Chapter 1 Java Building Blocks Flashcards
Java Basics Working with Java Data Types
What is an object?
An object is a runtime instance of a class in memory.
What are the two primary elements in Java classes?
Methods and fields. Together these are called the members of the class.
[Blank] holds the state of the program.
Variables (Fields)
[Blank] operates on the state of the program.
Methods
Java calls a word with special meaning a [Blank].
Keyword
Public classes in Java means?
The class can be used by other classes.
A method is
an operation that can be called again.
Return type void means
that no value at all is returned.
Methods that require information be supplied to it from the calling method is called a [Blank]
parameter
The full declaration of a method is called [Blank]
method signature
What are the the three type of comments?
Single line comments: //
Multiple line comment:
/*
*
*/
Javadoc comment /** * * */
What is a main() method?
A Java program begins execution with its main() method. It is the gateway between the startup of a Java process, which is managed by the JVM (Java Virtual Machine).
The file must have what extension to be able to be compiled as Java code?
.java
The name of the file must also match the name of the class.
The compiler turns a .java file into what type of file?
.class
What is bytecode?
Bytecode is a class file which consists of instructions that the JVM knows how to execute.
What is method access modifier?
It declares the method’s level of exposure to potential callers in the program.
The keyword [Blank] binds a method to its class so it can be called by just the class name.
static
[Blank] is a fixed size list of items that are all the same type.
Arrays
Command line arguments are treated as [Blank] objects, even if they represent another data type.
String
You need to have [Blank] to compile because it includes a compiler.
JDK (Java Development Kit)
You need [Blank] to run the code.
JRE (Java Run-time Environment)
Java class runs on [Blank] and therefore runs on any machine with Java rather than just the machine or operating system they happened to have been compiled on.
JVM (Java Virtual Machine)
Java code is organized into folders called [Blank]
packages
What does import statement do?
Import statement tell Java which packages to look in for classes.
Wildcard (*) imports
all packages that matches classes in the package
What package is automatically imported?
java.lang
package contains the classes that are fundamental to the design of the Java programming language.
[Blank] keyword is needed to create an instance of a class.
new
[Blank] is a special type of method that creates a new object.
Constructor
Two key points to note about a constructor:
Name of constructor must match the name of the class.
There are no return type.
What is the purpose of a constructor.
Initialize fields.
Are you required to code a constructor?
No, the compiler will supply a “do nothing” default constructor for you.
Codes between braces are call a
code block
Instance initializers are
code blocks that appears outside a method.
What are the two type of data?
Primitive type
Reference type
What are the 8 primitive data type?
Byte Short Int Float Double Long Boolean Char
Boolean Type
True and False
Byte Type and Size
Integral Value and 8-bit
Short Type and Size
Integral Value and 16-bit
Int Type and Size
Integral Value and 32-bit
Long Type and Size
Integral Value and 64-bit
Float Type and Size
Floating Point Value and 32-bit
Double Type and Size
Floating Point Value and 64-bit
Char Type and Size
Unicode Value and 16-bit
Reference types refer to the [Blank]
Object (An instance of a class)
A reference “points” to an object by storing the [Blank] where the object is located/
memory address
A value can be assigned to the reference in which two way?
- A reference can be assigned to another object of the same type.
- A reference can be assigned to a new object using the new keyword.
Can a reference type be assigned to null?
Yes, it means it does not currently refer to an object.
Can a primitive type by assigned to null?
No, it will give you a compiler error.
Primitive type naming convention
lowercase type names
Classes naming convention
Class names begin with an uppercase letter followed by CamelCase. Don’t start any
identifiers with $. The compiler uses this symbol for some files.
A [Blank] is a name for a piece of memory that stores data.
variable
When you declare a variable, you need to
state the variable type along with giving it a name.
Can you declare and initialize a variable in the same statement?
Yes, as long as it is all the same type. You can also initialize any or all of those value inline.
3 rules of identifier names.
- The name must begin with a letter or the symbol $ or _.
- Subsequent characters may also be numbers.
- You cannot use the same name as a Java reserved word.
What is a reserved word?
A reserved word is a keyword that Java has reserved so that you are not allowed to use it.
Name some reserved words:
abstract assert boolean break byte case catch char class const* continue default do double else enum extends false final finally float for goto* if implements import instanceof int interface long native new null package private protected public return short static strictfp super switch synchronized this throw throws transient true try void volatile while
Method and variables naming convention:
Method and variables names begin with a lowercase letter followed by CamelCase.
