Basics

Question 1

When is a prefix operator evaluated?

Answer 1

Before its FIRST use in the expression

Question 2

When is a postfix operate evaluated?

Answer 2

After it is FIRST used in the expression

Question 3

Which packages are automatically inherited into every Java source file?

Answer 3

java.lang. This does not mean that you can’t import it explicitly, its just not required.

Question 4

How many public classes can appear at the top level of a source file?

Answer 4

Only 1. There can be public nested classes, but not more than 1 top-level public class. You can have more than 1 class at the top level so long as only 1 is public

Question 5

Does an abstract class need abstract methods to be abstract?

Answer 5

No. An abstract class simply means it is not a concrete class and cannot be instantiated.

Question 6

Can you import a class statically?

Answer 6

No, only static methods can be imported. “import static a.b.C” where C is a class is not permitted

Question 7

Can you import a method statically?

Answer 7

Yes. To import all static methods of a class use the .* notation or just reference the method name to import only that method

Question 8

Does importing a class give you direct access to its members?

Answer 8

No, you must explicitely import them.

Question 9

Is it valid to use () in front of a method name when importing that static method

Answer 9

No, this is not allowed.

Question 10

What is the order for declaration of class/interface definitions, import statements and package statements?

Answer 10

(1) Package statement (2) import statements (3) class/interface declarations

Question 11

How many package statements can occur in a source file?

Answer 11

1 and only 1. If there is no package statement then the classes/interfaces defined in the class can NEVER be referenced by a class in another package

Question 12

What java statements can be annotated with the abstract keyword and what does it mean in each case?

Answer 12

(1) abstract class - it cannot be instantiated (2) abstract method - needs to be implemented by child classes and the class must be declared abstract. You CANNOT have abstract variables.

Question 13

Where can the final keyword appear and what does it mean?

Answer 13

(1) final class - you cannot extend the class (2) final variable - you cannot change the variable once its value is set either in the declaraiton or the constructor (3) final method - you cannot override the method

Question 14

Where can the keyword static appear and what does it mean?

Answer 14

(1) nested classes NOT top-level classes (2) variables (3) methods - in each case, only 1 entity appears per class in the JVM

Question 15

Define what class cohesiveness means?

Answer 15

It is ensuring that each of your classes only does what it is intended for with clear boundaries and responsibilities

Question 16

Descrbe each of the rules used by the Java compiler to determine if an identifier is legal?

Answer 16

(1) The identifier can start with loweror upper case alphabet, (2) can’t start with numbers, (3) can start with underscore or connecting symbols, (4) can start with any currency-based symbol. (5) Can be any length in theory. (6) cannot be a java keyword, and is case-sensitive (7) the characters used can be any letter, currency characters, connecting characters and numbers

Question 17

Are interfaces objects?

Answer 17

No.

Question 18

What are the 8 rules of source file declarations?

Answer 18

(1) There can only be 1 public class in a source code file (2) Comments can appear anywhere
(3) Package statements, if included must be the first line of the file
(4) If there is a public class, it must match the name of the source code file
(5) If there are import statements they must appear between the package statement and class/interface declarations
(6) import and package statementa apply to all class in the file - you cannot declare classes in the same source code file in different packages or with different import statements
(7) A file can have more than one nonpublic class
(8) Files with no public class do not have to have a class whose name matches the source code file

Question 19

What are the 5 requirements for a method to be executable in a class?

Answer 19

(1) call main
(2) return type is void
(3) accepts an array of strings
(4) be static
(5) be public

Note this means the main method can be final

Question 20

Can the main() method be overloaded?

Answer 20

Yes, but any other main method will not be executable. It will just be treated as a normal main method.

Question 21

Can a main method accept a varargs?

Answer 21

Yes. The following is perfectly legitimate -

public static void main (String .. args)

Question 22

What is a classes fully qualified name? Give an example

Answer 22

The fully qualified name of a class is its class name plus the package path to get to it e.g. java.util.ArrayList and not just ArrayList.

