Initialize an ArrayList in Java

Initialize in one line with Java 9+ List.of and Set.of

• Provide either Set.of or List.of factory method, since Java 9+, to the ArrayList(Collection) constructor to create and init an ArrayList in one line at the creation time

@Test
public void initWithListOfAndSetOf() {  
    List<Integer> arrayList1 = new ArrayList<>(List.of(3, 1, 2));
    assertThat(arrayList1).contains(3, 1, 2);

    List<Integer> arrayList2 = new ArrayList<>(Set.of(5, 4, 6));
    assertThat(arrayList2).contains(5, 4, 6);
}

• List.of permits duplicated elements while Set.of doesn't, it throws IllegalArgumentException

  Both List.of and Set.of don't permit null element, they throw NullPointerException

Initialize in one line with Arrays.asList

• Provide Arrays.asList to the ArrayList(Collection) constructor to create and init an ArrayList in one line

  Array.asList accepts duplicated and null elements

@Test
public void initWithArraysAsList() {  
    List<Integer> set = new ArrayList<>(Arrays.asList(3, 1, 2, null));
    assertThat(set).contains(3, 1, 2, null);
}

Initialize in one line from an existing Collection

• Provide an existing Collection to the ArrayList(Collection) constructor to create and init a ArrayList in one line

@Test
public void initFromAnExistingCollection() {  
    List<Integer> arrayList1 = new ArrayList<>(Arrays.asList(3, 1, 2, null));

    List<Integer> arrayList2 = new ArrayList<>(arrayList1);
    assertThat(arrayList2).contains(3, 1, 2, null);
}

Initialize by using add and addAll methods

• You can use add or addAll method to initialize an ArrayList after the creation time

@Test
public void initWithAddAndAddAll() {  
    List<Integer> arrayList1 = new ArrayList<>(Set.of(1, 2, 3));

    List<Integer> arrayList2 = new ArrayList<>();
    arrayList1.add(1);
    arrayList1.add(1); // duplicated element is ignored
    arrayList1.add(null); // permits null element
    arrayList2.addAll(arrayList1);

    assertThat(arrayList2).contains(1, 2, 3, null);
}

Double brace initialization should not be used in practice

• Double brace initialization can cause a memory leak as it creates anonymous classes with an hidden reference to the owner instance object

@Test
public void initWithDoubleBrace() {  
    List<Integer> arrayList = new ArrayList<>(){{
        add(3);
        add(1);
        add(2);
    }};

    assertThat(arrayList).contains(1, 2, 3);
}

Initialize with capacity

• Use this to improve performance when you need to add a large number of items than the ArrayList default capacity (10)
• Internally, an ArrayList is backed by a fixed size array. When its capacity is full, Java needs to create a new array with larger capacity and copies old items to that

@Test
public void initWithCapacity() {  
    List<Integer> arrayList = new ArrayList<>(1000);

    for (int i = 0; i < 1000; i++) {
        arrayList.add(i);
    }

    assertThat(arrayList).hasSize(1000);
}

Conclusion

In this tutorial, we learned some ways to create and initialize an ArrayList in Java. You can find the full source code as below