Kotlin vs Java: Important Differences That You Must Know

Kotlin and Java are both two extremely popular programming languages. Especially in the world of Android development, you’ll see that the two completely dominate. If you’re looking to develop Android apps, and get a position in this vast field, then it’s vital that you know the differences between Kotlin and Java.

That’s what we cover here. This Kotlin vs Java head-to-head comparison breaks down the similarities and differences between the two. If you’re eager for a quick summary of the differences, we’ve posted a table below. After that, you‘ll find a more detailed explanation of the differences.

Kotlin vs Java: Head to Head Comparison

What is Kotlin?

Kotlin is a cross-platform, statically typed, general-purpose language developed by JetBrains.

Similar to Java, Kotlin has become a top choice for developing Android applications. This is evident from the fact that Android Studio comes with inbuilt support for Kotlin, as it has for Java.

Kotlin is mostly used to develop Android applications, while Java has more expansive use cases. It is best thought of as a more feature-filled Java for Android development. It is also fully compatible with Java, which makes switching over much easier.

What is Java?

Java is a high-level, class-based, object-oriented programming language that was created in 1995. It is managed by Oracle and runs on over 3 billion devices across the world. It is extremely prevalent in Android applications, web and desktop applications, and web servers.

The language is very popular and has a great deal of community support, as well as many career opportunities. Java is frequently used in the sectors of banking, financial services, big data, and retail stores.

Java vs Kotlin

Choosing Kotlin or Java as a language to learn depends on certain preferences. However, before making the switch it’s important to understand the differences between Kotlin and Java, which we detail below.

Checked Exceptions

One major difference between Java and Kotlin is that the latter has no provision for checked exceptions. Therefore, there is no need to catch or declare any exceptions.

If a developer working in Java finds it infuriating to use try/catch blocks in the code then they may find Kotlin much better. However, if the developer believes that checked exceptions encourage error recovery and the creation of robust code, they would prefer Java.

Code Conciseness

Comparing a Java class with an equivalent Kotlin class demonstrates the conciseness of Kotlin code. A Kotlin class necessitates less code than a Java class does for the same operation.

For example, a particular segment where Kotlin can significantly reduce the total amount of boilerplate code is findViewByIds.

Kotlin Android Extensions permit importing a reference to a View into the Activity file. This allows for working with that View as if it was part of the Activity.

Coroutines

CPU-intensive work and network I/O are long-running operations. The calling thread is blocked until the whole operation completes. As Android is single-threaded by default, an app’s UI gets completely frozen as soon as the main thread is blocked.

The traditional solution for the problem in Java is to create a background thread for the long-running or intensive work. However, managing multiple threads leads to an increase in complexity as well as errors in the code.

Kotlin also allows the creation of additional threads. However, there is a better way of managing intensive operations in Kotlin, known as coroutines. Coroutines are stackless, which means they demand lower memory usage as compared to threads.

Coroutines are able to perform long-running and intensive tasks by suspending execution without blocking the thread and then resuming the execution at some later time. It allows the creation of non-blocking asynchronous code that appears to be synchronous.

Data Classes

Full-size projects have several classes that are solely meant to hold data. Though these classes have very little to no functionality, a developer needs to write a lot of boilerplate code in Java.

Usually, a developer needs to define a constructor, several fields to store the data, getter and setter functions for each of the fields, and equals(), hashCode(), and toString() functions.

Kotlin has a very simple way of creating such classes. The developer needs to only include the data keyword in the class definition, and that’s it. The compiler will take care of the entire task on its own.

Extension Functions

Kotlin allows developers to extend a class with new functionality via extension functions. These functions, available in other programming languages like C#, aren't available in Java.

Creating an extension function is easy in Kotlin. It is done by prefixing the name of the class that needs to be extended to the name of the function being created. In order to call the function on the instances of the extended class, one needs to use the ‘.’ notation.

Higher-Order Functions and Lambdas

In Java, higher-order functions are made possible through Callables, and lambdas were introduced with Java 8. In Kotlin, they come pre-built.

A higher-order function is one that takes functions as parameters or returns a function. Lambas are essentially anonymous methods. There are a few things worth knowing about these when it comes to Kotlin.

Kotlin functions are first-class. This means that they can be stored in data structures and variables, which can be passed as arguments to and returned from other higher-order functions.

As a statically typed programming language, Kotlin makes use of a range of function types for representing functions. Moreover, it comes with a set of specialized language constructs, such as the lambda expressions.

Implicit Widening Conversions

There is no support for implicit widening conversions for numbers in Kotlin. So, smaller types aren’t able to get converted to bigger types. Whereas Java has support for implicit conversions, Kotlin requires an explicit conversion in order to achieve the conversion.

Inline Functions

Variables that are accessed in the body of the function are known as closures. Making use of higher-order functions can impose several runtime penalties. Every function in Kotlin is an object and it captures a closure.

Both classes and function objects call for memory allocations. These along with virtual calls introduce runtime overhead. This additional overhead can be avoided by inlining the lambda expressions in Kotlin. One such example is the lock() function.

Unlike Kotlin, Java doesn’t provide support for inline functions. Nonetheless, the Java compiler is capable of performing inlining using the final method. This is so because final methods cannot be overridden by subclasses. Also, calls to a final method are resolved at compile time.

Native Support for Delegation

In programming terminology, delegation represents the process where a receiving object delegates operations to a second delegate object.

Kotlin supports composition over inheritance design pattern by means of the first-class delegation, also known as implicit delegation.

Class delegation is an alternative to inheritance in Kotlin. This makes it possible to use multiple inheritances. Kotlin’s delegated properties also prevent the duplication of code.