Java supports which character set:
Unicode: which has more than 45,000 characters
local variable
is a variable defined within a method. Local variables must be initialized
before use. They do not have a default value and contain garbage data until initialized. The
compiler will not let you read an uninitialized value.
Variables that are not local variables are known as
Instance variables or class variables
Instance variables are also called [Blank]
fields
[Blank] variables are shared across multiple objects
Class
Variables are class variables if they have the keyword [Blank] before it.
Static
Does instance and class variables needs initialization
No, as soon as you declare these variables, they are given a default value.
Boolean default initialization value
false
byte, short, int, long default initialization value
0 (in the type’s bit-length)
float, double default initialization value
0.0 (in the type’s bit-length)
char default initialization value
‘\u0000’ (NUL)
All object references (everything else) default initialization value
null
Methods in scope local
Cannot be used outside the method
Scope
Local Variables: in scope from declaration to end of block
instance Variables: in scope from declaration until object garbage collected
Class Variables: in scope from declaration until program ends
Is package declaration required and where does it go in the file. How do you declare a package?
No, first line in the file, package abc;
Is import declaration required and where does it go in the file. How do you declare an import?
No, immediately after a package, import java.util.*;
Is class declaration required and where does it go in the file. How do you declare a class?
Yes, Immediately after the import, public class c;
Is field declaration required and where does it go in the file. How do you declare a field?
No, anywhere inside a class, int value;
Is method declaration required and where does it go in the file. How do you declare a method?
No, anywhere inside a class, void method();
How does Java destroy objects?
Java automatically take care of it for you as it provides a garbage collector to automatically look for objects that aren’t needed anymore.
Where in memory are Java objects stored?
Heap
Is System.gc() guaranteed to run?
No. System.gc() simplys suggests that now might be a good time for Java to kick off a garbage collection run. Java is free to ignore the request.
An object is no longer reachable when
- The object no longer has any references pointing to it.
2. All references to the object have gone out of scope.
The reference is
a variable that has a name and can be used to access the contents of an object.
[Blank] can be assigned to another reference, passed to a method, or returned from a method.
A reference
[Blank] are the same size, no matter what their type is.
All references
[Blank] sits on the heap and does not have a name. Therefore, you have no way to access an object except through a reference.
An object
[Blank] cannot be assigned to another object, nor can an object be passed to a method or returned from a method. It is the object that gets garbage collected, not its reference.
An object
This method gets called if the garbage collector tries to collect the object.
finalize()
If the garbage collector doesn’t run, the method doesn’t get called. If the garbage collector fails to collect the object and tries to run it again later, the method doesn’t get called a second time.
[Blank] is only run when the object is eligible for garbage collection.
finalize()
Benefits of Java
Object Oriented Encapsulation Platform Independent Robust Simple
What is object oriented?
All code is defined in classes and most of those classes can be instantiated into objects.
Encapsulation
Java supports access modifiers to protect data from unintended access and modification.
Platform Independent
Java is an interpreted language because it gets compiled to bytecode. A key benefit is that Java code gets compiled once rather than needing to be recompiled for different operating systems.
How is Java Robust?
One of the major advantages of Java over C++ is that it prevents memory leaks. Java manages memory on its own and does garbage collection automatically.
How is is Java Simple?
Java was intended to be simpler than C++. In addition to eliminating pointers, it got rid of operator overloading. In C++, you could write a + b and have it mean almost anything.
How is Java Secure?
Java code runs inside the JVM. This creates a sandbox that makes it hard for Java code to do evil things to the computer it is running on.
How do you reference classes from other packages?
You use import statements
A wildcard ending an import statement means
you want to import all classes in that package.
Constructors create [Blank]
Java Objects
When an object is instantiated, [Blank] and [Blank] of codes are initialized first. Then the constructor is run.
fields and blocks
[Blank] are the basic building blocks of Java
Primitive types
Declaring a variable involves stating the [blank] type and giving the variable a name,
data
[Blank] variables must be specifically initialized.
Local
[Blank] maybe contain letters, numbers, $, or _.
Identifiers
[Blank] may not begin with numbers.
Identifiers
[Blank]
refers to that portion of code where a variable can be accessed.
Depending on scope in Java, what are the three kind of variables?
Instance, class and local.
[Blank] are nonstatic fields in your class
Instance variables
[Blank] are the static field within a class
Class variables
[Blank] are declared within a method.
