Java Scjp Certification (Chap 6) - Strings, I/o, Formatting, And Parsing

Front 1

What's the big different between StringBuilder and StringBuffer

Back 1

StringBuilder isn't thread-safe so it's faster.

StringBuffer and StringBuilder have EXACTLY THE SAME INTERFACE!

Front 2

String x = "hello ";
x.concat("world");
System.out.println(x);

What prints?

Back 2

"hello "

Most (all?) methods called on a string to change that string create a new String object which must be assigned to a reference --- notice that the original reference points to the unchanged String.

Front 3

String s1 = "spring ";
String s2 = s1 + "summer ";
s1.concat("fall ");
s2.concat(s1);
s1 += "winter ";
System.out.println(s1 + " " + s2);

What is the output?
How many String objects were created prior to the println statement?

Back 3

output:
spring winter spring summer

String objects created (8):
1: "spring " (s1)
2: "summer " (lost)
3: "spring summer " (s2)
4: "fall " (lost)
5: "spring fall " (lost)
6: "spring summer spring " (lost)
7: "winter " (lost)
8: "spring winter " (s1, "spring " now lost)

Strings are immutable so they can't be updated. Instead when a string is "updated" - a brand new string is actually created.

Front 4

String s1 = new String("test");
String s2 = new String("test");

How many string objects are created by this statement?

Back 4

3.

1: the literal "test" gets turned into a String object and placed in the "String constant pool"

2: a new String object with the value "test" placed in the non-pool.

3: another new String object with the value "test" also placed in the non-pool.

Front 5

String method - returns the character located at the specified index?

Back 5

public char charAt(int index)

Front 6

String method - appends one String to the end of another?

Back 6

public String concat(String s)

Front 7

String method - determine the equality of two Strings, ignoring case?

Back 7

public boolean equalsIgnoreCase(String s)

Front 8

String method - returns the number of characters in the String?

Back 8

public int length()

Front 9

String method - swaps occurrences of some character with a new character?

Back 9

public String replace(char old, char new)

Front 10

String method - returns part of a string?

Back 10

public String substring(int begin)
public String substring(int begin, int end)

NOTE: "substring" is all lower case

NOTE: begin is inclusive and end is exclusive

Front 11

String method - return string with uppercase converted to lowercase?

Back 11

public String toLowerCase()

NOTE: starts with "to"

NOTE: IS camel cased!

Front 12

String method - return string with lowercase converted to uppercase?

Back 12

public String toUpperCase()

Front 13

String method - return string with whitespaces removed from front AND end?

Back 13

public String trim()

Front 14

StringBuffer str = "";

What is wrong with this?

Back 14

Only String objects use the shortcut of creation by assignement of a literal.

To create a StringBuffer object you must explicitly build an object:

new StringBuilder("...");

Front 15

Name 2 big differences between a String and a StringBuffer

Back 15

A String is immutable while a StringBuffer isn't - it can be changed.

This leads to the second big difference. String methods do not change the original string and instead only return new strings. StringBuffer methods, though, do change the object on which the method is called.

String s = "test";
s = s.concat(" one"); // calling object unchanged. new String object is created and s now points at it.

StringBuffer sb = "test";
sb.append(" two"); // change calling object. sb still points at same object which has had it's contents changed

Front 16

StringBuffer method - add to the end?

Back 16

public synchronized StringBuffer append(String s)

NOTE: Returns a reference to itself for chaining of calls.

NOTE: StringBuilder method is same except it is NOT synchronized.

Front 17

StringBuilder method - return with a substring removed?

Back 17

public StringBuilder delete(int begin, int end)

NOTE: object itself is changed as with all StringBuilder/StringBuffer methods

NOTE: begin is inclusive, end is exclusive

NOTE: there is no overloaded type with only a single begin argument

Front 18

StringBuilder method - insert a string at a certain offset point?

Back 18

public StringBuilder insert(int offset, String s)

NOTE: all insert methods require an offset

NOTE: offset is inclusive

NOTE: everything at and above insert point is moved over to make way for the inserted string

Front 19

StringBuffer method - reverse contents?

Back 19

public synchronized StringBuffer reverse()

Front 20

What's the basic difference between InputStream/OutputStream and Reader/Writers?

