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101 Cards in this Set

  • Front
  • Back

Identifiers:What can they begin with?

Identifiers can begin with a letter, an underscore, or a currency character.

Identifiers:Can they include digits?

After the first character, identifiers can also include digits

Identifiers: How Long?

Identifiers can be of any length.
Executable Java Files and main() (OCA Objective 1.3): How to compile execute command line?
You can compile and execute Java programs using the command-line programs javac and java, respectively. Bothprograms support a variety of command-line options.

Executable Java Files and main() (OCA Objective 1.3) : Which main() methods have special powers?

The only versions of main() methods with special powers are those versions with method signatures equivalent to publicstatic void main(String[] args).

Executable Java Files and main() (OCA Objective 1.3) : Can Main() be overloaded?

main() can be overloaded.
Imports (OCA Objective 1.4) :what is the job of an import statement?
An import statement's only job is to save keystrokes.
Imports (OCA Objective 1.4) :what is the job of an import statement? What is the * used for?
You can use an asterisk (*) to search through the contents of a single package.

Imports (OCA Objective 1.4) : What is the syntax when using the keyword static?

Although referred to as "static imports," the syntax is import static….

Imports (OCA Objective 1.4) : What can you import?

You can import API classes and/or custom classes
Source File Declaration Rules (OCA Objective 1.2) : How many public classes can a source file have?

A source code file can have only one public class.

Source File Declaration Rules (OCA Objective 1.2) : What must the filename be with respect to the public class?
If the source file contains a public class, the filename must match the public class name
Source File Declaration Rules (OCA Objective 1.2) :How many package statements/ import statements can a file have?
A file can have only one package statement, but it can have multiple imports.
Source File Declaration Rules (OCA Objective 1.2) :Where do you put the package statement?
The package statement (if any) must be the first (non comment) line in a source file.
Source File Declaration Rules (OCA Objective 1.2) :Where do you put the import statement?
The import statements (if any) must come after the package and before the class declaration.
Source File Declaration Rules (OCA Objective 1.2) :If there are no package statement where is the import statement placed in the source file?
If there is no package statement, import statements must be the first (noncomment) statements in the source file.
Source File Declaration Rules (OCA Objective 1.2) :What do the package and import statements apply to?
package and import statements apply to all classes in the file.
Source File Declaration Rules (OCA Objective 1.2) : How many nonpublic classes can a file have?
A file can have more than one nonpublic class.
Source File Declaration Rules (OCA Objective 1.2) Are there naming restrictions for files with no public classes?
Files with no public classes have no naming restrictions.
Class Access Modifiers (OCA Objective 6.6) : What are the access modifiers?
There are three access modifiers: public, protected, and private.

Class Access Modifiers (OCA Objective 6.6) : What are the access levels?

There are four access levels: public, protected, default, and private.
Class Access Modifiers (OCA Objective 6.6) : What are the types of access a class can have?
Classes can have only public or default access.
Class Access Modifiers (OCA Objective 6.6) : What classes can see another class with default access?
A class with default access can be seen only by classes within the same package
Class Access Modifiers (OCA Objective 6.6) : What classes can see another class with public access?
A class with public access can be seen by all classes from all packages.

Class Access Modifiers (OCA Objective 6.6) : Class visibility effects when code in one class can do these things with another class:

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) (OCA Objective 7.6) What are the keywords that can modify a class?
Classes can also be modified with final, abstract, or strictfp.
Class Modifiers (Nonaccess) (OCA Objective 7.6) Can a class be both final and abstract?
A class cannot be both final and abstract.
Class Modifiers (Nonaccess) (OCA Objective 7.6) Can a final class be subclassed?
A final class cannot be subclassed
Class Modifiers (Nonaccess) (OCA Objective 7.6) Can an abstract class be instantiated?
An abstract class cannot be instantiated.
Class Modifiers (Nonaccess) (OCA Objective 7.6) If a single method is abstract does that make the whole class abstract?
A single abstract method in a class means the whole class must be abstract.
Class Modifiers (Nonaccess) (OCA Objective 7.6) Can abstract class have both abstract and non abstract methods?
An abstract class can have both abstract and nonabstract methods.
Class Modifiers (Nonaccess) (OCA Objective 7.6) Must a concrete class that extend an abstract class implement all of its abstract methods?
The first concrete class to extend an abstract class must implement all of its abstract methods.
Interface Implementation (OCA Objective 7.6) What are interfaces?
Interfaces are contracts for what a class can do, but they say nothing about he way in which the class must do it.
Interface Implementation (OCA Objective 7.6) How are interfaces implemented?
Interfaces can be implemented by any class, from any inheritance tree
Interface Implementation (OCA Objective 7.6) How are interfaces implemented?What is an interface with respect to an abstract class?
An interface is like a 100-percent abstract class and is implicitly abstract whether you type the abstract modifier in thedeclaration or not.
Interface Implementation (OCA Objective 7.6) Can an interface have concrete methods?
An interface can have only abstract methods, no concrete methods allowed.
Interface Implementation (OCA Objective 7.6) Can an interface have concrete methods? What are the access modifiers for an interface.
Interface methods are by default public and abstract—explicit declaration of these modifiers is optional.
Interface Implementation (OCA Objective 7.6) Can an interface have constants? What access modifiers do they have if so?
Interfaces can have constants, which are always implicitly public, static, and final.
Interface Implementation (OCA Objective 7.6) Are constant declarations optional?
Interface constant declarations of public, static, and final are optional in any combination
Interface Implementation (OCA Objective 7.6) What are the legal nonabstract implementing class properties?

1 concrete implementations for the interface's methods.


2 must follow all legal override rules for the methods it implements


3 must not declare any new checked exceptions for an implementation method.


4 must not declare any checked exceptions that are broader than the exceptions declared in the interface method.


5 may declare runtime exceptions on any interface method implementation regardless of the interface declaration.


6 must maintain the exact signature (allowing for covariant returns) and return type of the methods it implements (butdoes not have to declare the exceptions of the interface).



Interface Implementation (OCA Objective 7.6) Can a class implementing and interface itself be abstract?
A class implementing an interface can itself be abstract.
Interface Implementation (OCA Objective 7.6) When must all the interface methods be implemented?
An abstract implementing class does not have to implement the interface methods (but the first concrete subclass must)
Interface Implementation (OCA Objective 7.6) How many classes can a class extend? How many interfaces?
A class can extend only one class (no multiple inheritance), but it can implement many interfaces.
Interface Implementation (OCA Objective 7.6) How many Interfaces can an interface extend?
Interfaces can extend one or more other interfaces.
Interface Implementation (OCA Objective 7.6) Can an interface extend a class or implement a class or interface?
Interfaces cannot extend a class or implement a class or interface
Interface Implementation (OCA Objective 7.6) When taking the exam, verify what is legal before verifying the other code?
When taking the exam, verify that interface and class declarations are legal before verifying other code logic.
Member Access Modifiers (OCA Objective 6.6) :

What variables are known as "members"

Methods and instance (nonlocal) variables are known as "members."
Member Access Modifiers (OCA Objective 6.6) :Can members use all 4 access levels?
Members can use all four access levels: public, protected, default, and private
Member Access Modifiers (OCA Objective 6.6) :What forms can member access come in?
Member access comes in two forms:

1 Code in one class can access a member of another class.


2 A subclass can inherit a member of its superclass.

Member Access Modifiers (OCA Objective 6.6) :If a class cant be accessed can its members be accessed?
If a class cannot be accessed, its members cannot be accessed.
Member Access Modifiers (OCA Objective 6.6) :Determine what before determining member visibility?
Determine class visibility before determining member visibility.
Member Access Modifiers (OCA Objective 6.6) :Can public members be accessed by all other classes even in other packages?
public members can be accessed by all other classes, even in other packages.
Member Access Modifiers (OCA Objective 6.6) :If a superclass member is public, does the subclass inherit it- regardless of the package?
If a superclass member is public, the subclass inherits it—regardless of package.
Member Access Modifiers (OCA Objective 6.6) :Must members accessed without the dot operator belong to the same class?
Members accessed without the dot operator (.) must belong to the same class.
Member Access Modifiers (OCA Objective 6.6) :What does this. refer to?
this. always refers to the currently executing object.
Member Access Modifiers (OCA Objective 6.6) :this.aMethod() is the same as invoking?
this.aMethod() is the same as just invoking aMethod().
Member Access Modifiers (OCA Objective 6.6) :How are private members accessed?
private members can be accessed only by code in the same class.
Member Access Modifiers (OCA Objective 6.6) :Can private members be inherited?
private members are not visible to subclasses, so private members cannot be inherited.
Member Access Modifiers (OCA Objective 6.6) :Members accessed without the dot operator must ...
Members accessed without the dot operator (.) must belong to the same class.
Member Access Modifiers (OCA Objective 6.6) : Default and protected members differ only when subclasses are involved:

1 Default members can be accessed only by classes in the same package.


2 protected members can be accessed by other classes in the same package, plus subclasses regardless of package.


3 protected = package + kids (kids meaning subclasses).


4 For subclasses outside the package, the protected member can be accessed only through inheritance; a subclassoutside the package cannot access a protected member by using a reference to a superclass instance. (In otherwords, inheritance is the only mechanism for a subclass outside the package to access a protected member of itssuperclass.)


5 A protected member inherited by a subclass from another package is not accessible to any other class in thesubclass package, except for the subclass' own subclasses.



Local Variables (OCA Objective 2.1) Can local variable declarations have access modifiers?
Local (method, automatic, or stack) variable declarations cannot have access modifiers
Local Variables (OCA Objective 2.1) What is the only modifier available to local variables?
final is the only modifier available to local variables.
Local Variables (OCA Objective 2.1) Can local variables get default values?
Local variables don't get default values, so they must be initialized before use
Other Modifiers—Members (OCA Objective 6.6): Can final methods be overridden in a subclass?
final methods cannot be overridden in a subclass.
Other Modifiers—Members (OCA Objective 6.6): What are abstract methods declared with? Are they implemented?
abstract methods are declared with a signature, a return type, and an optional throws clause, but they are notimplemented.
Other Modifiers—Members (OCA Objective 6.6): How do abstract methods end?
abstract methods end in a semicolon—no curly braces.
Other Modifiers—Members (OCA Objective 6.6): Three ways to spot a nonabstract method:

1. The method is not marked abstract.


2. The method has curly braces.


3. The method MIGHT have code between the curly braces.

Other Modifiers—Members (OCA Objective 6.6): Must the first concrete class to extend and abstract class implement all of the abstract class' methods?



The first nonabstract (concrete) class to extend an abstract class must implement all of the abstract class' abstractmethods.
Other Modifiers—Members (OCA Objective 6.6): What kind of access control can synchronized methods have? Can they be marked final?
synchronized methods can have any access control and can also be marked final.
Other Modifiers—Members (OCA Objective 6.6): abstract methods must be implemented by a subclass, so they must be inheritable. For that reason:
1 abstract methods cannot be private.

2 abstract methods cannot be final.

Other Modifiers—Members (OCA Objective 6.6): The native modifier applies to what?
The native modifier applies only to methods.
Other Modifiers—Members (OCA Objective 6.6): What does the strictfp modifier apply to?
The strictfp modifier applies only to classes and methods.
Methods with var-args (OCP Only, OCP Objective 1.3) What is the var-arg method?
As of Java 5, methods can declare a parameter that accepts from zero to many arguments, a so-called var-arg method.
Methods with var-args (OCP Only, OCP Objective 1.3) How is the var-arg parameter declared?
A var-arg parameter is declared with the syntax type... name; for instance: doStuff(int... x) { }.
Methods with var-args (OCP Only, OCP Objective 1.3) How many var-arg parameters can a var-arg method have?
A var-arg method can have only one var-arg parameter.
Methods with var-args (OCP Only, OCP Objective 1.3) If a method has a normal parameter and a var-arg.. the var-arg must come...
In methods with normal parameters and a var-arg, the var-arg must come last
Variable Declarations (OCA Objective 2.1):

Instance variables can ...

