Oracle Certified Associated Java Se Programmer I Notes

Front 1

Common Java legacy packages

Back 1

java.lang

java.util

java.io

java.net

java.awt

javax.swing

(javax = Java standard Extensions)

Front 2

Classes in java.util

Back 2

Comparator

Date

Calendar

TimeZone

Locale

Currency

Random

StringTokenizer

java.util.regex.Matcher

java.util.regex.Pattern

Front 3

Classes in java.io

Back 3

byte-stream:

InputStream

OutputStream

character-stream:

Reader - BufferedReader - InputStreamReader - FileReader

Writer

File

FileDescriptor

FlenameFilter

RandomAccessFile

Front 4

Classes in java.net

Back 4

Classes for network applications

Front 5

Java Naming Conventions:

which elements begin uppercase and continue camel case?

Back 5

Class names

Interface names

Enumeration names

Front 6

Java Naming Conventions:

which elements begin lowercase and continue camel case?

Back 6

Method names

Instance, static and local variables

Parameters

Front 7

Java Naming Conventions:

which elements are uppercase and with compound words separated with underscore?

Back 7

Constants
Enumeration constants

Front 8

Java Naming Conventions:

which elements are lowercase and with compound words separated with underscore?

Back 8

Package names

Front 9

Java Naming Conventions:

which elements have one single uppercase letter?

Back 9

Generic type parameters

Front 10

javac options

Back 10

-d

destination of compiled classes

-classpath -cp

Front 11

java or javaw options

Back 11

-classpath -c

-D=

properties

java -Dprop1="hello world" -Dprop2=pippo ClassToExecute

-version

after it the interpreter exits

if a class is written before it is executed and this parameter ignored

Front 12

How to read the properties of an application

Back 12

Properties props = System.getProperties();

props.getProperty("property name");

props.setProperty("property_name", "property_value");

props.list(System.out);

Front 13

What are the type of statements in Java?

Back 13

expression

conditional

iteration

transfer of control

(Exception handling)

Front 14

What are the expression statements?

Back 14

assignment

expressions

empty statement (;)

Front 15

What are the transfer of control statements?

Back 15

assert

break

case

continue

return

Front 16

What are the exception handling statements?

Back 16

throw

try-catch-finally

Front 17

What are the iteration statements?

Back 17

while

do-while

for loop

enhanced for loop

Front 18

What are the conditional statements?

Back 18

if

if-then

if-then-else

switch

Front 19

What is the structure of the if, if-then and if-then-else statements?

Back 19

if

if

[else]

if-then

if

(else if)+

if-then-else

if

(else if)+

else

Front 20

What are the admitted types for the result of the expression in a switch statement?

Back 20

byte

short

int

char

(and their wrappers)

enum

String (only for Java > Java 7)

Front 21

What are the parts of a for loop?

Back 21

initialization
expression
iteration
loop block

Front 22

What is the structure of the enhanced for loop?

Back 22

for ( type variable : collection ) loop-block

Front 23

What types of element can be used to iterate with the enhanced for loop?

Back 23

array

collection

objects that implements the interface Iterable

Front 24

In what statements is used the break statement?

Back 24

switch

for loop

enhanced for

while

do-while

Front 25

In what statements is used the continue statement?

Back 25

for loop

enhanced for

while

do-while

Front 26

What are the usage of the return statement?

Back 26

exit a method with a value

return ;

exit a method without returning a value

return void;

The return statement is not needed if it is the last statement of a method and if it returns nothing.

Front 27

How can be used a labeled statement?

Back 27

break ;

terminates an outer statement

The break statement terminates the labeled statement; it does not transfer the flow of control to the label. Control flow is transferred to the statement immediately following the labeled (terminated) statement

continue ;

skips the current iteration of an outer loop marked with the given label

Front 28

What are the compound assignment operator?

Back 28

+=

-=

*=

/=

%=

&= (and)

|= (or)

^= (xor)

<<= (signed left bit shift)

>>= (signed right bit shift)

>>>= (unsigned right bit shift)

Front 29

What are the numeric promotion rules used when an operation is applied to operands of different types?

Back 29

1) if one and only one is a double, convert the non-double to double

2) if one and only one is a float, convert the non-float to float

3) if one and only one is a long, convert the non-long to long

4) convert both to int

Front 30

What are the operator precedence in Java?

Back 30

1. [] () .

2. postfix ++ --

3. prefix ++ --, !, ~

4. cast, new

5. * / %

6. + - concatenation

7. < <= > >= instanceof

8. == !=

9. &

10. ^

11. |

12. &&

13. ||

14. ? :

15. assignment, compound assignment

Front 31

Strings are mutable or immutable objects?

Back 31

immutable!

Front 32

StringBuilder are mutable or immutable objects?

