Java: Data Types, Classes, and Methods

• Java is extensible, meaning programmers can create custom data-types (classes).
• A class provides one or more methods.
• A class can possess attributes specified by instance variables.
• Instance variables are carried within an object.

Java Data Types

• Primitive types include: boolean, byte, char, short, int, long, float, double.
• Reference types (non-primitive types) include objects and interfaces.
• The default value for reference types is null.
• A class defines a kind of object by specifying its attributes (data) and methods (actions).
• An object is an instance of a class.
• Example: "Person" class has attributes such as name, date of birth, and methods such as change address.

Methods

• Methods represent tasks in a program.
• Methods describe the mechanisms that perform tasks and hide complex tasks.
• A method call tells the target method to execute.

Return Types

• Return types are declared in the method header.
• Return types indicates the type of item returned by a method.
• Void functions do not have a data type.
• Value-returning functions have a data type.

Constructor

• Java requires a constructor, a special method for every class.
• Constructors initialize objects and share the class name.
• Constructors may take no arguments (default constructor) or many arguments (overloaded constructors).
• Java provides a default no-argument constructor if none is provided.
• To create a class instance: use 'new' with the class name and parentheses i.e Person p = new Person();

Variables

• Variables declared inside a class but outside methods are called instance variables or fields.
• Each class instance has its own instance variables.
• Variables declared within a method are called local variables and can only be used within that method.
• The JVM marks an object for garbage collection, when there are no more references.
• The JVM's garbage collector empties those objects' memory.
• Class variables (fields or attributes) can be initialized with a default value upon declaration.

Default Values

• Reference types default to null.
• Primitive numerical types (int, double, etc.) default to 0.
• Booleans default to false.
• Characters default to the null-character '\u0000'.
• The compiler does not provide default values for local variables in methods and throws a compilation error if uninitialized variables are used.

Access Modifiers

• Access modifiers are keywords: private and public.
• Access modifiers are used for instance variables/methods
• Private variables/methods are only accessible to methods of the class where they are declared, also known as data-hiding.
• Using public or private modifiers on local variables causes compilation error.
• Attributes should be private; methods should be public.
• Declaring attributes as private protects data integrity and makes code easier to maintain.

UML Class Diagrams

• Unified Modeling Language (UML) is a general-purpose visual modeling language, used for documenting software architecture and defining a standard for diagrams.
• UML class diagrams display classes and their relationships and give information about a class, its attributes, and methods.
• Each class is represented by a box with 3 compartments, and the class name is listed in the top compartment.
• Attributes are listed in the middle compartment in the format attribute name: data-type.
• A "-" symbol denotes private attributes; "+" denotes public attributes.
• Methods are listed in the bottom compartment.
• Input parameters are specified by name followed by a colon and type i.e. "name: String"
• Return types are indicated after the method's parentheses and a colon i.e. (): String
• Constructors are in the bottom compartment and placed first by convention, with "<>" before the name.
• If no constructor is defined, courseName is considered "null."

The this Reference

• Using this keyword, objects can access their own reference.
• this can access instance variables that are shadowed by local variables or method parameters.
• Non-static methods implicitly use this to refer to instance variables and methods.
• One ".java" file may contain multiple classes.
• Only one class with the same name as the ".java" file can be public.
• Each class in the file is compiled into a separate ".class" file.

Conditional Operator

• This operator uses a boolean condition to determine which of two expressions is evaluated.
• Syntax: condition ? expression1 : expression2
• If the condition is true, expression1 is evaluated, and if false, expression2 is evaluated.
• The selected expression's value becomes the overall value of the conditional operator.

The toString() Method

• All Java classes directly or indirectly inherit from class Object (java.lang.Object).
• The toString() method is inherited from class Object.
• toString() returns a String representation of an object.
• The default format is: Package.ClassName@hashCode.
• The toString() method is implicitly called, when an object (instance of a class) is printed and when an object is concatenated with a String.
• The behavior of toString() is changeable by overriding it to print customized formats.

Overloaded Constructors

• An overloaded constructor is an initialization method that is implicitly called when an object of the class is created.
• The no-argument constructor, also known as the default constructor, is invoked without arguments.
• Overloaded constructors provide multiple constructor definitions with different signatures.
• The this reference invokes another constructor, but must be as the first statement in a constructor's body.

Copy Constructor

• The copy constructor creates a new object using another object of the same class.
• This mechanism achieves a deep copy (independent copy).
• Copy constructors help copy complex objects with several fields.

Composition

• Composition occurs when a class holds references to objects of other classes as members.
• It is described as a "has-a" relationship.
• For example, an Employee has-a birthdate of Date type
• The toSting() method may be overriden.
• There are implicit calls to hireDate and birthDate's toString methods.

Static Fields And Methods

• Static fields represent class-wide information.
• A static field is useful when multiple class instances should share the same copy of field.
• A static field is also useful when a field should be accessible even when no instances of the class exist.
• Static fields are accessed through the class name using the dot operator. i.e. ClassName.staticFieldName or Math.PI
• Static fields can also be accessed through an instance; but it generates a warning since it is irrelevant.
• Need not access it via an instance, the appropriate way is static access.
• Static data members are set to zero by default and only be returned by static methods.
• Static has it most common use to access to static fields.

Static vs Instance Methods

• A static method

  • Belongs to a class rather than an instance of a class
  • Is callable without creating a class instance.

• An instance method can

  • Invoke an instance or static method
  • Access an instance or static data field

• A static method

  • Can invoke a static method and access a static data field
  • Cannot invoke an instance method or access an instance data field

• In UML Diagrams

  • getCount is a static method that returns the static attribute Count

Final Instance Variables

• Declaring an instance variable as final prevents modification.

• This also applies to static variables, however, final static means variable is compile-type constant

• The final keyword specifies that an instance variable is not modifiable (is a constant).

• Final instance variables are initialized at their declaration or in all constructors.