Local variables
Orders within file: Class declaration, fields, import statement, methods, package statement
package statement import statement class fields / methods
Garbage collectors are responsible for
removing objects from memory when they can never be used again.
An object becomes eligible for garbage collection when there are
no more references to it or its references have all gone out of scope.
Java code is [Blank], meaning all code is defined in class
object oriented
[Blank] allows classes to encapsulate data.
Access modifiers
Java is [Blank], compiling to bytecode.
platform independent
Java is secure because it runs in a [Blank]
virtual environment
Java is robust and simple because it does not
provide pointers or operator overloading
Accessing arguments that wasn’t passed in will cause the code to
throw an exception
A constructor has the same name as the
class
Can multiple variables be declared and initialized in the same statement?
As long as they share the same type
[Blank] variables require an explicit initialization
Local Variables
Local variables go out of scope when
the block they are declared in ends.
Instance variables go out of scope when
the object is garbage collected.
Class variables go out of scope when
the program stops running.
Which of the following are valid Java identifiers? A. A$B B. _helloWorld C. true D. java.lang E. Public F. 1980_s
A, B, E. Option A is valid because you can use the dollar sign in identifiers. Option B is
valid because you can use an underscore in identifiers. Option C is not a valid identifier
because true is a Java reserved word. Option D is not valid because the dot (.) is not
allowed in identifiers. Option E is valid because Java is case sensitive, so Public is not
a reserved word and therefore a valid identifier. Option F is not valid because the first
character is not a letter, $, or _.
What is the output of the following program?
1: public class WaterBottle {
2: private String brand;
3: private boolean empty;
4: public static void main(String[] args) {
5: WaterBottle wb = new WaterBottle();
6: System.out.print(“Empty = “ + wb.empty);
7: System.out.print(“, Brand = “ + wb.brand);
8: } }
A. Line 6 generates a compiler error.
B. Line 7 generates a compiler error.
C. There is no output.
D. Empty = false, Brand = null
E. Empty = false, Brand =
F. Empty = null, Brand = null
D. Boolean fields initialize to false and references initialize to null, so empty is false
and brand is null. Brand = null is output.
Which of the following are true? (Choose all that apply) 4: short numPets = 5; 5: int numGrains = 5.6; 6: String name = "Scruffy"; 7: numPets.length(); 8: numGrains.length(); 9: name.length(); A. Line 4 generates a compiler error. B. Line 5 generates a compiler error. C. Line 6 generates a compiler error. D. Line 7 generates a compiler error. E. Line 8 generates a compiler error. F. Line 9 generates a compiler error. G. The code compiles as is.
B, D, E. Option A (line 4) compiles because short is an integral type. Option B (line
5) generates a compiler error because int is an integral type, but 5.6 is a floating-point
type. Option C (line 6) compiles because it is assigned a String. Options D and E (lines
7 and 8) do not compile because short and int are primitives. Primitives do not allow
methods to be called on them. Option F (line 9) compiles because length() is defined
on String.
Given the following class, which of the following is true? (Choose all that apply)
1: public class Snake {
2:
3: public void shed(boolean time) {
4:
5: if (time) {
6:
7: }
8: System.out.println(result);
9:
10: }
11: }
A. If String result = "done"; is inserted on line 2, the code will compile.
B. If String result = "done"; is inserted on line 4, the code will compile.
C. If String result = "done"; is inserted on line 6, the code will compile.
D. If String result = "done"; is inserted on line 9, the code will compile.
E. None of the above changes will make the code compile.A, B. Adding the variable at line 2 makes result an instance variable. Since instance
variables are in scope for the entire life of the object, option A is correct. Option B is
correct because adding the variable at line 4 makes result a local variable with a scope
of the whole method. Adding the variable at line 6 makes result a local variable with
a scope of lines 6–7. Since it is out of scope on line 8, the println does not compile and
option C is incorrect. Adding the variable at line 9 makes result a local variable with
a scope of lines 9 and 10. Since line 8 is before the declaration, it does not compile and
option D is incorrect. Finally, option E is incorrect because the code can be made to
compile.
