initialize arraylist with null or zeros

core-java-modules/core-java-collections-array-list-2/pom.xml

@@ -0,0 +1,32 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns="http://maven.apache.org/POM/4.0.0"
+         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+    <parent>
+        <artifactId>parent-modules</artifactId>
+        <groupId>com.baeldung</groupId>
+        <version>1.0.0-SNAPSHOT</version>
+        <relativePath>../../pom.xml</relativePath>
+    </parent>
+    <modelVersion>4.0.0</modelVersion>
+
+    <artifactId>core-java-collections-array-list-2</artifactId>
+    <build>
+        <plugins>
+            <plugin>
+                <groupId>org.apache.maven.plugins</groupId>
+                <artifactId>maven-compiler-plugin</artifactId>
+                <configuration>
+                    <source>9</source>
+                    <target>9</target>
+                </configuration>
+            </plugin>
+        </plugins>
+    </build>
+
+    <properties>
+        <maven.compiler.source>17</maven.compiler.source>
+        <maven.compiler.target>17</maven.compiler.target>
+    </properties>
+
+</project>

core-java-modules/core-java-collections-array-list-2/src/main/java/com/baeldung/InitializeArrayListWithNullOrZeros.java

@@ -0,0 +1,14 @@
+package com.baeldung;
+
+import java.util.ArrayList;
+
+public class InitializeArrayListWithNullOrZeros {
+
+     public static void main(String[] args) {
+
+         ArrayList<Integer> arrayList = new ArrayList<>();
+         for (int i = 0; i< 10; i++) {
+             arrayList.add(null);
+         }
+     }
+}

core-java-modules/core-java-collections-array-list-2/src/test/java/com/baeldung/InitializeArrayListWithNullOrZerosUnitTest.java

@@ -0,0 +1,75 @@
+package com.baeldung;
+
+import org.junit.Test;
+import org.junit.jupiter.api.Assertions;
+
+import java.util.*;
+import java.util.stream.Collectors;
+import java.util.stream.IntStream;
+import java.util.stream.Stream;
+
+
+public class InitializeArrayListWithNullOrZerosUnitTest {
+
+    @Test
+    public void whenInitializingListWithNCopies_thenListIsCorrectlyPopulated() {
+        // when
+        ArrayList<Integer> list = IntStream.of(new int[10])
+                .boxed()
+                .collect(Collectors.toCollection(ArrayList::new));
+
+        // then
+        Assertions.assertEquals(10, list.size());
+        Assertions.assertTrue(list.stream().allMatch(elem -> elem == 0));
+    }
+
+    @Test
+    public void whenInitializingListWithStream_thenListIsCorrectlyPopulated() {
+
+        // when
+        ArrayList<Integer> listWithZeros = Stream.generate(() -> 0)
+                .limit(10).collect(Collectors.toCollection(ArrayList::new));
+
+        ArrayList<Object> listWithNulls = Stream.generate(() -> null)
+                .limit(10).collect(Collectors.toCollection(ArrayList::new));
+
+        // then
+        Assertions.assertEquals(10, listWithZeros.size());
+        Assertions.assertTrue(listWithZeros.stream().allMatch(elem -> elem == 0));
+
+        Assertions.assertEquals(10, listWithNulls.size());
+        Assertions.assertTrue(listWithNulls.stream().allMatch(Objects::isNull));
+    }
+
+    @Test public void whenInitializingListWithIntStream_thenListIsCorrectlyPopulated() {
+        // when
+        ArrayList<Integer> list = IntStream.of(new int[10])
+                .boxed()
+                .collect(Collectors.toCollection(ArrayList::new));
+
+        // then
+        Assertions.assertEquals(10, list.size());
+        Assertions.assertTrue(list.stream().allMatch(elem -> elem == 0)); }
+
+    @Test
+    public void whenInitializingListWithAsList_thenListIsCorrectlyPopulated() {
+        // when
+        Integer[] integers = new Integer[10];
+        Arrays.fill(integers, 0);
+        List<Integer> integerList = new ArrayList<>(Arrays.asList(integers));
+
+        // then
+        Assertions.assertEquals(10, integerList.size());
+        Assertions.assertTrue(integerList.stream().allMatch(elem -> elem == 0));
+    }
+
+    @Test
+    public void whenInitializingListWithVector_thenListIsCorrectlyPopulated() {
+        // when
+        List<Integer> integerList = new Vector<>() {{setSize(10);}};
+
+        // then
+        Assertions.assertEquals(10, integerList.size());
+        Assertions.assertTrue(integerList.stream().allMatch(Objects::isNull));
+    }
+}