Back 20

Streams are about moving bytes while Reader/Writers are about moving characters.

Front 21

What does the File class represent?

Back 21

An abstract represenation of a file and directory pathnames.

Front 22

What are FileReader/FileWriter used for?

Back 22

Read and Write characters out of files.

Front 23

Describe how BufferedReader/BufferedWriter are used.

Back 23

Both take lower level Reader and Writer classes (i.e. FileReader & FileWriter) easier to manipulate.

NOTE: BufferedWriter provides method: newLine() which allows for creation of platform specific line separators (newLine is camel cased)

NOTE: BufferedReader has a readLine() method (readLine is camel cased)

Front 24

PrintWriter

Back 24

Enhanced in Java 5. In past could only construct with OutputStream or a Writer. As of 1.5 can also create with a File and String.

close()
flush()
format()
print()
printf()
println()
write()

Front 25

What package do you find java I/O in?

Back 25

java.io.*

Front 26

File f = new File("temp.txt");

If temp.txt doesn't exist, is it created automatically?

Back 26

No. Creating a File object is not the same as creating a file on disk. Simply creating a File has nothing to do with any existing (or not existing) file.

You must MANIPULATE/USE the File object for actual files to be affected.

Front 27

How do you check to see if a file exists?

Back 27

File f = new File("myfile.txt");
if (f.exists()) {}

Front 28

How do you create a new file on disk?

Back 28

File f = new File("temp.txt");
try {
f.createNewFile();
}
catch (IOException e) {}

NOTE: There are other classes that automatically create files as part of their object creation (i.e. BufferedReader / BufferedWriter)

NOTE: return true if file doesn't exist and was created, return false if already exists.

NOTE: createNewFile()might throw an IOException as do most I/O operations.

Front 29

What are the 3 key methods of the FileWriter?

Back 29

close()
flush()
write()

Front 30

What is the 1 key method of the FileReader?

Back 30

read()

Front 31

What method does BufferedReader add to a Reader it wraps?

Back 31

readLine();

NOTE: notice that camel case!

Front 32

What are the key constructor argument types of a File?

Back 32

(String)
(String, String)
(File, String)

Front 33

What are the key constructor argument types of a FileWriter/FileReader?

Back 33

File or String

Front 34

How do you create a new directory?

Back 34

File f = new File("foo");
f.mkdir();

Front 35

Assume temp.txt does not exist.

File f = new File("temp.txt");
FileWriter fw = new FileWriter(f);

Compiles? Runs?

Back 35

Yes and yes. temp.txt will actually be automatically created on disk when the FileWriter (or a FileReader) object is created.

Front 36

File f1 = new File("foo");
File f2 = new File(f1, "foo");
f2.createNewFile();

Compiles? Runs?

Back 36

Compiler error. The createNewFile() has to be wrapped in a try clause because IOException might be thrown.

If it's wrapped thusly, it will throw an IOException if the directory "foo" doesn't exist.

If directory "foo" exists, but the file "foo" doesn't it will create the file and return true.

Front 37

Assume relative directory foo/foo doesn't exist.

File f1 = new File("foo/foo");
try{
f1.mkdir();
}
catch (IOException e){}
Compiles? Runs?

Back 37

Yes and yes. Running does NOT mean the directory gets created, though.

If the first foo directory already exists then the second WILL get created and true is returned.

If the first foo directory does NOT exist then the mkdir() method creates no directories and false is returned.

An exception is NOT thrown!

mkdir() can only create one directory at a time.

Front 38

Assume directory foo does not exist nor does foo.txt

FileWriter f = new FileWriter("foo/foo.txt");

Compiles? Runs?

Back 38

During runtime an IOException will be thrown because while FileWriter/FileReader can create files when their objects are created... they cannot also create directories.

if foo directory did exist this would run w/out throwing an exception and the foo.txt would get automatically created.

Front 39

How do you know when you're at the end of a file using a BufferedFileReader?

Back 39

when readLine() returns null.

NOTE: readLine is camel cased.

Front 40

File dir = new File("foo");
dir.mkdir();
File f = new File("foo/temp.txt");
try{
f.createNewFile();
}
catch(IOException e) {}
dir.delete();

Compiles? Runs?