Given the following classes, which of the following can independently replace INSERT
IMPORTS HERE to make the code compile? (Choose all that apply)
package aquarium;
public class Tank { }
package aquarium.jellies;
public class Jelly { }
package visitor;
INSERT IMPORTS HERE
public class AquariumVisitor {
public void admire(Jelly jelly) { } }
A. import aquarium.*;
B. import aquarium.*.Jelly;
C. import aquarium.jellies.Jelly;
D. import aquarium.jellies.*;
E. import aquarium.jellies.Jelly.*;
F. None of these can make the code compile.C, D. Option C is correct because it imports Jelly by classname. Option D is correct because it imports all the classes in the jellies package, which includes Jelly. Option A is incorrect because it only imports classes in the aquarium package—Tank in this case—and not those in lower-level packages. Option B is incorrect because you cannot use wildcards anyplace other than the end of an import statement. Option E is incorrect because you cannot import parts of a class with a regular import statement. Option F is incorrect because options C and D do make the code compile.
Given the following classes, what is the maximum number of imports that can be removed
and have the code still compile?
package aquarium; public class Water { }
package aquarium;
import java.lang.*;
import java.lang.System;
import aquarium.Water;
import aquarium.*;
public class Tank {
public void print(Water water) {
System.out.println(water); } }
A. 0
B. 1
C. 2
D. 3
E. 4
F. Does not compile.E. The first two imports can be removed because java.lang is automatically imported.
The second two imports can be removed because Tank and Water are in the same package,
making the correct answer E. If Tank and Water were in different packages, one of
these two imports could be removed. In that case, the answer would be option D.
Given the following classes, which of the following snippets can be inserted in place of
INSERT IMPORTS HERE and have the code compile? (Choose all that apply)
package aquarium;
public class Water {
boolean salty = false;
}
package aquarium.jellies;
public class Water {
boolean salty = true;
}
package employee;
INSERT IMPORTS HERE
public class WaterFiller {
Water water;
}
A. import aquarium.*;
B. import aquarium.Water;
import aquarium.jellies.*;
C. import aquarium.*;
import aquarium.jellies.Water;
D. import aquarium.*;
import aquarium.jellies.*;
E. import aquarium.Water;
import aquarium.jellies.Water;
F. None of these imports can make the code compile.A, B, C. Option A is correct because it imports all the classes in the aquarium package
including aquarium.Water. Options B and C are correct because they import Water by
classname. Since importing by classname takes precedence over wildcards, these compile.
Option D is incorrect because Java doesn’t know which of the two wildcard Water classes to use. Option E is incorrect because you cannot specify the same classname in
two imports.
Given the following class, which of the following calls print out Blue Jay? (Choose all that
apply)
public class BirdDisplay {
public static void main(String[] name) {
System.out.println(name[1]);
} }
A. java BirdDisplay Sparrow Blue Jay
B. java BirdDisplay Sparrow "Blue Jay"
C. java BirdDisplay Blue Jay Sparrow
D. java BirdDisplay "Blue Jay" Sparrow
E. java BirdDisplay.class Sparrow "Blue Jay"
F. java BirdDisplay.class "Blue Jay" Sparrow
G. Does not compile.B. Option B is correct because arrays start counting from zero and strings with spaces
must be in quotes. Option A is incorrect because it outputs Blue. C is incorrect because
it outputs Jay. Option D is incorrect because it outputs Sparrow. Options E and F are
incorrect because they output Error: Could not find or load main class Bird-
Display.class.
Which of the following legally fill in the blank so you can run the main() method from the command line? (Choose all that apply) public static void main( ) A. String[] _names B. String[] 123 C. String abc[] D. String _Names[] E. String... $n F. String names G. None of the above.
A, C, D, E. Option A is correct because it is the traditional main() method signature
and variables may begin with underscores. Options C and D are correct because the
array operator may appear after the variable name. Option E is correct because
varargs are allowed in place of an array. Option B is incorrect because variables are
not allowed to begin with a digit. Option F is incorrect because the argument must be
an array or varargs. Option F is a perfectly good method. However, it is not one that
can be run from the command line because it has the wrong parameter type.
Which of the following are legal entry point methods that can be run from the command
line? (Choose all that apply)
A. private static void main(String[] args)
B. public static final main(String[] args)
C. public void main(String[] args)
D. public static void test(String[] args)
E. public static void main(String[] args)
F. public static main(String[] args)
G. None of the above.
E. Option E is the canonical main() method signature. You need to memorize it.
Option A is incorrect because the main() method must be public. Options B and F
are incorrect because the main() method must have a void return type. Option C is
incorrect because the main() method must be static. Option D is incorrect because the
main() method must be named main.
