SCJP

• Identifiers must start with a letter, a currency character ($), or a connecting character such as the underscore ( _ ). Identifiers cannot start with a number!
  

• After the first character, identifiers can contain any combination of letters, currency characters, connecting characters, or numbers.
  

• In practice, there is no limit to the number of characters an identifier can contain.
  

• You can't use a Java keyword as an identifier. Table 1-1 lists all of the Java keywords including one new one for 5.0, enum.
  

• Identifiers in Java are case-sensitive; foo and FOO are two different identifiers.Examples of legal and illegal identifiers follow, first some legal identifiers:
  

int _a;
int $c;
int ______2_w;
int _$;
int this_is_a_very_detailed_name_for_an_identifier;

The following are illegal (it's your job to recognize why):

int :b;
int -d;
int e#;
int .f;
int 7g;

Classes and interfaces The first letter should be capitalized, and if several words are linked together to form the name, the first letter of the inner words should be uppercase (a format that's sometimes called "camelCase"). For classes, the names should typically be nouns. For example:

Dog
Account
PrintWriter

For interfaces, the names should typically be adjectives like

Runnable
Serializable



Variables Like methods, the camelCase format should be used, starting with a lowercase letter. Sun recommends short, meaningful names, which sounds good to us. Some examples:

buttonWidth
accountBalance
myString


Constants Java constants are created by marking variables static and final. They should be named using uppercase letters with underscore characters as separators:

MIN_HEIGHT


JavaBean Property Naming Rules

• If the property is not a boolean, the getter method's prefix must be get. For example, getSize()is a valid JavaBeans getter name for a property named "size." Keep in mind that you do not need to have a variable named size (although some IDEs expect it). The name of the property is inferred from the getters and setters, not through any variables in your class. What you return from getSize() is up to you.

• If the property is a boolean, the getter method's prefix is either get or is. Forexample, getStopped() or isStopped() are both valid JavaBeans names fora boolean property.
  

• The setter method's prefix must be set. For example, setSize() is the valid JavaBean name for a property named size.

• To complete the name of a getter or setter method, change the first letter of the property name to uppercase, and then append it to the appropriate prefix (get, is, or set).

• Setter method signatures must be marked public, with a void return type and an argument that represents the property type.

• Getter method signatures must be marked public, take no arguments, and have a return type that matches the argument type of the setter method for that property.

JavaBean Listener Naming Rules

• Listener method names used to "register" a listener with an event source must use the prefix add, followed by the listener type. For example, addActionListener() is a valid name for a method that an event source will have to allow others to register for Action events.

• Listener method names used to remove ("unregister") a listener must use the prefix remove, followed by the listener type (using the same rules as the registration add method).

• The type of listener to be added or removed must be passed as the argument to the method.

• Listener method names must end with the word "listener".

Examples of valid JavaBean method signatures are

public void setMyValue(int v)
public int getMyValue()
public boolean isMyStatus()
public void addMyListener(MyListener m)
public void removeMyListener(MyListener m)

Examples of invalid JavaBean method signatures are

void setCustomerName(String s) // must be public
public void modifyMyValue(int v) // can't use 'modify'
public void addXListener(MyListener m) // listener type mismatch .


Source File Declaration Rules

• There can be only one public class per source code file.

• Comments can appear at the beginning or end of any line in the source code file; they are independent of any of the positioning rules discussed here.

• If there is a public class in a file, the name of the file must match the name of the public class. For example, a class declared as public class Dog { } must be in a source code file named Dog.java.

• If the class is part of a package, the package statement must be the first line in the source code file, before any import statements that may be present.

• If there are import statements, they must go between the package statement (if there is one) and the class declaration. If there isn't a package statement, then the import statement(s) must be the first line(s) in the source code file. If there are no package or import statements, the class declaration must be the first line in the source code file.

• import and package statements apply to all classes within a source code file. In other words, there's no way to declare multiple classes in a file and have them in different packages, or use different imports.

• A file can have more than one nonpublic class.

• Files with no public classes can have a name that does not match any of the classes in the file.

Class Declarations and Modifiers

The following code is a bare-bones class declaration:

class MyClass { }

This code compiles just fine, but you can also add modifiers before the class declaration. Modifiers fall into two categories:

• Access modifiers: public, protected, private.

• Non-access modifiers (including strictfp, final, and abstract).

package name convention

Sun recommends that developers use reverse domain names, appended with division and/or project names. For example, if your domain name is geeksanonymous.com, and you're working on the client code for the TwelvePointOSteps program, you would name your package something like com.geeksanonymous.steps.client.That would essentially change the name of your class to com.geeksanonymous.steps.client.Utilities. You might still have name collisions within your company, if you don't come up with your own naming schemes, but you're guaranteed not to collide with classes developed outside your company (assuming they follow Sun's naming convention, and if they don't, well, Really Bad Things could happen).


Class Access
What does it mean to access a class? When we say code from one class (class A) has access to another class (class B), it means class A can do one of three things:

• Create an instance of class B.

• Extend class B (in other words, become a subclass of class B).

• Access certain methods and variables within class B, depending on the access

control of those methods and variables. In effect, access means visibility. If class A can't see class B, the access level of the methods and variables within class B won't matter; class A won't have any way to access those methods and variables.