Back 40

Yes and yes. A directory named "foo" is created on disk and then in that directory a file called temp.txt is also successfully created on disk.

When an attempt to delete the directory is made, it simply returns false because you cannot delete a non-empty directory. Notices that it does NOT throw an exception in this case.

Front 41

How do you rename a File?

Back 41

Use the renameTo(File f) method on the File you want to change.

Front 42

What is the difference between these 2 declarations? How do they function differently?

File f1 = new File("foo", "temp.txt");
File f2 = new File("foo/temp.txt");

Back 42

They are the same. They function exactly the same.

Front 43

File dir = new File("foo");
dir.mkdir();
File f = new File ("foo/stuff.txt");
try {
f.createNewFile();
}
catch (IOException e){}
File dir2 = new File("fee");
dir.renameTo(dir2);

Compiles? Runs? Exception thrown?

Back 43

It compiles, runs, and no exception is thrown (assuming system allows file creation)

Directory foo gets created. Then stuff.txt gets put inside of it. Then directory foo simply gets renamed to fee -- it doesn't matter that foo had stuff in it. The is now no foo directory, just a fee directory with a file called stuff.txt

Front 44

How do you generate a list of all the files in directory?

Back 44

File dir = new File("foo");
[]String files = dir.list();

Front 45

What I/O streams are used to serialize and then unserialize objects? What methods are called to perform these serialization behaviors?

Back 45

ObjectOutputStream.writeObject();

ObjectInputStream.readObject();

Front 46

What argument does the ObjectOutputStream constructor include?

Back 46

An OutputStream or no arguments(used by children of this class).

There is not constructor that takes a File or a String.

Front 47

What is definition of the interface used to serialize?

Back 47

public interface Serializable{}

NOTE: Serializable is a "marker" interface -- just marks any implementing objects as a certain kind of thing (in this case objects mark themselves as Serializable things)

Front 48

What will you probably have to do after calling readObject() on an ObjectInputStream?

Back 48

Cast the Object to it's actual class.

Front 49

Does serialization make deep or shallow copies of everything it stores?

Back 49

Deep. All the objects that the top level objects owns are (and that they then own, and so on) are also serialized. Primitives are also serialized.

Front 50

If a object marked Serializable owns an instance of another object type that is NOT serializable, what happens when you call on writeObject() on the top level object?

Back 50

Runtime error. NotSerializableException

Front 51

How to you keep an instance variable from being serialized?

Back 51

Use the transient modifier.

Front 52

Give an example of why you'd want to keep an instance variable from being serialized along with it's owning object?

Back 52

If that ivar is of a type that is not serializable, a NotSerializableException will be thrown unless it's maked transient.

Front 53

Let's say an object has to mark some of its ivars transient? How might it restore those ivars when it gets unserialized?

Back 53

An object marked Serializable can implement it's own special writeObject() and readObject() methods, which allow it to save and restore additional information on top of default serialization behavior.

import java.io.*;

class A {
private int size = 1;

public int getSize(){
return size;
}

public void setSize(int size) {
this.size = size;
}
}

class B implements Serializable {
transient A a = new A();

private void writeObject(ObjectOutputStream os) {
try{
os.defaultWriteObject();
os.writeInt(a.getSize());
System.out.println("WRITING:" + a.getSize());

}
catch (Exception e) {}
}

private void readObject(ObjectInputStream is) {
try{
is.defaultReadObject();
a = new A();
a.setSize(is.readInt());
System.out.println("READING:" + a.getSize());
}
catch (Exception e) {}
}
}


class Temp {

public static void main(String[] args) {
B obj1 = new B();
B obj2 = new B();

obj1.a.setSize(999);

File f = new File ("temp.txt");

try {
FileOutputStream fos = new FileOutputStream(f);
ObjectOutputStream oos = new ObjectOutputStream(fos);

oos.writeObject(obj1);
oos.flush();
oos.close();

FileInputStream fis = new FileInputStream(f);
ObjectInputStream ois = new ObjectInputStream(fis);

obj2 = (B) ois.readObject();
ois.close();

System.out.println (obj2);
System.out.println (obj2.a);

}
catch(Exception e) {}
}
}

Front 54

If a class is not marked Serializable, can it be serialized?

