Unit tests just for understand: What is a true way of unit testing

gistfile1.txt

package how.to.use.mockito;

import static org.hamcrest.CoreMatchers.is;
import static org.junit.Assert.assertThat;
import static org.mockito.Mockito.mock;
import static org.mockito.Mockito.verify;
import static org.mockito.Mockito.when;

import java.io.ByteArrayInputStream;
import java.io.FileOutputStream;
import java.io.InputStream;
import java.net.URL;
import java.nio.channels.FileChannel;

import org.junit.Test;
import org.junit.runner.RunWith;
import org.powermock.api.mockito.PowerMockito;
import org.powermock.core.classloader.annotations.PrepareForTest;
import org.powermock.modules.junit4.PowerMockRunner;

@RunWith(PowerMockRunner.class)
@PrepareForTest(Util.class)
public class UtilTest
{
  @Test
	public void many_free_memory() throws Exception {
		Runtime runtimeMock = mock(Runtime.class);
		when(runtimeMock.freeMemory()).thenReturn((long) 800);
		when(runtimeMock.totalMemory()).thenReturn((long) 1000);
		
		PowerMockito.mockStatic(Runtime.class);
		PowerMockito.when(Runtime.getRuntime()).thenReturn(runtimeMock);
		
		assertThat(Util.isMemoryEnough(), is(true));
	}
	
	@Test
	public void no_free_memory() throws Exception {
		Runtime runtimeMock = mock(Runtime.class);
		when(runtimeMock.freeMemory()).thenReturn((long) 0);
		when(runtimeMock.totalMemory()).thenReturn((long) 1000);
		
		PowerMockito.mockStatic(Runtime.class);
		PowerMockito.when(Runtime.getRuntime()).thenReturn(runtimeMock);
		
		assertThat(Util.isMemoryEnough(), is(false));
	}
	
	@Test
	public void downloadInFile_channel_was_invoked() throws Exception {
		InputStream stream = new ByteArrayInputStream("Good luck Mockito!".getBytes());
		
		URL urlMock = PowerMockito.mock(URL.class);
		PowerMockito.whenNew(URL.class).withAnyArguments().thenReturn(urlMock);
		when(urlMock.openStream()).thenReturn(stream);
		
		FileOutputStream outputMock = mock(FileOutputStream.class);
		when(outputMock.getChannel()).thenReturn(mock(FileChannel.class));
		PowerMockito.whenNew(FileOutputStream.class).withAnyArguments().thenReturn(outputMock);
		
		Util.downloadInFile("http://www.google.com", "google.html");
		
		verify(outputMock).getChannel();
	}

}