[list-with-default] Set Default Value for Elements in List (#14125)

* [list-with-default] Set Default Value for Elements in List

* [list-with-default] remove the ncopies and stream methods.

* [list-with-default]  remove irrelevant codes

core-java-modules/core-java-collections-list-5/src/test/java/com/baeldung/java/listwithdefault/ListWithDefaultValuesUnitTest.java

@@ -0,0 +1,92 @@
+package com.baeldung.java.listwithdefault;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import java.time.Instant;
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.Date;
+import java.util.List;
+import java.util.function.Supplier;
+
+import org.junit.jupiter.api.Test;
+
+import com.google.common.collect.Lists;
+
+public class ListWithDefaultValuesUnitTest {
+    private static final List<String> EXPECTED_LIST = Lists.newArrayList("new", "new", "new", "new", "new");
+    private static final Date DATE_EPOCH = Date.from(Instant.EPOCH);
+    private static final Date DATE_NOW = new Date();
+
+    static <T> List<T> newListWithDefault(T value, int size) {
+        List<T> list = new ArrayList<>(size);
+        for (int i = 0; i < size; i++) {
+            list.add(value);
+        }
+        return list;
+    }
+
+    static <T> List<T> newListWithDefault2(Supplier<T> supplier, int size) {
+        List<T> list = new ArrayList<>(size);
+        for (int i = 0; i < size; i++) {
+            list.add(supplier.get());
+        }
+        return list;
+    }
+
+    @Test
+    void whenUsingArraysFill_thenGetExpectedList() {
+        String[] strings = new String[5];
+        Arrays.fill(strings, "new");
+        List<String> result = Arrays.asList(strings);
+        assertEquals(EXPECTED_LIST, result);
+
+        //result is a fixed size list
+        assertThrows(UnsupportedOperationException.class, () -> result.add("a new string"));
+        assertThrows(UnsupportedOperationException.class, () -> result.remove(0));
+
+        //result's element can be "set"
+        result.set(2, "a new value");
+        assertEquals("a new value", result.get(2));
+
+        Date[] dates = new Date[2];
+        Arrays.fill(dates, Date.from(Instant.EPOCH));
+        List<Date> dateList = Arrays.asList(dates);
+        assertEquals(Lists.newArrayList(DATE_EPOCH, DATE_EPOCH), dateList);
+        dateList.get(0)
+          .setTime(DATE_NOW.getTime());
+        assertEquals(Lists.newArrayList(DATE_NOW, DATE_NOW), dateList);
+
+    }
+
+    @Test
+    void whenUsingNewListWithDefault_thenGetExpectedList() {
+        List<String> result = newListWithDefault("new", 5);
+        assertEquals(EXPECTED_LIST, result);
+
+        List<Integer> intList = newListWithDefault(42, 3);
+        assertEquals(Lists.newArrayList(42, 42, 42), intList);
+
+        List<Date> dateList = newListWithDefault(Date.from(Instant.EPOCH), 2);
+        assertEquals(Lists.newArrayList(DATE_EPOCH, DATE_EPOCH), dateList);
+        dateList.get(0)
+          .setTime(DATE_NOW.getTime());
+        assertEquals(Lists.newArrayList(DATE_NOW, DATE_NOW), dateList);
+    }
+
+    @Test
+    void whenUsingNewListWithDefault2_thenGetExpectedList() {
+        List<String> result = newListWithDefault2(() -> "new", 5);
+        assertEquals(EXPECTED_LIST, result);
+
+        List<Integer> intList = newListWithDefault2(() -> 42, 3);
+        assertEquals(Lists.newArrayList(42, 42, 42), intList);
+
+        List<Date> dateList = newListWithDefault2(() -> Date.from(Instant.EPOCH), 2);
+        assertEquals(Lists.newArrayList(DATE_EPOCH, DATE_EPOCH), dateList);
+        dateList.get(0)
+          .setTime(DATE_NOW.getTime());
+        assertEquals(Lists.newArrayList(DATE_NOW, DATE_EPOCH), dateList);
+    }
+}