Util class for unit tests (gist:5759950) just for understand: What is a true way of unit testing

gistfile1.txt

package how.to.use.mockito;

import java.io.FileOutputStream;
import java.io.IOException;
import java.net.URL;
import java.nio.channels.Channels;
import java.nio.channels.ReadableByteChannel;

public class Util {
  
	public static void downloadInFile(String url, String file) {
		try (
			ReadableByteChannel byteChannel = Channels.newChannel(new URL(url).openStream());
			FileOutputStream fileOutputStream = new FileOutputStream(file);
		) {
			fileOutputStream.getChannel().transferFrom(byteChannel, 0, Long.MAX_VALUE);
		} catch (IOException e) {
			e.printStackTrace();
		}
	}
	
	public static boolean isMemoryEnough() {
		long totalMemory = Runtime.getRuntime().totalMemory();
		long freeMemory = Runtime.getRuntime().freeMemory();
		double freeMemoryInPercent = (double) freeMemory/(double) totalMemory;
		if (freeMemoryInPercent > AppConfig.MEMORY_ENOUGH_IN_PERCENT_LIMIT) {
			return true;
		} else {
			return false;
		}
	}
	
	class AppConfig {
		public static final double MEMORY_ENOUGH_IN_PERCENT_LIMIT = 0.48;
	}
	
}