Back 54

Maybe. If it descends from an ancestor that implements Serializable then it can be.

Front 55

What must a Serializable object, which inherits from non-serializable classes, do to serialize without throwing runtime errors?

Back 55

It's ivars might cause NotSerializableExceptions - but it's ancestors will cause no problem. The first ancestor (and all ancestors above that) will simply be rebuilt by the calling of constructors. The values that come from these ancestor classes will be just as were at the point of the original object creation

All classes that are serializable in the ancestral tree do NOT have their constructors called - and instead state is restored accordingly.

Front 56

If a class is not marked Serializable, can it be serialized?

Back 56

Maybe. If it descends from an ancestor that implements Serializable then it can be.

Front 57

What must a Serializable object, which inherits from non-serializable classes, do to serialize without throwing runtime errors?

Back 57

It's ivars might cause NotSerializableExceptions-s but it's ancestors will cause no problem. The first ancestor (and all ancestors above that) will simply be rebuilt by the calling of constructors. The values that come from these ancestor classes will be just as were at the point of the original object creation

All classes that are serializable in the ancestral tree do NOT have their constructors called - and instead state is restored accordingly.

Front 58

Which are marked serializable: the collection interfaces or the concrete collection classes?

Back 58

The classes.

Front 59

What must be true of an array and a collection before either can be serialized?

Back 59

All their elements must be serializable.

Front 60

What packages are Dates, Numbers Currencies found in?

Back 60

java.util.*
-Date
-Calendar
-Locale

java.text.*
-DateFormat
-NumberFormat

Front 61

Get current date and time.

Back 61

Date d = new Date();
System.out.println(d.toString());

OR

Calendar c = Calendar.getInstance();
Date d = c.getTime();
System.out.println(d.toString());

Front 62

Get an object that lets you perform date and time calculations in a different locale.

Back 62

Locale l = new Locale(language); or
Locale l = new Locale(language, country);

Calendar c = Calendar.getInstance(l);

c.add(...) & c.roll(...) to add and subtract time to the existing date and time.

c.set(...) & c.setTime(...) to set the date and time anew.

Front 63

Get an object that lets you perform data and time calcs, and then format it for output in different locales with different date styles.

Back 63

Calendar c = Calendar.getInstance();

Locale l = new Locale(language); or
Locale l = new Locale(language, country);

Date d = c.getTime();

DateFormat df = DateFormat.getInstance(style, l);

String s = df.format(d);

Front 64

Get an object that lets you format numbers or currencies accross many different locales.

Back 64

Locale l = new Locale(...);
NumberFormat nf = NumberFormat.getInstance(l); or
NumberFormat nf = NumberFormat.getCurrencyInstance(l);

String s = nf.format(someNumber);

Front 65

What's the relationship between the Date and Calendar classes?

Back 65

Date has for the most part been depricated and replaced by Calendar. There are times when Date bridges gaps between Calendar and other classes (specifically DateFormat class)

Front 66

What is notable about instantiating Calendar objects vs. Date objects?

Back 66

While new Date() works fine, Calendar is ABSTRACT! Thus you cannot directly create a Calendar object. You have to do it indirectly using a static method:

Calendar c = Calendar.getInstance();

Front 67

What does a Date represent?

Back 67

The number of milliseconds(internally as a long) since January 1st, 1970, 12am.

I sniff a unix timestamp! ;P

Front 68

Besides toString(), what are the 2 methods we want to know about in the Date class?

Back 68

getTime() <-- returns milliseconds

setTime(long milliseconds)


NOTE: working in milliseconds scale and NOT the seconds scale!

BTW: 1000 milliseconds = 1 second

Front 69

What Calendar methods should you be familiar with?

Back 69

Assume: Calendar c = Calendar.getInstance();

getTime(); // Returns a Date obj
setTime(Date d);

set(...); // numeric args to set up a date & time
getFirstDayOfWeek()
-returns value from Calendar.SUNDAY to Calendar.SATURDAY

get(c.DAY_OF_WEEK)

add(field, amount) where field =
Calendar.SECOND
Calendar.MINUTE
Calendar.HOUR
Calendar.YEAR
Calendar.DAY_OF_WEEK
etc.

roll()
acts like add() except when a part of Date get incremented or decrmented, larger parts of the Date will not change

Front 70

What Class do you ideally use to create a date so that it can be manipulated?