Question 23

Can you use a static import statement to import non-static entities?

Answer 23

No - you can only use static imports to import static members of a class

Question 24

What is the order of the static and import keywords in a static import statement?

Answer 24

always, always “import static”. You cannot have static import instead

Question 25

Describe import statements

Answer 25

Import statements allow you to save typing by importing classes that are not part of this classes package. In order to use a class you must explicitly import it or it is included as part of a wildcard. ``` e.g. import java.util.*; or import java.util.ArrayList will both give you access to ArrayList. You can also import all static methods of a class by doing static import java.lang.System.*; ``` Bear in mind that if you are importing two classes with the same name, the compiler will complain and you will have to explcitely use their fully qualified names or rename the classes

Question 26

What will java.*; do?

Answer 26

This will import all non-static classes and interfaces in the java package if there are any. It will compile even if the package contains no classes or interfaces.

Question 27

What are the two other names typically associated with inner classes?

Answer 27

Nested classes and anonymous classes

Question 28

What is a top-level class?

Answer 28

It is a class that appears at the top level of a Java source code file

Question 29

What are the two types of modifiers and what modifiers fall into each type?

Answer 29

Access and non-access. Access modifiers - public, protected, private; Non-access - final, static, abstract

Question 30

What are the two access control modifiers that can be used on a top-level class?

Answer 30

public and none. (package private).

Question 31

Why can't you have a protected top-level class in a source code file?

Answer 31

This is semantically equivalent to using the public access modifier. It makes sense that the class is only visible in the package but not for sub-classing, as it would need to be made public to be visible.

Question 32

Can you combine final and abstract? Why?

Answer 32

Final prevents the class from being subclasses, but asbtract means the class has to be subclassed because it cannot be instantiated, hence they cannot be used together.

Question 33

What Java entities can strictfp be used on?

Answer 33

A class and method, but never a variable. OCP to explain why

Question 34

Why would you use final on a class?

Answer 34

Final means that you cannot subclass that class and is only used when you want to guarentee the implementation of that class where none of your methods are overriden in the entire class. If it is not the entire class, then use final on the method which is more common. Ask - If this class is subclassed, then could this change the expected behaviour in a bad way? Use it carefully, as you are effectively saying that this class cannot be improved upon or substituted with a subclass if a problem is found with it. Using final stops your code from being extensible which is bad OO design.

Question 35

What is an abstract class and why would you use it?

Answer 35

Abstract classes cannot be instantiated. They don't require abstract methods. They exist soley to be subclassed and provide a way to create general/abstract behaviour to all subclasses. NOTE : You can compile and even execute an abstract class - you just can't instantiate it.

Question 36

Can abstract apply to methods and variables?

Answer 36

Only classes and methods. It doesn't make sense to have an abstract variable. If a class has abstract methods, then it must also be declared as abstract.

Question 37

When marking a method as abstract what else must you do?

Answer 37

Not provide a method body defined by the curly brackets. You do, however, provide any parameters for the signature including () if that is necessary.

Question 38

Are interfaces abstract?

Answer 38

Yes, but it is redundant to mark an interface as abstract as it is implied in the nature of the entity. All interfaces are abstract! Likewise, you don't have to mark a method as being abstract in an interface, even though an interface contains only abstract entities (methods).

Question 39

What is an interface?

Answer 39

An interface is a contract that can be implemented by any class. It is a 100% abstract class. NOTE : The compiler adds "public abstract" to all method declarations in an interface and the class is implicitely public.

Question 40

What are the 9 rules of an interface?

Answer 40

(1) All interface methods are implictely public and abstract (2) Any variables declared must be public, static and final (3) Interface methods cannot be static (4) Methods cannot be marked as final because they are inheritently abstract (5) An interface can extend one or more other interfaces (6) An interface is not an object and cannot extend an object (7) An interface cannot implement another interface (8) Declared with the keyword interface rather than class (9) Interface types can be used polymorphically

Question 41

Can you explicitely declare an interface as abstract?

