Singleton Supplier

Singleton.java

package com.kaffeinelabs.function;

import java.util.function.Supplier;

public class Singleton<T> implements Supplier<T> {
  private final Supplier<? extends T> factory;
  private T instance = null;

  private Singleton(final Supplier<? extends T> factory) {
    this.factory = factory;
  }

  public static <T> Supplier<T> of(final Supplier<? extends T> factory) {
    return new Singleton<>(factory);
  }

  @Override
  public T get() {
    if (this.instance == null) {
      this.instance = this.factory.get();
    }
    return this.instance;
  }
}

SingletonTest.java

package com.kaffeinelabs.function;

import org.junit.jupiter.api.Test;

import java.util.concurrent.atomic.AtomicInteger;
import java.util.function.Supplier;

import static org.junit.jupiter.api.Assertions.assertEquals;

public class SingletonTest {
  final AtomicInteger instanceCount = new AtomicInteger(1);

  @Test
  void itCreatesAnInstanceOfTheObject() {
    final Supplier<TestService> singleton = Singleton.of(() -> new TestService(instanceCount.getAndIncrement()));

    final TestService service = singleton.get();

    assertEquals(new TestService(1), service);
  }

  @Test
  void itCreatesASingleInstanceOfTheObject() {
    final Supplier<TestService> singleton = Singleton.of(() -> new TestService(instanceCount.getAndIncrement()));

    final TestService firstService = singleton.get();
    final TestService secondService = singleton.get();

    assertEquals(firstService, secondService);
  }

  record TestService(int number) {
  }
}