. The main difference between primitive and non-primitive data types are:

• Java already defines primitive types, but it does not predefine non-primitive types (except for String).  The programmer creates non-primitive types. 
• You can use non-primitive types to call methods to perform certain operations, while primitive types cannot.
• A primitive type always has a value, while non-primitive types can be null.
• A primitive type starts with a lowercase letter, while non-primitive types start with an uppercase letter.
• The size of a primitive type depends on the data type, while non-primitive types are all the same size.

8. Operators

Operators are symbols that perform logical and mathematical operations on variables or identifiers. These operators help in performing operations. Java supports various operators, which include the following:

• Arithmetic operators: Used in mathematical expressions
• Shift operators: “<<” shifts a bit pattern to the left, and the signed right shift operator “>>” shifts a bit pattern to the right. The left-hand operand gives the bit pattern, and the right-hand operand gives the number of positions to shift. The unsigned right shift operator “>>>” shifts a zero into the leftmost position, while the leftmost position after “>>” depends on sign extension.
• Bitwise operators: Act upon the individual bits of their operands.
• Ternary operators: Used as if then else statement
• Boolean Logical operators: Used only on Boolean operands
• Instance of operator: “~” inverts a bit pattern; it applies to any of the integral types. (for example, making every “0” a “1” and every “1” a “0”).
• Unary operators: The unary operators require only one operand. They perform various operations. (For example, incrementing/decrementing a value by one, negating an expression, or inverting the value of a Boolean).
• Assignment operator: Uses a single equal sign.
• Relational operator: Determines the relationship that one operand has to the other.

9. Classes and Objects

To put it simply:

• You can define a class as a design prototype or a blueprint. 
• Those blueprints help create objects. 
• The instances of classes are called objects.

In greater detail, classes are supported by every object-oriented programming language. In addition, you can use them to group related data. Data variables (also known as member variables) and methods depict the behavior of objects. These are known as instances of the class. In other words, an object defines the state of the entity represented by a class at any given instant. An object typically includes:

• State: The properties or attributes of an object at a particular time.
• Behavior: Methods represent the behavior of an object and define how the objects communicate.
• Identity: Identifies the object by giving it a unique name.

To understand the relationship between classes and objects in Java, you should consider the following:

• A class is like a blueprint, and you can create many objects using that class.
• The objects within one class have the same properties, meaning it copies the properties from that class.
• You can create multiple objects of a class, and multiple objects can be of the same class.
• Every object should belong to a class because you cannot create objects without a class.
• A class is also referred to as a type. 
• An object is also referred to as an instance.

10. Packages

You can group classes and interfaces that have similar functionality or dependency to form a package. Packages give you more flexibility to group and organize your Java code.

11. Fields 

A field is a variable that belongs to a class or object. For instance, the Car Class could define a field called brand. All Car objects would have this brand field, but the value of this field can be different for each instance of the class. This allows Car objects to be different from one another while still sharing the basic attributes inherent to all cars.

12. Constructors

In Java, a constructor is a special method you can use to initialize objects. Java calls the constructor after you create an object of a class. You can also use constructors to set initial values for object attributes.

The purpose of constructors is to assign values to the fields that are specific to that particular instance of the object. For instance, if you are creating a Car object, you may need to tell the program what brand the car is, its color, how fast it is going, and in what direction it is traveling. A constructor allows you to do these things easily when the object is created during program execution.

13. Methods

Java supports the creation of methods. A method is a group of operations that carry out a certain function together. Methods take advantage of the OOP concept, encapsulation. Methods are useful for when you want to:

• Remove duplicate code.
• Reduce code complexity.
• Make it easier to maintain your code.

When you declare a method, it contains a method name, input parameters, and return type. This is known as a method signature. 

When you are learning about methods, note that it is very important to understand these two core Java concepts:

• How the parameters are passed from the calling method to the called method
• How the return value returns to the calling method

Abstract Class and Methods

In Java, every class is a concrete class by default. To implement the abstract class, you need to use the abstract keyword before the class. This sets the class to an abstract class. 

Abstract classes in Java do not map to real-life objects. They cannot exist on their own unless they take the form of their respective concrete subclasses.

Using an abstract class in Java offers several advantages, which include:

• You can create both reference and object for any concrete class.
• You can assign a concrete subclass reference to abstract super-class reference.

It is important to note that while you can create a reference for an abstract class, you cannot create an object of an abstract class. An additional benefit of using abstract classes is that it gives you the ability to define abstract methods.

Abstract methods are methods that have only the declaration and do not have a definition. In Java, declaration and definition entail the following: 

• Declaration: Creating only the method signature but no method body.
• Definition: Creating both the method signature and method body.

As a friendly reminder, the method signature includes the method name, input parameters, and return type. 

Once you have a basic understanding of these principles, you can start creating your own Java programs. Many of these concepts may seem slightly abstract when they are new to you. After you start coding in Java, all the pieces will come together. Java programming offers many benefits that will lay the groundwork for your success as a Java programmer.