Manning OCA Java SE 7 Reading Note: Chapter 1

  |   Source

This is a post that I moved from my old blog site. This is the reading note for OCA Java SE 7 Certification Guide from Manning. Please note that all definations and examples in this post are taken from the book.

Chapter 1: Java Basics

Topics Covered in Chapter 1

Understanding the structure and components of a Java class

Understanding executable Java applications

Understanding Java packages

Importing Java packages into your code

Applying access and nonaccess modifiers

1.1: The Structrue of a Java Class and Source Code File

1.1.1 Struture of a Java Class

Java source code file: xxx.java. A Java source code file (.java file) can define multiple classes and interfaces.

in Java compiler: javac (.exe)

then output Java bytecode (compiled code for JVM): xxx.class

List of Components in a class

  • The package statement

  • The import statement

  • Comments

  • Class declarations and definitions

  • Variables

  • Methods

  • Constructors

Package Statement All Java classes are part of a package.

A Java class can be explicitly defined in a named package; otherwise it becomes part of a default package, which doesn’t have a name.

The package statement should be the first statement in a class.

The package statement cannot appear within a class declaration or after the class declaration.

The package statement must appear exactly once in a class.

Import Statement

Classes and interfaces in the same package can use each other without prefixing their names with the package name.

Code will fail compiling because an import statement can’t be placed before a package statement.

The import statement can’t be used to import multiple classes or interfaces with the same name.

If a class includes a package statement, all the import statements should follow the package statement.

Comments

Multiline comments span multiple lines of code. They start with /* and end with */.

Multiline comments can contain any special characters (including Unicode characters).

People use an asterisk (*) to start the comment in the next line. Please note that this isn’t required—it’s done more for aesthetic reasons.

End-of-line comments start with // and, as evident by their name, they are placed at the end of a line of code.

A comment can precede a package statement.

Class Declaration

The class declaration marks the start of a class. It can be as simple as the keyword class followed by the name of a class.

The declaration of a class is composed of the following parts:

  • Access modifiers

  • Nonaccess modifiers

  • Class name

  • Name of the base class, if the class is extending another class

  • All implemented interfaces, if the class is implementing any interfaces

  • Class body (class fields, methods, constructors), included within a pair of curly braces, {}

To declare a class, you must have:

  • Keyword class

  • Name of the class

  • Class body, marked by the opening and closing curly braces, {}

  • The property of an object: variable

  • The behavior of an object: method

  • The state of a class: attribute or instance variable

Points to remember:

A class name starts with the keyword class. Watch out for the case of the keyword class. Java is case sEnSiTivE. class (lowercase c) isn’t the same as Class (uppercase C). You can’t use the word Class (capital C) to define a class.

The state of a class is defined using attributes or instance variables.

The behavior is defined using methods

A class definition may also include comments and constructors

A class is a design from which an object can be created.

Variables

Variables that store the state of an object (also called an instance), they are called instance variables or instance attributes.

Each object has its own copy of the instance variables.

The instance variables are defined within a class but outside all methods in a class.

A class may define an instance variable before or after the definition of a method and still use it.

A single copy of a class variable or static variable is shared by all the objects of a class.

Methods

Instance methods are generally used to manipulate the instance variables.

A class method or static method is used to work with the static variables.

Constructors

A class constructor is used to create and initialize the objects of a class.

A class can define multiple constructors that accept different sets of method parameters.

1.1.2 Structure and Components of a Java Source Code File

A Java source code file is used to define classes and interfaces.

An interface is a grouping of related methods and constants, but the methods in an interface cannot define any implementation. An interface specifies a contract for the classes to implement.

The definition of an interface starts with the keyword interface. An interface can define constants and methods. You can’t use the word Interface (with a capital I) to define an interface.

You can also define a combination of classes and interfaces in the same Java source code file, in any order.

If you define a public class or an interface in a class, its name should match the name of the Java source code file. Also, a source code file can’t define more than one public class or interface.

When you use a package or import statement within such Java files, both the package and import statements apply to all of the classes and interfaces defined in that source code file.

Classes and interfaces defined in the same Java source code file can’t be defined in separate packages.

Classes and interfaces imported using the import statement are available to all the classes and interfaces defined in the same Java source code file.

1.2 Executable Java Applications

1.2.1 Executable Java Classes versus Non-executable Java Classes

The JVM executes the code that is defined in the main method.

The main method should comply with the following rules:

  • The method must be marked as a public method.

  • The method must be marked as a static method.

  • The name of the method must be main.

  • The return type of this method must be void.

  • The method must accept a method argument of a String array or a variable argument of type String. i.e. String... args and String[] args are all acceptable.

To define a variable argument variable, the ellipsis (...) should follow the type of the variable and not the variable itself. i.e. String args... is not compiling.

To define an array, the square brackets, [], can follow either the variable name or its type. i.e., String[] args and String args[] are all acceptable.

The placement of the keywords public and static can be interchanged. i.e. public staic and static public are all acceptable. You cannot move void!

Important: As long as you do java class_name_of_main (parameters) in command line, you are calling main method. If you do java name_of_other_class (parameters), you will be recogized as calling other methods.