Answer 41

Yes, but it is redundant.

Question 42

what access can modifiers can be used on an interface?

Answer 42

Either public or default access. Protected and private are not allowed for similar reasons that a protected and private top-level class are not allowed.

Question 43

How many top-level classes/interfaces can youhave?

Answer 43

As many as you like, but you can only have one public class/interface and its name must be th same as the source code file.

Question 44

Can an interface have static methods?

Answer 44

No, an interface only defines instance methods.

Question 45

Can interface methods have access modifiers other than public?

Answer 45

Declaring an interface method as private or protected will not compile. They don't make sense in the context of an interface.

Question 46

Describe interface constants

Answer 46

In the same way that you don't define a method as public and abstract but it is, an varibles you declare will also be public, static and final and treated as such. You cannot have a variable declared as being private or protected. they are not allowed in an interface.

Question 47

What are the only 2 access modifiers than can be used for a class? (Note - not a nested class that is treated as a different entity in Java)

Answer 47

public and default access

Question 48

What are the two different relationship types between classes?

Answer 48

Is-a and has-a. If a class has access to another class, then it is either through inheritance or via the package structure. Access across packages is provided by the dot operator and access of super classes is provided through inheritance

Question 49

What does it mean for code in one class to have access to code in another class?

Answer 49

It means it is visible.

Question 50

When a superclass declares a private method whose signature is mimicked in a subclass is that subclass overriding that private super method?

Answer 50

No - if it is private, then it is invisible to any subclasses.

Question 51

What are the 3 ways for accessing a method?

Answer 51

(1) using a method in the same class (2) via the dot operator (3) via inheritance

Question 52

If a method is declared protected, can a subclass access that method through an object reference?

Answer 52

No, only through inheritance. For this to work, the member needs to be declared public. i.e. publicly visible outside the package.

Question 53

Can access modifiers be applied to local variables?

Answer 53

No

Question 54

What is the one access modifier that can be applied to local variables?

Answer 54

Final. Once initialised, the local variable cannot be changed.

Question 55

What are the two different types of members of a class?

Answer 55

variables and methods.

Question 56

What are the 7 different non-access modifiers?

Answer 56

final, abstract, static, synchronised, native, strictfp, transient.

Question 57

What are the 4 different access modifiers?

Answer 57

public, protected, private and default access

Question 58

What does final mean in the context of a method and when would you use it?

Answer 58

A final method cannot be overridden. It is used when you want to guarantee the functionality of the method. It is often done for safety and security reasons. The reasons for not doing it are it goes against being extensible and is not maintainable.

Question 59

Which other Java entity behaves similarly to local variables?

Answer 59

Method arguments. They behave exactly as if they were local variables.

Question 60

Describe what final means in the context of a method argument?

Answer 60

A final method argument cannot have its value changed during the execution of that method. If it is an object type, it can still have its member variables changed and methods invoked, but its object reference cannot be changed.

Question 61

What does abstract mean in the context of a method?

Answer 61

The method will have no body and subclasses are forced to implement it who wish to be concrete classes. It is ended with a semicolon and forces the enclosing class to also be declared abstract. The reverse, however is not true - an abstract class does not need to have abstract methods.

Question 62

What does concrete mean?

Answer 62

Non-abstract

Question 63

Is it possible to overload methods from a super class?

Answer 63

Yes. If you match the method name but have a different parameter list, then you are overloading the method, not overriding it.

Question 64

What is the definition of a method's signature?

Answer 64

It is the combination of a method name and its parameter list. If it is the same from a super class, then you are overriding that method.

Question 65

Why can't a method or class be declared as both abstract and final?

Answer 65

Because they are opposite each other. One says it must be subclassed or overridden to be of any use and the other says that you cannot subclass or override.

Question 66

Can private methods be declared as abstract? Why?

Answer 66

No - because they are private to that class, and any subclass will not be able to see that private method. It won't compile.

