random element from list guide (#690)

* random element from list guide

* refactor

* with removing

* refactor + thread optimized index generation

* refactor

* final review refactor

core-java-8/src/test/java/com/baeldung/RandomListElementTest.java

@@ -3,15 +3,71 @@ package com.baeldung;
 import org.junit.Test;
 
 import java.util.Arrays;
+import java.util.Collections;
 import java.util.List;
 import java.util.Random;
+import java.util.concurrent.ThreadLocalRandom;
 
 public class RandomListElementTest {
 
     @Test
-    public void givenList_whenRandomNumberChosen_shouldReturnARandomElement() {
+    public void givenList_whenRandomIndexChosen_shouldReturnARandomElementUsingRandom() {
         List<Integer> givenList = Arrays.asList(1, 2, 3);
         Random rand = new Random();
+
         givenList.get(rand.nextInt(givenList.size()));
     }
+
+    @Test
+    public void givenList_whenRandomIndexChosen_shouldReturnARandomElementUsingMathRandom() {
+        List<Integer> givenList = Arrays.asList(1, 2, 3);
+
+        givenList.get((int)(Math.random() * givenList.size()));
+    }
+
+    @Test
+    public void givenList_whenNumberElementsChosen_shouldReturnRandomElementsRepeat() {
+        Random rand = new Random();
+        List<String> givenList = Arrays.asList("one", "two", "three", "four");
+
+        int numberOfElements = 2;
+
+        for (int i = 0; i < numberOfElements; i++) {
+            int randomIndex = rand.nextInt(givenList.size());
+            givenList.get(randomIndex);
+        }
+    }
+
+    @Test
+    public void givenList_whenNumberElementsChosen_shouldReturnRandomElementsNoRepeat() {
+        Random rand = new Random();
+        List<String> givenList = Arrays.asList("one", "two", "three", "four");
+
+        int numberOfElements = 2;
+
+        for (int i = 0; i < numberOfElements; i++) {
+            int randomIndex = rand.nextInt(givenList.size());
+            givenList.get(randomIndex);
+            givenList.remove(randomIndex);
+        }
+    }
+
+    @Test
+    public void givenList_whenSeriesLengthChosen_shouldReturnRandomSeries() {
+        List<Integer> givenList = Arrays.asList(1, 2, 3, 4, 5, 6);
+        Collections.shuffle(givenList);
+
+        int randomSeriesLength = 3;
+
+        givenList.subList(0, randomSeriesLength - 1);
+    }
+
+    @Test
+    public void givenList_whenRandomIndexChosen_shouldReturnElementThreadSafely() {
+        List<Integer> givenList = Arrays.asList(1, 2, 3, 4, 5, 6);
+        int randomIndex = ThreadLocalRandom.current().nextInt(10);
+
+        givenList.get(randomIndex);
+    }
+
 }