Back 70

Calendar

Calendar c = Calendar.getInstance(); // date AND time


Calendar c = Calendar.getDateInstance(); // date only

Front 71

DateFormat df = new DateFormat();

Compiles? If not how would you make it compile?

Back 71

Fails to compile.

DateFormat is abstract. Need to call one of the following static methods:

getInstance();
- unlike getDateInstance(), includes time as well
- is the default and uses a SHORT formatter for both date and time
- a SHORT time formatter means that there will be no milliseconds

getDateInstance();

getDateInstance(style);
-where style is
DateFormat.SHORT
DateFormat.MEDIUM
DateFormat.LONG
DateFormat.FULL

getDateInstance(field, locale);

Front 72

Methods of a DateFormat object?

Back 72

getInstance()
getDateInstance()
getDateInstance(SHORT/MEDIUM/LONG/FULL)
getDateInstance(SHORT/MEDIUM/LONG/FULL, Locale)

format(Date d)
parse(String dateAsString)
-might throw a ParseException!

Front 73

How does Date connect Calendar and DateFormat?

Back 73

DateFormat method format() does not take a Calendar object. It only takes a Date or in some cases a Number object. So if you convert your Calendar to a Date then you can use DateFormat.

Front 74

What happens when you call Calendar's toString() method?

Back 74

You get lots of crud! Better to output as a Date (getTime() will output Date object) and either use Date's toString() or use Date as argument of DateFormat's format() method.

Front 75

When you send a bad string to a DateFormat object's parse() method, what happens?

Back 75

a CHECKED exception gets thrown (meaning you MUST do a parse inside a try block or declare that the current method throws the exception)

This is what might be thrown:

ParseException

Front 76

Calendar c = Calendar.getInstance();
Date d = c.getTime();
System.out.println(d);

DateFormat df = DateFormat.getDateInstance(DateFormat.SHORT);
String s = df.format(d);
d = df.parse(s);
System.out.println(d);


Compile? If so, will the printed outputs match?

Back 76

This will NOT compile if if the DateFormat parse() method call is not inside a try statement or part of a method that declares it throws the checked ParseException.

If the above condition is met, then NO, the 2 strings printed will not be the same (probably ;P ). When the Date is formatted with the DateFormat.SHORT field, the formatted string doesn't include any time information. When that string get's parsed back into a Date object it now is missing any time info. So, unless this code is run EXACTLY at midnight, the 2 strings output will be different.

Front 77

If the Locale class is abstract, how do you instantiate a Locale object?

Back 77

The Locale class IS NOT abstract (so you directly instantiate)

new Locale(String language);
new Locale(String language, String country);

ISO 639 = language codes
ISO ? = country codes

(you don't need to know any codes for the exam)

Front 78

What happens when you try to format with a locale language or country that's not supported on the local machine?

Compiles? Runs?

Back 78

Yes and yes. But the output will be question marks.

I.E. ??????, ?? ??????, ????

Front 79

When you want to change locales in a DateFormat or NumberFormat object, what method to you call on the formatting object?

Back 79

None. You must create a new formatting object if you want to change locales. Once a formatting object is built with a Locale object, that Locale object is immutably set.

Front 80

What are the primary methods you'll call on a locale object?

Back 80

Locale l1 = new Locale(String language, String country);

l1.getDisplayCountry(); or
l1.getDisplayCountry(Locale l2)
// The actual instace of the locale will be displayed in default locale format if no arg version called, and will display in the locale format of the argument if a Locale argument is provided.

l1.getDisplayLanguage()
l1.getDisplayLanguage(Locale l2)

Front 81

How do you get an instance of the NumberFormat class?

Back 81

NumberFormat.getInstance();
NumberFormat.getInstance(Locale l);

NumberFormat.getNumberInstance();
NumberFormat.getNumberInstance(Locale l);

NumberFormat.getCurrencyInstance();
NumberFormat.getCurrencyInstance(Locale l);

Front 82

What are the important methods of the NumberFormat class?