1.3 Java Packages

If you don’t include an explicit package statement in a class or an interface, it’s part of a default package.

1.3.2 Defining classes in a package using the package statement

It’s common for organiza- tions to use subpackages to define all their classes, such as com.test.example.

A few of important rules about packages:

  • Per Java naming conventions, package names should all be in lowercase.

  • The package and subpackage names are separated using a dot (.).

  • Package names follow the rules defined for valid identifiers in Java.

  • For packaged classes and interfaces, the package statement is the first statement in a Java source file (a .java file). The exception is that comments can appear before or after a package statement.

  • There can be a maximum of one package statement per Java source code file(.java file).

  • All the classes and interfaces defined in a Java source code file will be defined in the same package. There is no way to package classes and interfaces defined within the same Java source code file in different packages.

A fully qualified name for a class or interface is formed by prefixing its package name with its name (separated by a period), such as packagename.Classname.

To enable the Java Runtime Environment (JRE) to find your classes, add the base directory that contains your packaged Java code to the classpath.

1.3.3 - 1.3.8

Importing more classes doesn’t increase the size of your own class.

It is possible to use a packaged class or interface without using the import statement, by using its fully qualified name.

You can’t use the import statement to access multiple classes or interfaces with the same names from different packages. For example, you cannot import java.util.Date and java.sql.Date at the same time.

By using the wildcard character, an asterisk (*), you can import all of the public members, classes, and interfaces of a package. However, you can only use * to import current level class. You cannot import sub-level classes.

The default package is automatically imported in the Java classes and interfaces defined within the same directory on your system. However, A class from a default package can’t be used in any named packaged class, regardless of whether they are defined within the same directory or not.

You can import an individual static member of a class or all its static members by using the import static statement. Note correct statement is import static, not static import.

1.4 Java Access Modifiers

A top-level class is a class that isn’t defined within any other class. A class that is defined within another class is called a nested or inner class.

Important: Local variables and method parameters can’t be defined using access modifiers.

Java defines four access modifiers:

  • public (least restrictive)

  • protected

  • default

  • private (most restrictive)

Think of Access Modifier from the following four perspectives:

  • Same Package/Different Package

  • Derived Classes/Unrelated Classes

Public:

  • The least restrictive access modifier.

  • Classes and interfaces defined using the public access modifier are accessible across all packages, from derived to unrelated classes.

  • All four perspeactives are avaiable.

Proctected:

The members of a class defined using the protected access modifier are accessible to:

  • Classes and interfaces defined in the same package

  • All derived classes, even if they’re defined in separate packages

  • Only unrelated/different package perspective cannot access.

  • i.e. the subclass of derived class in different package cannot access.

Default:

  • The members with package access are only accessible to classes and interfaces defined in the same package.

  • Only same package perspective get access.

  • Default access can be compared to package-private (accessible only within a package) and protected access can be compared to package- private + kids (“kids” refer to derived classes)

Private:

  • The members of a class defined using the private access modifier are accessible only to themselves.

  • None of the four perspectives are accessiable.

1.5 Nonaccess Modifiers

]Access modifiers control the accessibility of your class and its members outside the class and the package.

Nonaccess modifiers change the default properties of a Java class and its members.

1.5.1 Abstract Modifier

Only variable cannot use abstract.

An abstract class can’t be instantiated. i.e You cannot do new abstract_class.

An abstract class may or may not define an abstract method; you can define an abstract class without any abstract methods. But a concrete class can’t define an abstract method.

An interface is an abstract entity by default.

You can add abstract keyword to interface if you like.

An abstract method doesn’t have a body.

Usually, an abstract method is implemented by a derived class.

A method with an empty body isn’t an abstract method. i.e. a method that havs {} is not a abstract method. a method that has abstract buy doesn't have {} is abstract method.

1.5.2 Final Modifier

Only interface cannot use final

A class that is marked final cannot be extended by another class.

An interface cannot be marked as final.

A final variable can’t be reassigned a value. It can be assigned a value only once.

You can call methods on a final variable, such as a.append(b),assuming b is a final variable. But you cannot reassign another object to a final variable.

A final method defined in a base class can’t be overridden by a derived class.

1.5.3 Static Modifier

Variables, methods, classes and interfaces can all use static modifier.

static variables belong to a class.

static attributes exist independently of any instances of a class and may be accessed even when no instances of the class have been created.

A static variable is shared by all of the objects of a class.**, i.e. if a instance change the value of a static variable, all instances that access this static variable after it will get same new value.

A static variable can be accessed using the name of the object reference variable or the name of a class.

static methods aren’t associated with objects and **can’t use any of theinstance variable`s of a class**.

You can use static methods to define utility methods, which are methods that usually manipulate the method parameters to compute and return an appropriate value.

The static members aren’t involved in runtime polymorphism. You can’t override the static members in a derived class, but you can redefine them.

Neither static methods nor static variables can access the non-static variables and methods of a class. But the reverse is true: non-static variables and methods can access static variables and methods because the static members of a class exist even if no instances of the class exist.

You can’t prefix the definition of a top-level class or an interface with the keyword static.

Comments powered by Disqus