Core Java APIs


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.


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");


  • 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.


  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


  • 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.


String string = "animals";
System.out.println(string.length()); // 7
  • Returns the number of characters in the String.


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.


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