BAEL-5678 - Creating Random Numbers With No Duplicates in Java (#12698)

* BAEL-5678 - Creating Random Numbers With No Duplicates in Java

Ready for review.

* BAEL-5678 - review

* using `isEmpty()` for `hasNext()`;
* using `assertEquals` instead of `assertTrue`;

core-java-modules/core-java-numbers-5/src/main/java/com/baeldung/uniquerng/BigUniqueRng.java

@@ -0,0 +1,36 @@
+package com.baeldung.uniquerng;
+
+import java.util.Iterator;
+import java.util.LinkedHashSet;
+import java.util.NoSuchElementException;
+import java.util.Random;
+import java.util.Set;
+
+public class BigUniqueRng implements Iterator<Integer> {
+
+    private Random random = new Random();
+    private Set<Integer> generated = new LinkedHashSet<>();
+
+    public BigUniqueRng(int size, int max) {
+        while (generated.size() < size) {
+            Integer next = random.nextInt(max);
+            generated.add(next);
+        }
+    }
+
+    @Override
+    public Integer next() {
+        if (!hasNext())
+            throw new NoSuchElementException();
+
+        Iterator<Integer> iterator = generated.iterator();
+        Integer next = iterator.next();
+        iterator.remove();
+        return next;
+    }
+
+    @Override
+    public boolean hasNext() {
+        return !generated.isEmpty();
+    }
+}

core-java-modules/core-java-numbers-5/src/main/java/com/baeldung/uniquerng/UniqueRng.java

@@ -0,0 +1,42 @@
+package com.baeldung.uniquerng;
+
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.Iterator;
+import java.util.List;
+import java.util.NoSuchElementException;
+
+public class UniqueRng implements Iterator<Integer> {
+
+    private int size;
+    private List<Integer> numbers = new ArrayList<>();
+
+    public UniqueRng(int size, boolean zeroBased) {
+        this.size = size;
+        int start = (zeroBased ? 0 : 1);
+        int limit = (zeroBased ? size - 1 : size);
+
+        for (int i = start; i <= limit; i++) {
+            numbers.add(i);
+        }
+
+        Collections.shuffle(numbers);
+    }
+
+    @Override
+    public Integer next() {
+        if (!hasNext())
+            throw new NoSuchElementException();
+
+        return numbers.remove(0);
+    }
+
+    @Override
+    public boolean hasNext() {
+        return !numbers.isEmpty();
+    }
+
+    public int getSize() {
+        return size;
+    }
+}

core-java-modules/core-java-numbers-5/src/test/java/com/baeldung/uniquerng/RngUtilsUnitTest.java

@@ -0,0 +1,56 @@
+package com.baeldung.uniquerng;
+
+import static org.junit.Assert.assertEquals;
+
+import java.util.Random;
+import java.util.Set;
+import java.util.TreeSet;
+import java.util.stream.Collectors;
+
+import org.junit.Test;
+
+public class RngUtilsUnitTest {
+
+    @Test
+    public void whenNUniqueNumRequested_thenNUniqueNumConsumed() {
+        TreeSet<Integer> set = new TreeSet<>();
+        int n = 5;
+        UniqueRng rng = new UniqueRng(n, true);
+
+        while (rng.hasNext()) {
+            set.add(rng.next());
+        }
+
+        assertEquals(n, set.size());
+    }
+
+    @Test
+    public void givenYRange_whenNUniqueNumRequested_thenNUniqueNumConsumed() {
+        TreeSet<Integer> set = new TreeSet<>();
+        int n = 5;
+        int y = n * 10;
+
+        BigUniqueRng rng = new BigUniqueRng(n, y);
+        while (rng.hasNext()) {
+            set.add(rng.next());
+        }
+
+        assertEquals(n, set.size());
+    }
+
+    @Test
+    public void givenIntStreamSizeN_whenCollected_thenSetSizeN() {
+        int n = 5;
+        int from = -5;
+        int to = n * 2;
+
+        Random r = new Random();
+        Set<Integer> set = r.ints(from, to)
+            .distinct()
+            .limit(n)
+            .boxed()
+            .collect(Collectors.toSet());
+
+        assertEquals(n, set.size());
+    }
+}