1 Have any access control


2 Be marked final or transient

Variable Declarations (OCA Objective 2.1):Instance variables cant be ...
Instance variables can't be abstract, synchronized, native, or strictfp.
Variable Declarations (OCA Objective 2.1):What is shadowing?
It is legal to declare a local variable with the same name as an instance variable; this is called "shadowing."
Variable Declarations (OCA Objective 2.1):final variables have the following properties:
❑ final variables cannot be reassigned once assigned a value.

❑ final reference variables cannot refer to a different object once the object has been assigned to the final variable.


❑ final variables must be initialized before the constructor completes.

Variable Declarations (OCA Objective 2.1): Is there such a thing as a final object? Can an object reference marked final mean the object itself cannot be changed?
There is no such thing as a final object. An object reference marked final does NOT mean the object itself can't change.
Variable Declarations (OCA Objective 2.1): What does the transient modifier apply to?
The transient modifier applies only to instance variables.
Variable Declarations (OCA Objective 2.1): What does the volatile modifier apply to?
The volatile modifier applies only to instance variables.
Array Declarations (OCA Objectives 4.1 and 4.2) Can arrays hold primitives or objects? Is the array itself always an object?
Arrays can hold primitives or objects, but the array itself is always an object.
Array Declarations (OCA Objectives 4.1 and 4.2) When you declare and array, where do you place the brackets with respect to the variable name?
When you declare an array, the brackets can be to the left or to the right of the variable name.
Array Declarations (OCA Objectives 4.1 and 4.2) Can you include the size of an array in the declaration?
It is never legal to include the size of an array in the declaration.
Array Declarations (OCA Objectives 4.1 and 4.2) Can you include the size of an array in the declaration? Can an array of objects hold any object that passes the IS-A (or instanceof) test for the declared type of the array?
An array of objects can hold any object that passes the IS-A (or instanceof) test for the declared type of the array. Forexample, if Horse extends Animal, then a Horse object can go into an Animal array.
Static Variables and Methods (OCA Objective 6.2) Are they tied to any particular instance of a class?
They are not tied to any particular instance of a class.
Static Variables and Methods (OCA Objective 6.2) Are class instances needed in order to use the static members of the class?
No class instances are needed in order to use static members of the class.
Static Variables and Methods (OCA Objective 6.2) How many copies of a static variable/class?
There is only one copy of a static variable/class and all instances share it.
Static Variables and Methods (OCA Objective 6.2) Do static methods have direct access to nonstatic members?
static methods do not have direct access to nonstatic members.
enums (OCA Objective 1.2 and OCP Objective 2.5) What does an enum specify?
An enum specifies a list of constant values assigned to a type.
enums (OCA Objective 1.2 and OCP Objective 2.5) Is an enum a String or an int?
❑ 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 theenum type Season.
enums (OCA Objective 1.2 and OCP Objective 2.5) Where can an enum can be declared?
An enum can be declared outside or inside a class, but NOT in a method.
enums (OCA Objective 1.2 and OCP Objective 2.5) What must an enum declared outside a class not be marked?
An enum declared outside a class must NOT be marked static, final, abstract, protected, or private.
enums (OCA Objective 1.2 and OCP Objective 2.5) What can enums contain?
enums can contain constructors, methods, variables, and constant-specific class bodies.
enums (OCA Objective 1.2 and OCP Objective 2.5) Can enum constants send arguments?
enum constants can send arguments to the enum constructor, using the syntax BIG(8), where the int literal 8 is passed tothe enum constructor.
enums (OCA Objective 1.2 and OCP Objective 2.5) Can enum constructors have arguments? Can they be overloaded?
enum constructors can have arguments and can be overloaded.
enums (OCA Objective 1.2 and OCP Objective 2.5) Can enum constructors be invoked directly in code?
enum constructors can NEVER be invoked directly in code.
enums (OCA Objective 1.2 and OCP Objective 2.5) Is the semicolon at the end of an enum declaration optional?
The semicolon at the end of an enum declaration is optional. These are legal:

❑ enum Foo { ONE, TWO, THREE}


❑ enum Foo { ONE, TWO, THREE};

enums (OCA Objective 1.2 and OCP Objective 2.5) What does MyEnum.values() return?
MyEnum.values() returns an array of MyEnum's values called automatically when an enum is initialized.