Back 32

mutable

Front 33

What is the difference between StringBuilder and StringBuffer?

Back 33

A StringBuffer object is thread-safe

Front 34

The string concatenation operator (+) is left or right associative?

Back 34

left

(a + b) + c

"abc" + 3 + 4 ==> "abc34"

3 + 4 + "abc" ==> "7abc"

Front 35

Among the following kind of initialization of a string variable, what is the most inefficient?

String s1 = new String("abc");

String s2 = "abc";

String s3 = "ab" + "c";

Back 35

The most inefficient manner of initialize a string is by calling its constructor.

Front 36

What classes implement the CharSequence interface?

Back 36

String

StringBuilder

StringBuffer

Front 37

List common String methods

Back 37

charAt(int)

indexOf(int ch)

indexOf(int ch, int indexFrom)

indexOf(String string)

indexOf(String string, int indexFrom)

length()

concat(String string)

replace(char old, char new)

replace(CharSequence old, CharSequence new)

startsWith(String prefix)

startsWith(String prefix, int offset)

endsWith(String suffix)

substring(int begin)

substring(int begin, int end) [begin, end[

trim()

toLowerCase()

toLowerCase(Locale locale)

toUpperCase()

toUpperCase(Locale locale)

equals(String string)

equalsIngoreCase(String string)

Front 38

List common StringBuilder methods

Back 38

append(...)

insert(...)

delete(int begin, int end) [begin, end[

deleteCharAt(int index)

reverse()

Front 39

What are the primitives in Java?

Back 39

boolean (1 bit)

char (16 bit unsigned)

byte (8 bit)

short (16 bit)

int (32 bit)

long (64 bit)

float (32 bit)

double (64 bit)

Front 40

How are initialized the primitives by default in a method? And as instance variable?

Back 40

In a method variables are not initialized by default, and so if they are used without initialization a compile time error will be generated.

In a class all the primitive instance variables are initialized by default with 0 or false.

Front 41

What are the range of numbers that can be saved in variables of the different primitives?

Back 41

boolean: true, false

char: 0, 65535

byte: -128, 128 (127?)

short: -32768, 32767

int:-2 000 000 000, 2 000 000 000 (circa)

long: -2^63, 2^63 -1

float: ???

double: ???

Front 42

What is the object to use in order to store a number without loosing precision?

Back 42

BigDecimal

Front 43

What are the wrapper classes for the primitives?

Back 43

Boolean

Character

Byte

Short

Integer

Long

Float

Double

Front 44

What are the mechanisms that convert automatically primitives to their wrapper and vice-versa?

Back 44

Autoboxing: primitive -> wrapper

Unboxing: wrapper -> primitive

Front 45

What is the syntax for defining and use an enumeration?

Back 45

enum Example{ EXAMPLE1, EXAMPLE2};

Example ex;

ex = Example.EXAMPLE1;

boolean test = ex == Example.EXAMPLE2; // false

Front 46

What is a literal?

Back 46

A literal is an hard-coded value.

It is any value that is not a variable.

Front 47

Is the following notation correct?

int number = 1_024;

Back 47

Yes.
In Java 7 has been introduced the possibility to insert underscore in the definition of numeric literal, for the sake of readability.

Front 48

What are the parts that compound a method declaration?

Back 48

[access-modifier] [return-type] ([parameter-list]) { [body] }

Front 49

What does mean "overloading" a method?

Back 49

Overloading is when in a class have been defined many methods with the same name but with different parameter list.

The parameter lists can be different in the number of parameters or in their types.

The return type can also be different among overloaded methods.

Front 50

When a method is called, what kind of variables are passed by value and what by reference?

Back 50

Primitives and enums are always passed by value.

All the other kind of variables are passed by reference?

Front 51

What is the scope of local variables, method parameters and instance variables?

Back 51

Local variable: the block of code in which has been declared and every nested block of code.

Method parameter: the entire method.

Instance variable: the entire class, it remains available until the object exists.

Front 52

What is the default constructor?

Back 52

The default constructor is added by the compiler only if a class doesn't specify any constructor.

The default constructor has no parameters and only calls the super constructor.

Front 53

For what is used the keyword "this"?

Back 53

Refer the current object, for accessing instance variables or for calling instance methods.

Referring to overloaded constructors.

The "explicit constructor invocation" must occur in the first line of the constructor.

Front 54

For what is used the keyword "super"?

Back 54

Refer to an object's superclass. For accessing to methods or constructors in the super class.

In a constructor must be the first line.

If in a constructor is not called super, the compiler adds a call to super with no arguments, and so, if the parent class has not such a type of constructor, a compile time error will be generated.

Can be used for calling an overridden method.

Front 55

For what is used the keyword "static"?

Back 55

For defining class methods and class variables, also known as static methods and static variables.

Front 56

What can be accessed inside a static method?