Question 67

Can methods and classes be both abstract and static?

Answer 67

No. top level classes and methods cannot be both static and abstract. static methods are shadowed, not overridden, and static classes are not allowed.

Question 68

Describe use of the synchronized keyword.

Answer 68

Only applicable to methods and not to classes or variables, it means only one thread can be in that method at any one time. It can be used in conjunction with all the other access modifiers for a method.

Question 69

Describe use of the word native

Answer 69

Native, like synchronized can only be used on methods - not classes or variables and means that the implementation of the method is platform specific often being written in C. Like an abstract method, the method signature ends with a semicolon and no body.

Question 70

Describe use of the word strictfp

Answer 70

strictfp is a non-access modifier for use on classes and methods. It enforces the entity to use floating point types and operations that strictly adhere to IEEE 754. It enforces common behaviour across different JVM. A variable can never be declared as strictfp.

Question 71

What is the definition of arguments?

Answer 71

Arguments are the variables or values used as part of a method call.

Question 72

What is the definition of parameters?

Answer 72

Parameters are the variables or values used as part of a method signature

Question 73

What are the 5 rules for using varargs?

Answer 73

(1) You must declare the type of the varargs which can be primitive or reference type (2) You follow the type with the ellipsis, a space and then the name of the array that will hold the parameters (3) The method signature can contain other parameters (4) the varargs must be the last parameter in the method signature if it is present at all. (5) You can only have 1 varargs in a method signature.

Question 74

Can you invoke a method with no arguments whose signature contains a varargs?

Answer 74

Yes, the array will not be null, but it will be of size 0.

Question 75

At what point in the code does a constructor get called?

Answer 75

When the object is created using the new operator.

Question 76

Describe constructors

Answer 76

(1) They are the same name as the class and are used to construct objects of that class type. (2) They cannot have a return type; (3) they can take multiple parameters.(4) They cannot be static (5) nor can they be overriden. (6) They cannot be final or abstract (cannot be overriden)

Question 77

Describe the two different variable types in Java

Answer 77

Primitive and reference variables. primitives can be 1 of 8 types - byte, short, int, long, float, double, char, boolean. Once declared its type can never be changed. Once declared, a reference type can not be changed; it refers to the memory address that contains the object. Both are passed by value - the value of the variable and the value of the memory reference.

Question 78

Describe what a signed integral or numeric type means?

Answer 78

A signed number means that it can either be positive or negative. The left most bit is used to represent the sign of the number. 1 means negative and 0 means positive. e.g. a byte has 7 bits for its value meaning a range of -128 to 127 which is 256 values (including the 0).

Question 79

List the 6 numeric types and their bit sizes

Answer 79

byte 8 short 16 int 32 long 64 float 32 double 64

Question 80

Compare all modifiers that can be used for class v local variables v variables v methods for both access/non-access

Answer 80

local variables - final variables - private, public, protected, final, static, transient, volatile methods - private, public, protected, abstract, final, static, synchronized, strictfp, native class - public, final, abstract, strictfp

Question 81

What does the keyword transient mean?

Answer 81

A transient variable is one in which you will not be serializing to a stream. Transient only applies to isntance variables (not class variables) and never to methods or classes.

Question 82

What does the keyword volatile mean?

Answer 82

It, like transient, can only be applied to instance variables

Question 83

when must the initial assignment of a final variable be completed by?

Answer 83

The end of the constructor. i.e. in the variable, static/instance initializers, static methods, and then the constructor.

Question 84

When implementing an interface method or overriding a method, can you introduce checked exceptions in the method?

Answer 84

No, these exceptions must be declared in the interface method. Same for overriding. It can throw or use exceptions which are subtypes of the exception in the abstract method it is overriding/implementing however. Also, you do not have to include the checked exceptions from the interface method.

Question 85

When implementing an interface method or overriding a method, can you introduce runtime exceptions in the method?

Answer 85

Yes. These are not requried to be declared in the abstract method.

