What are Static Methods on interface in Java?

In Java SE 8 and later, you can define static methods on interfaces. A static method is a method associated with the class, not the instance. This means you can call a static method without creating an instance of the class.

This feature can be particularly useful when providing utility methods that act on instances of the interface. You would normally keep these in a separate utility class, but by having these on the interface itself can lead to more readable and maintainable code.

Here is a simple example:

interface MyInterface {
    static void myStaticMethod() {
        System.out.println("Static Method on Interface");

public class Main {
    public static void main(String[] args) {
        MyInterface.myStaticMethod(); // Call without creating instance

In this example, myStaticMethod() is a static method defined on MyInterface. You call it using the interface name (MyInterface.myStaticMethod()), without needing to create an instance of MyInterface.

Keep in mind that static methods in interfaces are not inherited by classes that implement the interface or sub-interfaces, so you always have to use the interface name when calling them.

The Stream interface in Java has several static methods that provide useful functionality for working with sequences of elements, such as collections. Here is an example that uses the Stream.of() static method, which allows you to create a Stream from a set of objects:

import java.util.stream.*;

public class Main {
    public static void main(String[] args) {
        Stream.of("Hello", "World", "Interface", "Static", "Methods")

In this example, we use Stream.of() to create a Stream from a set of String objects. We then use map() to convert each string in the stream to uppercase, and forEach() to print out each string.

Here is another example, this time using the IntStream.range() static method:

import java.util.stream.*;

public class Main {
    public static void main(String[] args) {
        IntStream.range(1, 6)

In this example, IntStream.range(1, 6) creates a stream of integers from 1 (inclusive) to 6 (exclusive). The forEach() method is then used to print out each integer in the stream.

What is Default Methods in Java?

Default methods are a feature introduced in Java 8, allowing the declaration of methods in interfaces, apart from abstract methods. They are also known as defender methods or virtual extension methods.

With the use of default keyword, these methods are defined within the interface and provide a default implementation. This means they can be directly used by any class implementing this interface without needing to provide an implementation for these methods.

The main advantage of default methods is that they allow the interfaces to be evolved over time without breaking the existing code.

Here’s an example of a default method in an interface:

interface MyInterface {
    void abstractMethod();

    default void defaultMethod() {
        System.out.println("This is a default method in the interface");

In the above example, any class implementing MyInterface needs to provide an implementation for abstractMethod(), but not for defaultMethod() unless it needs to override the default implementation.

Before Java 8, we could declare only abstract methods in interfaces. It means that classes which implement the interface were obliged to provide an implementation of all methods declared in an interface. However, this was not flexible for developers, especially when they wanted to add new methods to the interfaces.

For instance, here is an interface used by multiple classes:

interface Animal {
    void eat();

Now, if we wanted to add a new method called run(), all classes that implement Animal would need to define this method, which could potentially introduce bugs and is quite cumbersome if we have many classes that implement the interface.

To mitigate such issues, Java 8 introduced default methods in interfaces. With default methods, we can now add new methods in the interface with a default implementation, thereby having the least impact on the classes that implement the interface.

interface Animal {
    void eat();

    default void run() {

So in the updated Animal interface, the run() method is a default method. Classes implementing Animal can choose to override this method, but they are not obliged to do so. If a class does not provide an implementation for this method, the default implementation from the interface will be used.

Here’s an example implementation of the Animal interface:

class Dog implements Animal {
    public void eat() {
        System.out.println("Dog is eating");

public class Main {
    public static void main(String[] args) {
        Dog dog = new Dog();
        dog.eat();  // Output: Dog is eating
        dog.run();  // Output: Running

As you can see, the Dog class didn’t implement the run method, but we’re still able to call dog.run() because of the default implementation in the Animal interface.

Note: In case a class implements multiple interfaces and these interfaces have default methods with identical signatures, the compiler will throw an error. The class must override the method to resolve the conflict.

What is a Functional Interface in Java?

A Functional Interface in Java is an interface that has exactly one abstract method. Apart from this abstract method, it can include default and static methods. Java 8 introduced the @FunctionalInterface annotation to ensure an interface follows the rules of Functional Interface. It’s optional but good practice to use this annotation.

Functional interfaces are extensively used in Java’s lambda expressions. The main purpose of a functional interface is to be used as Lambda Expressions or Method References.

Here’s a basic example of defining a functional interface:

interface GreetingService {
    void sayMessage(String message);

You could use it in conjunction with a lambda like this:

GreetingService greetService = message -> System.out.println("Hello " + message);

In the code above, message -> System.out.println("Hello " + message) is a lambda expression that provides the implementation of the abstract method sayMessage(String message).

