I’m preparing the OCAJP: Oracle Certified Associate Java SE 8 Programmer. Here’s the second review of the certification training. In this review, I’ll talk about the Java Core APIs, including operations of String, StringBuiler, Arrays, ArrayList, and Java Time in Java 8.

Concatenation with String

Placing one String before the other String and combining them together is called string concatenation. In Java, the concatenation is called by using the + operator. There’re several roles to remember for this operator:

  1. If both operands are numeric, + means numeric addition.
  2. If either operand is a String, + means concatenation.
  3. The expression is evaluated left to right.

Here’re some examples:

System.out.println(1 + 2);           // 3
System.out.println(1 + 2 + ".");     // 3.
System.out.println("It's " + true);  // It's true
System.out.println("It's " + 1);     // It's 1
System.out.println("It's " + "OK");  // It's OK

The String Pool

Since strings are everywhere in Java, they uses up a lot of memory. In some production applications, they can use up 25-40 percent of the memory in the entire program. Therefore, JVM optimizes the usage of strings by introducing the string pool concept: the idea is to reuse the common ones the program in an internal location of JVM (Java Virtual Machine). The string pool contains literal values appeared in the program. For example, "name" is a literal value, but myObject.toString() isn’t.

String literal = "name";
String nonLiteral1 = new String("name");
String nonLiteral2 = myObject.toString();

Mutability and Chaining of StringBuilder

Previously, we saw the usage of String. Actually, String is immutable, which means that if we keep using a lot of strings as intermediate references for concatenation, it will be very inefficient: almost all of them are immediately eligible for garbage collection after their creation.

Another solution is to use StringBuilder. The class StringBuilder is mutable, and more interestingly, it can be used for chaining multiple concatenations, which avoids the creation of string interims:

String str = new StringBuilder()

Understanding Java Arrays

Now, let’s take a look in Arrays, an ordered list in Java. It can be created in several ways:

int[] numbers = new int[3];
int[] numbers = new int[]{1, 2, 3};
int[] numbers = {1, 2, 3};  // only works during asssignment

We can use equals() to compare two arrays because an array is an object. However, the equals() method on arrays does not look at the elements of the array. As for primitives, their array is still an object.

int one = 1;                // primitive
int[] numbers = {1, 2, 3};  // object

There’re still many things to explore about array, but I can’t explain more here because of the time limit.

Converting Between Array and List

There’re several ways to convert between an array and an ArrayList. Now, let’s take a look about how to convert a List to an array.

List<String> list = new ArrayList<>();
String[] array = list.toArray(new String[0]);  // ["one", "two"]

You might ask: why we need to specify the size of 0 for the array input, is it incorrect? Actually, ArrayList will create a new array of the proper size for the return value, if the input size does not fit the return one. Here’s the source code of ArrayList#toArray(T[] a) in Java 8:

public <T> T[] toArray(T[] a) {
    if (a.length < size)
        // Make a new array of a's runtime type, but my contents:
        return (T[]) Arrays.copyOf(elementData, size, a.getClass());
    System.arraycopy(elementData, 0, a, 0, size);
    if (a.length > size)
        a[size] = null;
    return a;

Now, let’s take a look how to convert an array to List through 3 ways: using Arrays#asList(T...), ArrayList, and Java stream.

String[] wordArray = {"one", "two"};

// 1. using Arrays#asList(T...)
List<String> wordList1 = Arrays.asList(wordArray);
wordList1.remove(1);     // unsupported operation
wordList1.add("three");  // unsupported operation

// 2. using ArrayList
List<String> wordListi2 = new ArrayList<>();
for (String word : wordArray) {

// 3. using Java stream:
List<String> wordList3 = Stream.of(wordArray).collect(Collectors.toList());

Working with Dates and Times

In Java 8, a famous Java Date library—Joda Time—has been integrated into the built-in package as java.time. Thanks to this library, handling date and time become much easier. Here’s a table listing the most important classes we need to remember:

Class Date Time Time Zone
LocalDate Yes No No
LocalTime No Yes No
LocalDateTime Yes Yes No
ZonedDateTime Yes Yes Yes

Exam Essentials

In this post, I reviewed the most difficult / tricky part of Java Core APIs, including operations of String, StringBuiler, Arrays, ArrayList, and Java Time in Java 8. As for the exam essentials, you need to know about:

  • Be able to determine the output of code using String.
  • Be able to determine the output of code using StringBuilder.
  • Understand the difference between == and equals.
  • Be able to determine the output of code using ArrayList.
  • Recognize invalid uses of dates and times.