Back 56

Only static variables and other static methods.

It cannot access any instance variable or instance method, neither use the keywords this and super.

Utility methods, perform a single task.

Front 57

Where can be accessed static variables?

Back 57

Any constructor and any other method can access static variables.

Front 58

How can be defined a constant?

Back 58

Using the keyword final.
private final int resultOK = 0;

Front 59

How are initialized the elements of an array?

Back 59

The elements of an array of primitives are initialized with 0, whereas the elements of an array of objects are initialized with null.

Front 60

Where can be used the initialization with curly brackets { }?

Back 60

Only in-line with the declaration of the array.

Front 61

How to initialize one-dimensional array?

Back 61

int [] b = new int [3];

int [] c = {1,2,3};

int [] d = new int [] {1,2,3}; // without size

Interface [] i = new Interface [3];

int [] a = new int []; **NO**

Object [] o = new Object [3] (); **NO**

Front 62

How to initialize a multi-dimensional array?

Back 62

int arr[][] = {{1,2,3}, {4,5,6}, {1,2,3,4,5,6}};

int arr[][] = new int[3][3];

int arr[][] = new int[2][];

arr[0] = new int[3];
arr[1] = new int[2];

Object o [] = {new Object[1], new Object[2]};

Front 63

How are performed the following common actions with array?

access

retrieve the size

copy

Back 63

var[index]

var.length

System.arraycopy(src, srcPos, dest, destPos, length);

Front 64

What are common methods of the class ArrayList?

Back 64

ArrayList()

ArrayList(int capacity)

get(int)

size()

add(E)

add(int, E)

ensureCapacity(int)

remove(int)

remove(Object)

Front 65

Can an object of type ArrayList be used in an enhanced for loop?

Back 65

Yes, because the class ArrayList implements the interface Iterable.

Front 66

When a class extends another class what does it inherits?

Back 66

All the public and protected methods and instance variables, and if they are in the same package all the package-private methods and instance variables.

Front 67

What is an overridden method?

Back 67

A class can override a method inherited from one of its parent classes redefining it with the same signature (name, parameters, return type)

The new method can call the overridden method by means of the super keyword.

A call to an overridden method is resolved at runtime and is based on the type of the object on which a method is called.

Front 68

An abstract class must have some abstract method?

Back 68

No, it may have some abstract method, but is not mandatory.

Front 69

A class that implements some interface must implements all its methods?

Back 69

Yes, if it is a concrete class.

No, if it is an abstract class.

Front 70

Can an interface extend only one other interface as happen with the inheritance of the classes?

Back 70

No, an interface can extend as many interfaces as it needs.

Front 71

What is the definition of Encapsulation?

Back 71

Encapsulation is the gathering of similar data and methods in single classes.

Note that Encapsulation is a programming language feature, whereas Information Hiding is a design principle.

For the context of the exam, encapsulation is defined as information hiding.

Front 72

What are the access modifiers?

Back 72

private

default (package-private)

protected

public

Front 73

A method or instance variable marked as private by what methods can be accessed?

Back 73

Only by methods in the same class.

The access is restricted also to its subclasses.

Front 74

A method or instance variable with the default access level (package-private) by what methods can be accessed?

Back 74

Only by methods on the same package.

Front 75

A method or instance variable marked as protected by what methods can be accessed?

Back 75

By methods on the same package and by the methods in the subclasses.

Front 76

A method or instance variable marked as public by what methods can be accessed?

Back 76

By any method.

There aren't any kind of restriction.

Front 77

What is the definition of Information Hiding?

Back 77

Information hiding is the technique of hiding the implementation details of a class using the most restrictive access modifiers that can be used.

The class must have a set of public methods that expose its functionality to other objects. Any other method should not be public

Front 78

What is the access modifier that must be used for the methods of an interface implemented in a class?

Back 78

public

Front 79

What is the meaning of Polymorphism?

Back 79

Polymorphism is when an object can take the place of an object with different type.

It is the capability of an object to be referred as its more general parent.

Front 80

What are the cases of polymorphism?

Back 80

When a class inherits another class or implements an interface.

Polymorphism uses the is-a relationship.

An object with an is-a relationship with another object can be used polymorphically as the second object, without being cast.

Front 81

In which cases there is the is-a relationship?

Back 81

When a class inherits another class, the first class "is-a" the second class.

A more specific class is-a more generic class.

The vice-versa is not valid.

Front 82

How to use practically polymorphism?

Back 82

Any subclass can be used interchangeably with its superclass.

If B extends A, to a variable of type A can be assigned an object of type B, but can be used only the methods defined in class A.

Any class that implements an interface can be used with its interface.

