What are the SOLID principles in Java?

SOLID principles are object-oriented design concepts relevant to software development. SOLID is an acronym for five other class-design principles: Single Responsibility Principle, Open-Closed Principle, Liskov Substitution Principle, Interface Segregation Principle, and Dependency Inversion Principle.

SOLID is a structured design approach that ensures your software is modular and easy to maintain, understand, debug, and refactor. Following SOLID also helps save time and effort in both development and maintenance. SOLID prevents your code from becoming rigid and fragile, which helps you build long-lasting software.

Examples

1. Single responsibility principle

Every class in Java should have a single job to do. To be precise, there should only be one reason to change a class. Here’s an example of a Java class that does not follow the single responsibility principle (SRP):

public class Vehicle {
    public void printDetails() {}
    public double calculateValue() {}
    public void addVehicleToDB() {}
}

The Vehicle class has three separate responsibilities: reporting, calculation, and database. By applying SRP, we can separate the above class into three classes with separate responsibilities.

Consider the following code example that fulfills the SRP. The Vehicle class is only responsible for printing the details of the Vehicle:

2. Open-closed principle

Software entities (e.g., classes, modules, functions) should be open for an extension, but closed for modification.

Consider the below method of the class VehicleCalculations:

public class VehicleCalculations {
    public double calculateValue(Vehicle v) {
        if (v instanceof Car) {
            return v.getValue() * 0.8;
        if (v instanceof Bike) {
            return v.getValue() * 0.5;

    }
}

Suppose we now want to add another subclass called Truck. We would have to modify the above class by adding another if statement, which goes against the Open-Closed Principle.
A better approach would be for the subclasses Car and Truck to override the calculateValue method:

Adding another Vehicle type is as simple as making another subclass and extending from the Vehicle class.

3. Liskov substitution principle

The Liskov Substitution Principle (LSP) applies to inheritance hierarchies such that derived classes must be completely substitutable for their base classes.

Consider a typical example of a Square derived class and Rectangle base class:

public class Rectangle {
    private double height;
    private double width;
    public void setHeight(double h) { height = h; }
    public void setWidht(double w) { width = w; }
    ...
}
public class Square extends Rectangle {
    public void setHeight(double h) {
        super.setHeight(h);
        super.setWidth(h);
    }
    public void setWidth(double w) {
        super.setHeight(w);
        super.setWidth(w);
    }
}

The above classes do not obey LSP because you cannot replace the Rectangle base class with its derived class Square. The Square class has extra constraints, i.e., the height and width must be the same. Therefore, substituting the Rectangle with the Square class may result in unexpected behavior.

The solution to the above example is to create a separate abstract Shape class and then have the Square and Rectangle classes inherit from it. Consider the following code examples:

4. Interface segregation principle

The Interface Segregation Principle (ISP) states that clients should not be forced to depend upon interface members they do not use. In other words, do not force any client to implement an interface that is irrelevant to them.

Suppose there’s an interface for vehicle and a Bike class:

public interface Vehicle {
    public void drive();
    public void stop();
    public void refuel();
    public void openDoors();
}
public class Bike implements Vehicle {

    // Can be implemented
    public void drive() {...}
    public void stop() {...}
    public void refuel() {...}
    
    // Can not be implemented
    public void openDoors() {...}
}

As you can see, it does not make sense for a Bike class to implement the openDoors() method as a bike does not have any doors! To fix this, ISP proposes that the interfaces be broken down into multiple, small cohesive interfaces so that no class is forced to implement any interface, and therefore methods, that it does not need.

5. Dependency inversion principle

The Dependency Inversion Principle (DIP) states that we should depend on abstractions (interfaces and abstract classes) instead of concrete implementations (classes). The abstractions should not depend on details; instead, the details should depend on abstractions.

Consider the example below. We have a Car class that depends on the concrete Engine class; therefore, it is not obeying DIP.

public class Car {
    private Engine engine;
    public Car(Engine e) {
        engine = e;
    }
    public void start() {
        engine.start();
    }
}
public class Engine {
   public void start() {...}
}

The code will work, for now, but what if we wanted to add another engine type, let’s say a diesel engine? This will require refactoring the Car class.
However, we can solve this by introducing a layer of abstraction. Instead of Car depending directly on Engine, let’s add an interface:

public interface Engine {
    public void start();
}

Now we can connect any type of Engine that implements the Engine interface to the Car class:

public class Car {
    private Engine engine;
    public Car(Engine e) {
        engine = e;
    }
    public void start() {
        engine.start();
    }
}
public class PetrolEngine implements Engine {
   public void start() {...}
}
public class DieselEngine implements Engine {
   public void start() {...}
}