Question 86

Can interfaces implement other interfaces?

Answer 86

No, they can extend multiple interfaces though, but not extend an object.

Question 87

Can interfaces extend mutiple interfaces?

Answer 87

Yes.

Question 88

What are the four basic variable scopes?

Answer 88

static, instance, local and block. Static is the longest and exists for as long as the class is loaded into the JVM; instance for as long as their is an instance of the object, local for as long as the method is on the call stack and block - only within the block.

Question 89

Define shadowing

Answer 89

Shadowing is similar to overriding. Rather than replace the variable, it shadows it for a time taking its place for that particular scope. e.g. local variable and an instance variable.

Question 90

What are the default values for all primitive types and object references?

Answer 90

``` int, byte, small, long - 0 object reference - null float, double - 0.0 boolean - false char '\u000' ```

Question 91

Explain the difference between an unitialized object and a null reference

Answer 91

The compiler will complain if a local object is not initialized (even with null) before use including checking for null.

Question 92

If you have two object references of the same type and copy one into the other, how many objects do you have?

Answer 92

1, but 2 object references to that object.

Question 93

What is the heap?

Answer 93

It is that area of memory that is used by the JVM for storing and retrieving objects. It is the only part of memory involved in garbage collection (no stack).

Question 94

What is the heap?

Answer 94

It is that area of memory that is used by the JVM for storing and retrieving objects. It is the only part of memory involved in garbage collection (no stack). For the purpose of garbage collection, the collector runs and looks for objects on the heap which are no longer *reachable*, *reachable* being the key word.

Question 95

What is the heap?

Answer 95

It is that area of memory that is used by the JVM for storing and retrieving objects. It is the only part of memory involved in garbage collection (no stack).

Question 96

Describe Garbage Collection

Answer 96

For the purpose of garbage collection, the collector runs and looks for objects on the heap which are no longer *reachable*, *reachable* being the key word. It is possible to call the JVM to ask it to run the GC but there are not gaurantees.

Question 97

When is an object eligible for GC>

Answer 97

When there are no live threads which have access to that object. We are really talking about having a reference variable that points to that object on the heap. GC does not ensure that you will not run out of memory - but only that the memory you have will be managed as efficiently as possible.

Question 98

How do you make an object eligble for garbage collection?

Answer 98

By assigning all references to that object to null.

Question 99

How do you make an object eligble for garbage collection?

Answer 99

By assigning all references to that object to null, or assign the reference to another object.

Question 100

How do you make an object eligble for garbage collection?

Answer 100

By assigning all references to that object to null, or assign the reference to another object. Local variables that are not returned or assigned to variables with bigger scope are eligible for GC.

Question 101

When is an object eligible for GC>

Answer 101

When there are no live threads which have access to that object. We are really talking about having a reference variable that points to that object on the heap. GC does not ensure that you will not run out of memory - but only that the memory you have will be managed as efficiently as possible. Just before throwing an OutofMemory error, the JVM will call the garbage collector. This is gauranteed, but it obviously does not gaurantee that you will always have enough memroy.

Question 102

What are islands of isolation?

Answer 102

When objects refer to each other in a cycilc manner, and noboby refers to anybody within their "inner circle", thus making an island of objects that nbody has reference to and is eligible for GC. The GC can normally detect this.

Question 103

Why is it recommended that you don't override finalize at all?

Answer 103

Because there are no gaurantees as to when the GC will run. It has non-deterministic behaviour, and so the finalize method on an object which is gauranteed to be called when you GC may or may not be called.

Question 104

Why is it recommended that you don't override finalize at all?

Answer 104

Because there are no gaurantees as to when the GC will run. It has non-deterministic behaviour, and so the finalize method on an object which is gauranteed to be called when you GC may or may not be called. The finalize method is only called once and could itself make the object ineligible for GC depending on the code you put into the finalize methd. It will only run ONCE though, and the GC will remember this and not run it more than once.