Declaration and Access Control

Identifiers (Objective 1.3)

•  Identifiers can begin with a letter, an underscore, or a currency character.
•  After the first character, identifiers can also include digits.
•  Identifiers can be of any length.
•  JavaBeans methods must be named using camelCase, and depending on the method's purpose, must start with set, get, is, add, or remove.

Declaration Rules (Objective 1.1)

• A source code file can have only one public class.
• If the source file contains a public class, the filename must match the public class name.
• A file can have only one package statement, but multiple imports.
• The package statement (if any) must be the first (non-comment) line in a source file.
• The import statements (if any) must come after the package and before the class declaration.
• If there is no package statement, import statements must be the first (noncomment) statements in the source file.
• package and import statements apply to all classes in the file.
• A file can have more than one nonpublic class.
• Files with no public classes have no naming restrictions.

Class Access Modifiers (Objective 1.1)

• There are three access modifiers: public, protected, and private.
• There are four access levels: public, protected, default, and private.
• Classes can have only public or default access.
• A class with default access can be seen only by classes within the same package.
• A class with public access can be seen by all classes from all packages.
• Class visibility revolves around whether code in one class can
• Create an instance of another class
• Extend (or subclass), another class
• Access methods and variables of another class

Class Modifiers (Nonaccess) (Objective 1.2)

• Classes can also be modified with final, abstract, or strictfp.
• A class cannot be both final and abstract.
• A final class cannot be subclassed.
• An abstract class cannot be instantiated.
• A single abstract method in a class means the whole class must be abstract.
• An abstract class can have both abstract and nonabstract methods.
• The first concrete class to extend an abstract class must implement all of its abstract methods.

 Member Access Modifiers (Objectives 1.3 and 1.4)

• Methods and instance (nonlocal) variables are known as "members."
• Members can use all four access levels: public, protected, default, private.
• Member access comes in two forms:
• Code in one class can access a member of another class.
• A subclass can inherit a member of its superclass.
• If a class cannot be accessed, its members cannot be accessed.
• Determine class visibility before determining member visibility.
• public members can be accessed by all other classes, even in other packages.
• If a superclass member is public, the subclass inherits it—regardless of package.
• Members accessed without the dot operator (.) must belong to the same class.
• this. always refers to the currently executing object.
• this.aMethod() is the same as just invoking aMethod().
• private members can be accessed only by code in the same class.
• private members are not visible to subclasses, so private members cannot be inherited.

Static Variables and Methods (Objective 1.4)

• They are not tied to any particular instance of a class.
• No classes instances are needed in order to use static members of the class.
• There is only one copy of a static variable / class and all instances share it.
• static methods do not have direct access to non-static members.

Enums (Objective 1.3)

• An enum specifies a list of constant values assigned to a type.
• An enum is NOT a String or an int; an enum constant's type is the enum type. For example, SUMMER and FALL are of the enum type Season.
• An enum can be declared outside or inside a class, but NOT in a method.
• An enum declared outside a class must NOT be marked static, final, abstract, protected, or private.
• Enums can contain constructors, methods, variables, and constant class bodies.
• enum constants can send arguments to the enum constructor, using the syntax BIG(8), where the int literal 8 is passed to the enum constructor.
• enum constructors can have arguments, and can be overloaded.
• enum constructors can NEVER be invoked directly in code. They are always called automatically when an enum is initialized.
• The semicolon at the end of an enum declaration is optional. These are legal:
• enum Foo { ONE, TWO, THREE}
• enum Foo { ONE, TWO, THREE};
• MyEnum.values() returns an array of MyEnum's values.

Methods with var-args (Objective 1.4)

• As of Java 5, methods can declare a parameter that accepts from zero to many arguments, a so-called var-arg method.
• A var-arg parameter is declared with the syntax type... name; for instance:                                                  doStuff(int... x) { }
• A var-arg method can have only one var-arg parameter.
• In methods with normal parameters and a var-arg, the var-arg must come last.

regards,
M.Agni..
Executive, 
Talent Sprint Edu. Pvt Ltd.
Hyderabad.


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