Back 82

NumberFormat nf = NumberFormat.getInstance();

nf.getMaximumFractionDigits()
nf.setMaximumFractionDigits(int i)
// fractions higher than max will be ROUNDED (not truncated)


nf.setParseIntegerOnly(boolean b)
- if set to true, then any decimals in parse string when parse() converts a string to a primitive number

nf.parse(String s)
- throws ParseException

nf.format(primitive number)

Front 83

What package is regex found in?

Back 83

java.util.regex.*

Front 84

How do you set up a pattern in Java's regex?

Back 84

Pattern p = Pattern.compile(String s);

NOTE: You don't directly instantiate a pattern object! Instead you call the static method compile(String s) which outputs a Pattern object

Front 85

How do you build an object that looks for matches?

Back 85

Pattern p = Pattern.compile(String lookForThis);
Matcher m = p.matcher(String lookInHere);

Front 86

How might you iterate through a source string to find all the matches of certain pattern?

Back 86

Pattern p = Pattern.compile("\\d*");
Matcher m = p.matcher("123abc456def");
boolean b;


System.out.println(p.pattern());
while (b = m.find()) {
System.out.println(m.start() + ": " + m.group());
}

Front 87

In regex, what do the following quantifiers mean?
+
?
*

Back 87

+ = 1 or more occurrences
? = zero or 1
* = 0 or more

Front 88

What are the 3 behavioral qualities of quantifiers in regex?

Back 88

greedy (default)
- eats entire source string and works backwards

reluctant (add a ?)
- eats as little of string as it can and works forward

possessive (add a +)
- eats entire string like greedy but it does not work backwards.

EXAMPLES:
Current REGEX is: .*foo // greedy quantifier
Current INPUT is: xfooxxxxxxfoo
I found the text "xfooxxxxxxfoo" starting at index 0 and ending at index 13.

Current REGEX is: .*?foo // reluctant quantifier
Current INPUT is: xfooxxxxxxfoo
I found the text "xfoo" starting at index 0 and ending at index 4.
I found the text "xxxxxxfoo" starting at index 4 and ending at index 13.

Current REGEX is: .*+foo // possessive quantifier
Current INPUT is: xfooxxxxxxfoo
No match found.

Front 89

String ps = "\d\d";
String source = "123cat346";
Pattern p = Pattern.compile(ps);
Matcher m = p.matcher(source);
boolean b;
while (b = m.find()) {
System.out.println(m.start());
}

What prints?

Back 89

Compiler error.

Compiler will read \d as an illegal escape. String literal needs to be \\d\\d

Front 90

Pattern p = Pattern.compile("\\d*");
Matcher m = p.matcher("123abc456def");
boolean b;


System.out.println(p.pattern());
while (b = m.find()) {
System.out.println(m.start() + ": " + m.group());
}


What prints?

Back 90

\d*
0: 123
3:
4:
5:
6: 456
9:
10:
11:
12:

NOTE: The star ("*") is greedy AND it will match 0 characters. At the 4th index... it first eats entire string and works backwards. When it finally works back to the "a" and that doesn't match, it DOES match on the empty string.

If you changed the * to a + you'd get what you might expect from this:

\d+
0: 123
6: 456

Front 91

What package is Scanner found in?

Back 91

java.util.*;

Front 92

What does Scanner do?

Back 92

It scans a File, InputStream, Readable, ReadableByteChannel, or String.

Often used for tokenizing it's source.

Front 93

How do you use Scanner for searching?

Back 93

Scanner s = new Scanner(System.in);
String token = "";
do {
token = s.findInLine("a");
}
while (token != null);

Front 94

Name 2 classes you can use for tokenizing?

Back 94

String and Scanner

Front 95

What's special about the String classes split method?

Back 95

It takes String that is a regex expression.

Front 96

What's the drawback of tokenizing with String's split(String regexPattern)?

Back 96

It tokenizes it's target all at once. You can't do stuff after finding each token.

Front 97

What is Scanner's default delimiter?

Back 97

whitespace

Front 98

List the Scanner methods you need to know():

Back 98

findInLine(String regexPattern);

hasNext()
- see if there is another token present (often use this in while loop along with next()
- outputs boolean value

next()
- outputs the next token

hasNextInt()
hasNextBoolean()
hasNextFloat()
etc.
- is there another token of this particular type?

nextInt()
nextBoolean()
nextFloat()
etc..
- get next token a particular type

useDelimiter(String regexPattern)

NOTE: there is NO hasNextXxx() nextXxx() for type char.