Java 8 has also defined several built-in functional interfaces. These built-in interfaces are packed in the java.util.function package. Some common ones include Predicate<T>, Function<T, R>, Supplier<T>, and Consumer<T>. Furthermore, BinaryOperator<T>, UnaryOperator<T>, BiFunction<T, U, R> are some other standard functional interfaces available.

Here are some examples of Java 8 built-in functional interfaces:

1. Predicate

Predicate<T> is a functional interface that takes a single input and returns a boolean value. It is located in java.util.function package.

Predicate<String> lengthCheck = s -> s.length() > 5;
System.out.println(lengthCheck.test("Hello"));  // Output: false

Predicate is often used when you need to pass some sort of condition or filter as a parameter. For example, you might be checking if the User inputs are valid:

Predicate<String> isValidEmail = email -> email.matches("^[A-Za-z0-9+_.-]+@(.+)$");
System.out.println(isValidEmail.test("[email protected]"));  // Output: true
System.out.println(isValidEmail.test("testgmail.com"));   // Output: false

2. Function

Function<T, R> is an interface that accepts one argument and produces a result.

Function<String, Integer> parse = Integer::parseInt;
System.out.println(parse.apply("123"));  // Output: 123

A Function<T, R> can be useful when you need to convert from one type to another, such as transforming a list of String into a list of Integer.

Function<String, Integer> stringToInteger = Integer::parseInt;
List<String> strings = Arrays.asList("1", "2", "3");
List<Integer> integers = strings.stream()

3. Consumer

Consumer<T> is an interface that takes one argument and returns no results. It is meant for implementing side effects.

Consumer<String> printer = System.out::println;
printer.accept("Hello");  // Output: Hello

The Consumer<T> interface is often used in conjunction with Java streams or Optional, where you have a collection of objects, and you want to perform a certain action on each of the objects.

Consumer<String> printUpperCase = str -> System.out.println(str.toUpperCase());
List<String> names = Arrays.asList("Jon", "Sansa", "Arya", "Bran");

4. Supplier

Supplier<T> is an interface that does not take any argument, but it produces a result.

Supplier<LocalDate> current = LocalDate::now;
System.out.println(current.get());  // Output: [current date]

Suppose you have a class RandomService that produces random numbers and is supposed to be used by other classes in your system.

class RandomService {
    Supplier<Double> getRandomNumber = Math::random;

// Usage in another class
RandomService rs = new RandomService();

5. BinaryOperator and UnaryOperator

UnaryOperator<T> takes one argument and returns a result of the same type. BinaryOperator<T> takes two arguments and returns a result of the same type.

UnaryOperator<String> upperifier = String::toUpperCase;
System.out.println(upperifier.apply("hello"));  // Output: HELLO

BinaryOperator<String> concatenator = String::concat;
System.out.println(concatenator.apply("Hello ", "World"));  // Output: Hello World

You have already learned about a few key functional interfaces in Java and how to use them with lambda expressions. Now, I’ll introduce you to a few more advanced topics about functional interfaces:

1. Custom Functional Interface

If the built-in functional interfaces in Java do not satisfy your requirements, you can define your own functional interfaces. Here is an example of a custom functional interface:

interface CustomInterface {
    String concatenateStrings(String s1, String s2);

You can now use it like this:

CustomInterface ci = (s1, s2) -> s1 + s2;
System.out.println(ci.concatenateStrings("Hello", " World")); // Output: Hello World

2. Method References

In some cases, lambdas just call an existing method. In those cases, we can use method references to make the code clearer. Here are some examples

Consumer<String> printer = System.out::println; // same as s -> System.out.println(s)

Predicate<String> lengthCheck = String::isEmpty; // same as s -> s.isEmpty()

Supplier<LocalDate> current = LocalDate::now; // same as () -> LocalDate.now()

3. Chaining Functional Interface Calls (Compose and AndThen)

You can chain multiple calls of Function, Consumer, and Predicate using default methods they provide, such as compose, andThen.

Function<Integer, Integer> multiplyBy2 = x -> x * 2;
Function<Integer, Integer> add1 = x -> x + 1;
Function<Integer, Integer> add1AndThenMultiplyBy2 = add1.andThen(multiplyBy2);

System.out.println(add1AndThenMultiplyBy2.apply(2)); // Output: 6

Remember that with compose, functions execute in reverse order.

Function<Integer, Integer> multiplyBy2ThenAdd1 = add1.compose(multiplyBy2);

System.out.println(multiplyBy2ThenAdd1.apply(2)); // Output: 5

Chaining calls this way leads to functional-style programming that can make your code more readable and maintainable by creating pipelines of transformations.

Real-world use of the functional interface is prevalent in Java library features such as Stream API, where they come together with lambda expressions to offer functional programming capabilities. They help contribute to writing clean, robust and concurrent code structures.