Non-private Fields

Encapsulation is essential in any program for achieving a desirable level of maintainability.

By means of encapsulating the representation of an object, it can be enforced how callers interact with it. It is possible to change the representation without the need to modify callers, provided the public API remains unchanged.

Non-private fields or public fields in Java are useful in scenarios where the callers of an object need to change according to its representation. It simply means that such fields expose the representation of an object to the callers.

Kotlin doesn’t have non-private fields.

Null Safety

One of the most infuriating issues concerning Java for developers is NullPointerExceptions. Java lets developers assign a null value to any variable.

However, if they try to use an object reference that has a null value, there comes the NullPointerException!

Unlike Java, all types are non-nullable in Kotlin by default. If developers try to assign or return null in the Kotlin code, it’ll fail at compile time. However, there’s a way around it. In order to assign a null value to a variable in Kotlin, it is required to explicitly mark that variable as nullable. This is done by adding a question mark after the type, for example:

val number: Int? = null

Thus, there are no NullPointerExceptions in Kotlin. If you do encounter such an exception in Kotlin then it is most likely that either you explicitly assigned a null value or because of some external Java code.

Primitive Types

There are 8 primitive data types, including char, double, float, and int. Unlike Kotlin, variables of a primitive type aren’t objects in Java. This means that they aren’t an object instantiated from a class or a struct.

Smart Casts

Before an object can be cast in Java, it is mandatory to check type. This is also true in scenarios where it’s obvious to cast the object.

Unlike Java, Kotlin comes with the smart casts feature, which automatically handles such redundant casts. You don’t need to cast inside a statement provided it is already checked with the ‘is operator’ in Kotlin.

Static Members

Kotlin has no provision for static members. However, in Java programming language the keyword static reflects that the particular member with which the keyword is used belongs to a type itself instead of an instance of that type.

It simply means that one and only one instance of that static member is created and shared across all instances of the class.

Support for Constructors

A Kotlin class, unlike a Java class, can have one or more secondary constructors in addition to a primary constructor. This is done by including these secondary constructors in the class declaration.

Ternary Operator

Unlike Kotlin, Java has a ternary operator. The Java ternary operator simply works like a basic if statement. It consists of a condition that evaluates to true or false.

Moreover, the Java ternary operator has two values. Only one of them is returned depending on whether the condition is true or false. The syntax for the Java ternary operator is:

(condition) ? (value1) : (value 2)

Wildcard Types

In generic code, ‘?’ represents an unknown type. It is known as the wildcard. There are several uses of a wildcard, including as the type of a field, local variable, or parameter.

While Java’s type system offers wildcard types, Kotlin doesn’t. However, it has two different things; declaration-site variance and type projections as an alternative to wildcard types.

Annotation Processing Libraries with Kotlin

In addition to providing support for existing Java frameworks and libraries, Kotlin also has a provision for advanced Java frameworks relying on annotation processing.

However, using a Java library that makes use of annotation processing in Kotlin requires adding the same to the Kotlin project in a bit different way than what is required for a Java library that doesn’t use annotation processing.

It is required to specify the dependency using the kotlin-kapt plugin. Afterward, the Kotlin Annotation processing tool needs to be used in place of the annotation Processor.

Apps Built Using Kotlin and Java

You might also want to know what apps have been built using Kotlin and Java. That will give you a better understanding of how the language affects the experience if you’ve used these apps yourself.

Some popular apps built using Kotlin are Coursera, Trello, Evernote, Pinterest, Uber, Square, Postmates, Airbnb, and Netflix.

Apps built using Java include Spotify, Google Earth, Uber, Opera Mini, NASA, Wikipedia Search, and Minecraft. Netflix uses Kotlin for its mobile apps, but it also uses Java for its backend.

The Bottom Line

So how do you know which to pick when it comes to choosing Java or Kotlin?

Obviously, some features are better in Kotlin while for others Java is advantageous to use. For those not willing to ditch any of the two leading programming languages for Android development, thankfully, there’s another way around.

Irrespective of the differences between Java and Kotlin, they are fully interoperable. Both Java and Kotlin compile to bytecode. This means it is possible to call Java code from Kotlin and vice-versa.

This flexibility has two advantages. First, it facilitates getting started with Kotlin by incrementally introducing the Kotlin code in a Java project. Second, both languages can be used simultaneously in any Android application development project.

For general-purpose programming, Java gains the upper hand. On the other hand, more developers and organizations are adopting Kotlin to rapidly develop Android applications.

The Complete Android 12 & Kotlin Development Masterclass

Frequently Asked Questions

1. Is Kotlin Better than Java?

Kotlin is better when it comes to Android development. However, Java does better when it comes to general-purpose programming. They’ve both got their pros and cons.

2. Is Kotlin easier than Java?

Both are fairly similar in terms of difficulty. Kotlin tends to be more concise though, so from that point of view, it might be easier as writing code is faster.

3. Is Kotlin Replacing Java?

No, Kotlin is not replacing Java. While it is increasingly a popular choice for Android development, Java’s stronger security standards and other features make it a mainstay in development. They both have their uses.

4. Can I Learn Kotlin Without Java?

Yes, you can learn Kotlin without Java — in fact, you need no Java knowledge at all. Kotlin is essentially an improved version of Java, though the latter lets you engage in general-purpose programming, whereas Kotlin is limited to Android development.

5. Should I Learn Java Before Kotlin?

If your focus is Android development, then you are better off learning Kotlin first. However, it helps to know some of the basics of Java so don’t shy away from learning a little Java on the side. If you have more general-purpose programming in mind, then consider learning Java first.

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