Front 99

What is the most important thing to remember about the relationship between java.io.PrintStream method format() and printf()?

Back 99

They are aliases of each other!

Front 100

What class are methods format() an printf() a part of?

Back 100

java.io.PrintStream

Front 101

When embedding argument-specfic formatting data in a string, what character must that data begin with?

Back 101

% (percent sign)

Front 102

What is the structure of a format string?

Back 102

%[arg_index$][flags][width][.precision]conversion char

Front 103

Is the argument index in a printf() formatting string zero based

Back 103

Nope.

Front 104

What are the possible flags that can be put in a formatting string?

Back 104

- left justify
+ include sign (+ or -)
0 padd this arg with zeros
, use locale specific separators
( enclose neg #s in paras

Front 105

What is the width declartion in a formatting string?

Back 105

The minimum number of charaters to print for the argument

Front 106

What is the precision declartion in a formatting string?

Back 106

When using floats, indicate number of digits to print after decimal point

Front 107

What are the conversion character codes in a formatting string?

Back 107

b boolean

c char
s string (NOT short)

d integer (NOT decimal)
f float

Front 108

System.out.format("%d" 12.3);

Compiles?

Back 108

Yes but you get a runtime error! 12.3 is more specific than integer (d)

Front 109

What is notable about an transient object ivar that has been instantiated to some object by it's constructor, then serialized, then deserialized?

Back 109

It's set to null...

UNLESS
1) A custom writeObject(ObjectOutputStream os) and custom readObject(ObjectInputStream) have been added to the class and recreate the object or....

2) The ivar is part of an ancestor of the actual serialized object, at which point it will get recreated when the ancestor's constructor is called.

Front 110

Class A implements Serializable {}
Class B extends A{}
Class C extends B implements Serializable {}

When serializing an object of Class C, will the ivars of class A be restored via serializiation or will they be initialized by the A() constructor?

Back 110

They'll be initialized by the constructor.

The key point is that at the very first ancestor that is NOT serializable, all ancestors above it will have their constructors called - regardless of whether they're marked Serializable or not.

NOTE: Think about the constructor chaining that occurs when an object is instantiated.

Front 111

Are static variables serialized?

Back 111

Never.

Front 112

What characters does the \w metacharacter represent?

Back 112

\w stands for Word characters.

They include:
letters
numbers
underscore

Front 113

What class is the split() method associated with? What does split() return?

Back 113

The String class.

It returns an array of Strings.

The argument to the string() method is a regex expression that serves to define the delimiter. The tokens are then put in the array.

Front 114

What is the difference between a token and a delimiter?

Back 114

Delimiters are seperators and the stuff between the separators are the tokens.

Front 115

BufferedWriter bf = new BufferedWriter("temp.txt");
Compiles?

Back 115

No.

A BufferedWriter is a decorator class that can only be instantiated by wrapping another Writer object. It cannot take a string, and turn that directly into a BufferredWriter.

Here is what would work:
try {
FileWriter fw = new FileWriter("temp.txt");
BufferedWriter bf = new BufferedWriter(fw);
}
catch (IOException e) {}

Front 116

Currency c = new Currency();

Compiles? Runs?

Back 116

It doesn't compile.

Currency does not have a public constructor. Currency objects are created using static mehtod (getInstance).

Currency is a singleton.

Front 117

NumberFormat nf = NumberFormat.getCurrencyInstance();
nf.parse("1234");


Compiles? What kind of object does parse function produce?

Back 117

It does not compile.

The parse method can throw a CHECKED exception, ParseException and needs to be wrapped accordingly.

If wrapped so and the string is a valid number, then the parse function outputs an object of class Number (NOT class String).

Front 118

System.out.format("%b", 123);

What prints?

Back 118

true

the boolean conversion char "b" returns true for any non null or non-boolean argument.

Front 119

System.out.format("%f", 123);

What prints?

Back 119

Runtime error.

int literals are not automatically promoted to floats.

Front 120

String a = new String("hello");
String b = a.toString();
System.out.println(a == b);

What prints?

Back 120

True.

toString() called on a String object returns the calling object.