Overall, functional interfaces bring the power of functional programming to Java and are extensively used for implementing simple callback-style interfaces, or for defining “thin” data structures used in control statements, among other uses.

What are Method References in Java?

Method references in Java are a feature that was introduced in Java 8. They provide a way to refer to a method without actually executing it. They are often used in conjunction with Java’s functional programming features, such as Streams and Lambdas, where a method to be executed is often expected as a parameter.

The syntax for a method reference is the name of the class (or the name of an object), followed by :: and the method’s name. Here’s an example:

List<String> words = Arrays.asList("Hello", "Method", "References", "In", "Java");

// Let's use a method reference to print each word in the list

In the above code, System.out::println is a method reference. The forEach method expects a lambda that takes a parameter and does something with it. Here, the println method of the System.out class is being referenced, and it will be used to print each word in the list.

There are four types of method references in Java:

  1. Static method reference: They refer to the static methods of a class. For example, ClassName::staticMethodName.
  2. Instance method reference of a particular object: They refer to the instance methods of a particular object. For example, in above code System.out::println.
  3. Instance method reference of an arbitrary object: They refer to the instance methods where the first parameter is the target of the method. For example, String::length.
  4. Constructor reference: They refer to the constructor of a class. For example, ClassName::new.

Let’s take a deeper look at the four kinds of method references with more elaborated examples.

1. Static method references:

Static method references can be used when the method to be invoked is a static method. For example:

package org.kodejava.basic;

import java.util.stream.Stream;

public class StaticMethodRef {
    public static void main(String[] args) {
        String[] array = {"Java", "Python", "Ruby", "JavaScript"};

    static void printStr(String str) {
        System.out.println("printStr method called with value: " + str);

In this example, the printStr method is a static method, and we reference this method using StaticMethodRef::printStr.

2. Instance method reference of a particular object:

Instance method references can be used when the method to be invoked is an instance method. For example:

package org.kodejava.basic;

import java.util.stream.Stream;

public class InstanceMethodRef {
    public static void main(String[] args) {
        InstanceMethodRef instance = new InstanceMethodRef();
        String[] array = {"Java", "Python", "Ruby", "JavaScript"};

    void printInstanceStr(String str) {
        System.out.println("printInstanceStr method called with value: " + str);

In this example, printInstanceStr is an instance method, and we create an instance of InstanceMethodRef and refer to an instance method instance::printInstanceStr.

3. Instance method reference of an arbitrary object:

We can do this when we have a collection of instances and want to invoke a method on them. For example:

package org.kodejava.basic;

import java.util.stream.Stream;

public class InstanceMethodReferenceArbitrary {
    public static void main(String[] args) {
        String[] array = {"Java", "Python", "Ruby", "JavaScript"};

In this example, String::toUpperCase invokes the toUpperCase method for every instance of the String in the Stream.

4. Constructor reference:

Constructor references are used for a constructor call. For example:

package org.kodejava.basic;

import java.util.stream.Stream;

class Student {
    String name;

    Student(String name) {
        this.name = name;

public class ConstructorReference {
    public static void main(String[] args) {
        Stream.of("John", "Martin", "Don")
                .forEach(student -> System.out.println("Student name is: " + student.name));

In the above example, Student::new creates a new instance of Student.

How do I create a string of repeated characters?

The following code demonstrates how to create a string of repeated characters. We use the String.repeat(int count) method introduced in Java 11. This method takes one parameter of type int which is the number of times to repeat the string. The count must be a positive number, a negative number will cause this method to throw java.lang.IllegalArgumentException.

In the snippet below, we use the method to repeat characters and draw some triangles. We combine the repeat() method with a for loop to draw the triangles.

package org.kodejava.basic;

public class StringRepeatDemo {
    public static void main(String[] args) {
        String star = "*";
        String fiveStars = star.repeat(5);
        System.out.println("fiveStars = " + fiveStars);

        String arrow = "-->";
        String arrows = arrow.repeat(10);
        System.out.println("arrows    = " + arrows);

The outputs of the code snippet above are:

fiveStars = *****
arrows    = -->-->-->-->-->-->-->-->-->-->
package org.kodejava.basic;

public class StringRepeatDemo {
    public static void main(String[] args) {
        String asterisk = "#";
        for (int i = 1; i <= 10; i++) {

The outputs of the code snippet above are:

package org.kodejava.basic;

public class StringRepeatDemo {
    public static void main(String[] args) {
        int height = 10;
        for (int i = 1, j = 1; i <= height; i++, j += 2) {
            System.out.println(" ".repeat(height - i) + "*".repeat(j));

The outputs of the code snippet above are: