java.md

Java notes

✨ Awesome Java: a curated list of awesome Java frameworks, libraries and software, by Andreas Kull.

 

🔍 Get the location of a class at runtime:

MyClass.class.getProtectionDomain().getCodeSource().getLocation()

 

One-liner to create a list initialized with some data (from Java 5 on):

List<String> list = Arrays.asList("a", "b", "c");

 

Use an ImmutableMap to translate arbitrary strings to enum values, instead of using switch/case or if/else structures:

public enum Fruit {
   APPLE, ORANGE, BANANA, RASPBERRY
}

import com.google.common.collect.ImmutableMap; // Google Guava
import java.util.Map;
import Fruit;
import static Fruit.*;

public class Greengrocer
{
   private static final Map<String, Fruit> FRUIT_MAP = ImmutableMap.of
      (
         "POMME", APPLE,
         "ORANGE", ORANGE,
         "BANANE", BANANA,
         "FRAMBOISE", RASPBERRY
      );
      
   public Fruit getFruitFromFrench(final String frenchFruit)
   {
      return FRUIT_MAP.get(frenchFruit);
      // Returns null if the input is null.
      // Returns null if the input is out of the mapping.
   }
}

 

Use an inner class when testing many repetitive unit test cases:

   @Test
   public void testManyCases()
   {
      class LocalTestCase {
         boolean isOrganic;
         String frenchFruit;
         Fruit expectedFruit;

         LocalTestCase(boolean o, String ff, Fruit ef) {
            isOrganic = o;
            frenchFruit = ff;
            expectedFruit = ef;
         }
      }

      List<LocalTestCase> testCaseList = Arrays.asList(
         new LocalTestCase(true, "POMME",     Fruit.APPLE),
         new LocalTestCase(true, "ORANGE",    Fruit.ORANGE),
         new LocalTestCase(true, "BANANE",    Fruit.BANANA),
         new LocalTestCase(true, "FRAMBOISE", Fruit.RASPBERRY),

         new LocalTestCase(false, "POMME",     Fruit.APPLE),
         new LocalTestCase(false, "ORANGE",    Fruit.ORANGE),
         new LocalTestCase(false, "BANANE",    Fruit.BANANA),
         new LocalTestCase(false, "FRAMBOISE", Fruit.RASPBERRY),

         new LocalTestCase(true, "APPLE",     null),
         new LocalTestCase(true, "BANANA",    null),
         new LocalTestCase(true, "RASPBERRY", null),

         new LocalTestCase(false, "APPLE",     null),
         new LocalTestCase(false, "BANANA",    null),
         new LocalTestCase(false, "RASPBERRY", null),

         new LocalTestCase(true, "pomme",       null),
         new LocalTestCase(true, "Orange",      null),
         new LocalTestCase(true, " BANANE",     null),
         new LocalTestCase(true, " FRAMBOISE ", null),

         new LocalTestCase(false, "Pomme",      null),
         new LocalTestCase(false, " ORANGE ",   null),
         new LocalTestCase(false, "banane",     null),
         new LocalTestCase(false, " FRAMBOISE", null),

         new LocalTestCase(true, "",       null),
         new LocalTestCase(true, " ",      null),
         new LocalTestCase(true, "FRAISE", null),
         new LocalTestCase(true, "P",      null),
         new LocalTestCase(true, null,     null),

         new LocalTestCase(false, "",       null),
         new LocalTestCase(false, " ",      null),
         new LocalTestCase(false, "CERISE", null),
         new LocalTestCase(false, "B",      null),
         new LocalTestCase(false, null,     null)
      );

      for (LocalTestCase testCase : testCaseList) {
         Greengrocer g = new Greengrocer(testCase.isOrganic);
         assertEquals(testCase.expectedFruit, g.getFruitFromFrench(testCase.frenchFruit));
      }
   }

 

Check only one specific test then build and skip all tests:

mvn clean verify -Dtest=ClassName#methodeName
mvn install -DskipTests=true

 

Get current class statically:

import java.lang.invoke.MethodHandles;

...

private static final Class currentClass = MethodHandles.lookup().lookupClass();

private static final String className = currentClass.getSimpleName();

 

Use Pattern.quote to escape text part of a regular expression:

import java.util.regex.Pattern;

String[] letters = "A|B|C".split(Pattern.quote("|"));

org.junit.Assert.assertEquals(3, letters.length);

 

Detect plain ASCII text:

import static java.nio.charset.StandardCharsets.US_ASCII;

/**
 * Check that a byte array is plain ASCII text.
 * See {@link java.lang.String#String(byte[], Charset)} to understand how it works.
 *
 * @param b a byte array
 * @return true if b is plain ASCII text, otherwise false.
 */
private static boolean isPlainAsciiText(final byte[] b)
{
   return Arrays.equals(b, new String(b, US_ASCII).getBytes(US_ASCII));
}

import com.google.common.base.Ascii; // from com.google.guava:guava:30.1-jre

/**
 * Check that a byte value belongs to the subset of ASCII printable characters, i.e.:
 * - LINE-FEED (10)
 * - CARRIAGE-RETURN (13)
 * - all characters from SPACE (32) to TILDE (126)
 *
 * @param b a byte
 * @return true if b belongs the subset of ASCII printable characters, otherwise false.
 */
private static boolean isByteInAsciiPrintableSubset(final byte b)
{
   return (b == Ascii.LF) || (b == Ascii.CR) || (b >= Ascii.SPACE && b < Ascii.MAX);
}