Figuring out the building of lists from functions and lazy lists with memoization.

LazyListTest.java

import static org.junit.Assert.*;

import java.util.List;

import org.junit.Test;

import com.google.common.base.Function;
import com.google.common.base.Supplier;
import com.google.common.base.Suppliers;
import com.google.common.collect.ImmutableList;
import com.google.common.collect.Lists;

class Counter {
	int count = 0;
}

public class LazyListTest {

	@Test
	public void test() {
		
		{
			final Counter count = new Counter();
			
			List<String> s = Lists.transform(
					(List<String>) ImmutableList.of("hello","world", "asd"),
					new Function<String, String>() {
						@Override
						public String apply(String input) {
							System.out.println("uppercasing " + input);
							count.count++;
							return input.toUpperCase();
						}
					});
			
			
			s.get(0); //each time this is called, "uppercasing hello" is printed.
			s.get(0);
			s.get(0);
			s.get(0);
			String str = s.get(0);
					
			assertEquals(5,count.count); //called five times
			assertEquals("HELLO",str);
		}
		
		{
			final Counter count = new Counter();
			
			List<String> s = 
				Lists.transform(
					ImmutableList.copyOf(
							Lists.transform(
									(List<String>) ImmutableList.of("hello","world", "asd"),									
									new Function<String,  Supplier<String>>() 
									{
										public Supplier<String> apply(String input) {
											return Suppliers.memoize(Suppliers.compose(
													new Function<String,String>() {
														@Override
														public String apply(String input) {
															System.out.println("uppercasing " + input);
															count.count++;
															return input.toUpperCase();
														}														
													},
													Suppliers.ofInstance(input)));
									}})),
									  new Function<Supplier<String>, String>() {
									    public String apply(Supplier<String> supplier) {
									      return supplier.get();
									    }
									  });

			s.get(1); //the FIRST time this is called, "uppercasing world" is called.
			s.get(1); //every other time is cached.
			s.get(1);
			s.get(1);
			String str = s.get(1);
					
			assertEquals(1,count.count); //called once
			assertEquals("WORLD",str);
		}
	}
	
}