How do I use the Supplier functional interface in Java?

The Supplier functional interface in Java is part of the java.util.function package and is used when we need to supply or produce a result without taking any input. It is a functional interface, meaning it can be represented as a lambda expression or method reference.

The Supplier interface has a single abstract method:

T get();

This method returns an object of type T and takes no arguments.

Usage of Supplier

Lambda Expression: We can use a lambda expression to define the logic for producing a result.
Method Reference: If we have an existing static or instance method that matches the Supplier signature (no parameters, return a value), we can use a method reference.

Here are a few examples to demonstrate how to use Supplier:

Example 1: Basic Supplier Usage

package org.kodejava.util.function;

import java.util.function.Supplier;

public class SupplierExample {
    public static void main(String[] args) {
        // Using a Supplier to generate a string
        Supplier<String> stringSupplier = () -> "Hello, Supplier!";
        System.out.println(stringSupplier.get());
        // Output: Hello, Supplier!
    }
}

Example 2: Supplier with Random Number

package org.kodejava.util.function;

import java.util.function.Supplier;
import java.util.Random;

public class RandomNumberExample {
    public static void main(String[] args) {
        // Using a Supplier to provide a random int
        // Random number between 0-99
        Supplier<Integer> randomSupplier = () -> new Random().nextInt(100);

        System.out.println("Random number: " + randomSupplier.get());
        System.out.println("Another random number: " + randomSupplier.get());
    }
}

Example 3: Method Reference with Supplier

package org.kodejava.util.function;

import java.util.function.Supplier;

public class SupplierMethodReferenceExample {
    public static void main(String[] args) {
        // Supplier using method reference
        Supplier<Double> piSupplier = Math::random;

        System.out.println("Random value using method reference: " + piSupplier.get());
    }
}

Example 4: Supplying an Object

package org.kodejava.util.function;

import java.util.function.Supplier;

class Person {
    String name;

    public Person(String name) {
        this.name = name;
    }

    @Override
    public String toString() {
        return "Person{name='" + name + "'}";
    }
}

public class ObjectSupplierExample {
    public static void main(String[] args) {
        // Using a Supplier to construct an object
        Supplier<Person> personSupplier = () -> new Person("Rosa");

        Person person = personSupplier.get();
        System.out.println(person);
        // Output: Person{name='Rosa'}
    }
}

Where to Use `Supplier`

Lazy Evaluation: To delay the execution of some logic until a value is needed.
Factories: To create new objects.
Caching or Computed Values: Use Supplier to generate values that are expensive to compute only when needed.

By using Supplier, we can encapsulate the logic of generating or supplying values while following the functional programming paradigm in Java.

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