Does Java 8 have cached support for suppliers?

Question

The guava library has it's own Supplier which does not extend Java 8 Supplier. Also guava provides a cache for suppliers - Suppliers#memoize.

Is there something similar, but for Java 8 Suppliers?

Answer 1

There's no built-in Java function for memoization, though it's not very hard to implement it, for example, like this:

public static <T> Supplier<T> memoize(Supplier<T> delegate) {
    AtomicReference<T> value = new AtomicReference<>();
    return () -> {
        T val = value.get();
        if (val == null) {
            val = value.updateAndGet(cur -> cur == null ? 
                    Objects.requireNonNull(delegate.get()) : cur);
        }
        return val;
    };
}

Note that different implementation approaches exist. The above implementation may call the delegate several times if the memoized supplier requested simultaneously several times from the different threads. Sometimes such implementation is preferred over the explicit synchronization with lock. If lock is preferred, then DCL could be used:

public static <T> Supplier<T> memoizeLock(Supplier<T> delegate) {
    AtomicReference<T> value = new AtomicReference<>();
    return () -> {
        T val = value.get();
        if (val == null) {
            synchronized(value) {
                val = value.get();
                if (val == null) {
                    val = Objects.requireNonNull(delegate.get());
                    value.set(val);
                }
            }
        }
        return val;
    };
}

Also note, as @LouisWasserman correctly mentioned in comments, you can easily transform JDK supplier into Guava supplier and vice versa using method reference:

java.util.function.Supplier<String> jdkSupplier = () -> "test";
com.google.common.base.Supplier<String> guavaSupplier = jdkSupplier::get;
java.util.function.Supplier<String> jdkSupplierBack = guavaSupplier::get;

So it's not a big problem to switch between Guava and JDK functions.