LazyLoading.java

///usr/bin/env jbang "$0" "$@" ; exit $? 

import java.util.function.Supplier;

public class LazyLoading {
    static int callCount = 0;

    /** This is the core lazy loading functionality */
    static class LazyLoader<T> {
        private final Supplier<T> sup;
        private T value;
        
        LazyLoader(Supplier<T> sup) {
            this.sup = sup;
        }

        T get() {
            if(value == null) {
                synchronized(this) {
                    if(value == null) {
                        value = sup.get();
                    }
                }
            }
            return value;
        }
    }

    /** Example using LazyLoader */
    static class LazyLoadingPojo {
        LazyLoader<Integer> count = new LazyLoader<>(this::computeCount);

        public int getCount() {
            return count.get();
        }

        private int computeCount() {
            return ++callCount;
        }
    }

    // THE REST IS TESTS //
    
    static void checkThat(String info, boolean value) {
        if(!value) {
            throw new RuntimeException("Assertion failed: " + info);
        }
    }

    public static void main(String [] args) {
        checkThat("Expecting initial callCount to be zero", callCount == 0);
        final LazyLoadingPojo p = new LazyLoadingPojo();
        checkThat("Expecting callCount still be zero", callCount == 0);
        assert(callCount == 0);
        for(int i=1; i <= 10; i++) {
            p.getCount();
            checkThat("At step " + i + ", expecting callCount to remain at 1", callCount == 1);
        }
        System.out.println("All tests passed, callCount=" + callCount);
    }
}