Core Java APIs

0430-0506

Exam objectives

1. Using Operators and Decision Constructs

   • Test equality between Strings and objects using == and equals().

2. Creating and Using Arrays

   • Declare, instantiate, initialize and use a one-dimensional array.

   • Declare, instantiate, initialize and use a multi-dimensional array.

3. Working with Selected classes from the Java API

   • Creating and manipulating Strings.

   • Manipulate data using the StringBuilder class and its methods.

   • Declare and use an ArrayList of a given type.

   • Create and manipulate calendar data using classes from java.time.LocalDateTime, java.time.LocalDate, java.time.Local-Time, java.time.format.DateTimeFormatter, java.time.Period.

4. Working with Java Data Types

   • Develop code that uses wrapper classes such as Boolean, Double, and Integer.

Notes

Creating and Manipulating Strings

• A string is basically a sequence of characters.

• String class is a special class, doesn't need to be instantiated with new.

  • String name = "Fluffy";

  • String name = new String("Fluffy");

Concatenation

• Placing String before the other String and combining them together is called string concatenating. The + operator can be used in two ways within the same line of code:

• If both operands are numeric, + means numeric addition.

• If either operand is a String, + means concatenation.

• The expression is evaluated left to right.

ie.

  System.out.println(1 + 2); // 2
  System.out.println("a" + "b"); // ab
  System.out.println("a" + "b" + 3); // ab3
  System.out.println(1 + 2 + "c"); // 3c

Immutability

• String objects are immutable, once they are created, they can not be made larger or smaller, and you cannot change one of the characters inside them.

The String Pool

• Strings are everywhere in Java, they use up a lot of memory, Java solves this issue by reusing common ones. The string pool, also known as the intern pool, is a location in the JVM that collects all these strings.

• The string pool contains literal values that appear in your program. Strings not in the string pool are garbage collected just like any other object.

String name = "Fluffy"; // this is a literal string.
String name = new String("Fluffy"); // this is not a literal string.

• The second way is less efficient than the first way, however, it is allowed.

Important String Methods

• A string is a sequence of characters and zero-indexed.

length()

String string = "animals";
System.out.println(string.length()); // 7

• Returns the number of characters in the String.

charAt()

String string = "animals";
System.out.println(string.charAt(0)); // a
System.out.println(string.charAt(6)); // s
System.out.println(string.charAt(7)); // throws exception

• Lets you query the string to find out what character is at a specific index.

• Indexes start counting with 0.

indexOf()