If A implements I, to a variable of type I can be assigned an object of type A, but can be used only the methods defined in the interface I

Access to static and instance fields and static methods depends on the class of the reference variable and not the actual object to which the variable points to. This is the opposite to what happens in the case of instance methods, the method of the actual class of the object is called.

Front 83

What is casting?

Back 83

Casting means converting an object to its original runtime type or any of its super classes.

Primitives can be cast to other primitives or compatible objects.

Casting is needed to convert primitives when the conversion causes data loss.

To perform a division on float variable is needed to cast the integer operands to float, e.g. float a = (float) b / (float)

An object can be cast to a type only if it was instantiated as this type or as one of its subclasses.

Front 84

What is the common parent class for exceptions and errors?

Back 84

Throwable

Front 85

What classes are direct children of the class Throwable?

Back 85

Exception and Error.

A child of Exception is the class RuntimeException

Front 86

What are checked exceptions?

Back 86

Checked exception are all the children of the class Exception.

They are checked at compile time.

Must be caught by catch statement or the thread will terminate.

Front 87

What are unchecked exceptions?

Back 87

Unchecked exception are all the children of the class RuntimeException.

They are checked at runtime, rather then compile time.

They don't need to be caught.

Front 88

What are unchecked errors?

Back 88

Unchecked errors are all the children of the class Error.

They are used in case of extreme conditions that cause the application to fail.

They shouldn't be handled.

Front 89

How to implement a custom exception or error?

Back 89

Extending the class Excpection or RuntimeException or Error.

Custom exceptions must terminate with Exception, whereas the custom errors with Error.

Custom exceptions and errors should implement a no-argument constructor and a constructor which takes a String argument.

Front 90

What are common methods of an exception class?

Back 90

These are the methods inherited from the Throwable class.

getMessage

returns a detailed message about the exception

toString

returns a detailed message about the exception and the class name

printStackTrace

prints a detailed message about the exception and the stack trace

Front 91

What type of object can be thrown with the throw statement?

Back 91

Every subtype of the Throwable class, otherwise a compile time error will occur.

throw new IllegalStateException();

Front 92

What is the usage of the keyword throws?

Back 92

It indicates that the method can send the specified exception to the caller.

Front 93

What kind of try statements exist?

Back 93

try-catch

try-finally

try-catch-finally

try-with-resources

The multi-catch clause can be used as catch clause.

Front 94

What is the best practise for sorting the catch statements?

Back 94

Subclasses first.

It is not a best practise, it is mandatory, otherwise there will be a compile time error telling that the exception has been already caught.

error: exception IllegalArgumentException has already been caught

Front 95

What does "silencing an exception" mean?

Back 95

It happens when a catch block is left empty. It is not a good practice.

Front 96

In which cases the finally clause is executed?

Back 96

It is always executed after the try clause, or any catch clause, even if try or catch return.

It is not executed only if the application terminates before the termination of the try block, except if an exception has been thrown.

Front 97

What is the usage of the try-with-resources statement?

Back 97

try(Resource r = new Resource()){

// code

} catch(Exception e){

// code

}

where the class Resource implements the interface AutoCloseable.

After the execution of the try block the resource is closed automatically, the same if an exception occurs.

The catch clause is not mandatory.

Front 98

What is the usage of the multi-catch clause?

Back 98

It permits to catch multiple exception in one catch clause.

} catch (ExceptionType1 | ExceptionType2 ... e ){

Front 99

List common checked exceptions.

Back 99

CloneNotSupportedException

ClassNotFoundException

NoSuchMethodException

IOException

FileNotFoundException

SQLException

InterruptedIOException

Front 100

List common unchecked exceptions.

Back 100

IllegalArgumentException

NumberFormatException

ArrayIndexOutOfBoundsException

IndexOutOfBoundsException

NullPointerException

IllegalStateException

ClassCastException

ArithmeticException

Front 101

List common errors.

Back 101

AssertionError (failed assertion)

ExceptionInInitializeError (unexpected exception in a static initializer)

VirtualMachineError

NoClassDefFoundError

Front 102

When is an object marked for garbage collection?

Back 102

When every reference to an object are put to null then the object is marked for garbage collection.

Front 103

When is the garbage collector executed?

Back 103

It is a low-priority thread and its execution depends on the OS.

You can never be sure about which objects have been garbage collected and when this happens.

Front 104

How many main methods used to start a Java application can be defined in a class?

Back 104

Only one.

(note that here we are talking about the main method used to start a Java application and not about a generic main method)

Front 105

In how many classes of an application can the main method be defined? One or many?

Back 105

Many.

Front 106

What type of arguments can accept the main method?

Back 106

String array

String... arg (varargs)

String **NO**

Front 107

Can the main method instantiate the current object?

Back 107

Yes.

There are no limitations on the kind of objects that can be instantiated in the main method.

Front 108

Can the value null be assigned to a primitive variable?

Back 108

NO