Which of the following are true? (Choose all that apply)
A. An instance variable of type double defaults to null.
B. An instance variable of type int defaults to null.
C. An instance variable of type String defaults to null.
D. An instance variable of type double defaults to 0.0.
E. An instance variable of type int defaults to 0.0.
F. An instance variable of type String defaults to 0.0.
G. None of the above.
C, D. Option C is correct because all non-primitive values default to null. Option D is
correct because float and double primitives default to 0.0. Options B and E are incorrect
because int primitives default to 0.
Which of the following are true? (Choose all that apply)
A. A local variable of type boolean defaults to null.
B. A local variable of type float defaults to 0.
C. A local variable of type Object defaults to null.
D. A local variable of type boolean defaults to false.
E. A local variable of type boolean defaults to true.
F. A local variable of type float defaults to 0.0.
G. None of the above.
G. Option G is correct because local variables do not get assigned default values. The
code fails to compile if a local variable is not explicitly initialized. If this question
were about instance variables, options D and F would be correct. A boolean primitive
defaults to false and a float primitive defaults to 0.0.
Which of the following are true? (Choose all that apply)
A. An instance variable of type boolean defaults to false.
B. An instance variable of type boolean defaults to true.
C. An instance variable of type boolean defaults to null.
D. An instance variable of type int defaults to 0.
E. An instance variable of type int defaults to 0.0.
F. An instance variable of type int defaults to null.
G. None of the above.
A, D. Options A and D are correct because boolean primitives default to false and
int primitives default to 0.
Given the following class in the file /my/directory/named/A/Bird.java:
INSERT CODE HERE
public class Bird { }
Which of the following replaces INSERT CODE HERE if we compile from /my/directory?
(Choose all that apply)
A. package my.directory.named.a;
B. package my.directory.named.A;
C. package named.a;
D. package named.A;
E. package a;
F. package A;
G. Does not compile.D. The package name represents any folders underneath the current path, which is
named.A in this case. Option B is incorrect because package names are case sensitive,
just like variable names and other identifiers.
Which of the following lines of code compile? (Choose all that apply) A. int i1 = 1_234; B. double d1 = 1_234_.0; C. double d2 = 1_234._0; D. double d3 = 1_234.0_; E. double d4 = 1_234.0; F. None of the above.
A, E. Underscores are allowed as long as they are directly between two other digits.
This means options A and E are correct. Options B and C are incorrect because the
underscore is adjacent to the decimal point. Option D is incorrect because the underscore
is the last character.
Given the following class, which of the following lines of code can replace INSERT CODE
HERE to make the code compile? (Choose all that apply)
public class Price {
public void admission() {
INSERT CODE HERE
System.out.println(amount);
} }
A. int amount = 9L;
B. int amount = 0b101;
C. int amount = 0xE;
D. double amount = 0xE;
E. double amount = 1_2_.0_0;
F. int amount = 1_2_;
G. None of the above.B, C, D. 0b is the prefix for a binary value and is correct. 0x is the prefix for a hexadecimal
value. This value can be assigned to many primitive types, including int and
double, making options C and D correct. Option A is incorrect because 9L is a long
value. long amount = 9L would be allowed. Option E is incorrect because the underscore
is immediately before the decimal. Option F is incorrect because the underscore is
the very last character.
Which of the following are true? (Choose all that apply)
public class Bunny {
public static void main(String[] args) {
Bunny bun = new Bunny();
} }
A. Bunny is a class.
B. bun is a class.
C. main is a class.
D. Bunny is a reference to an object.
E. bun is a reference to an object.
F. main is a reference to an object.
G. None of the above.A, E. Bunny is a class, which can be seen from the declaration: public class Bunny. bun is a reference to an object. main() is a method.
Which represent the order in which the following statements can be assembled into a program
that will compile successfully? (Choose all that apply)
A: class Rabbit {}
B: import java.util.*;
C: package animals;
A. A, B, C
B. B, C, A
C. C, B, A
D. B, A
E. C, A
F. A, C
G. A, BC, D, E. package and import are both optional. If both are present, the order must be package, then import, then class. Option A is incorrect because class is before package and import. Option B is incorrect because import is before package. Option F is incorrect because class is before package. Option G is incorrect because class is before import.
Suppose we have a class named Rabbit. Which of the following statements are true?
(Choose all that apply)
1: public class Rabbit {
2: public static void main(String[] args) {
3: Rabbit one = new Rabbit();
4: Rabbit two = new Rabbit();
5: Rabbit three = one;
6: one = null;
7: Rabbit four = one;
8: three = null;
9: two = null;
10: two = new Rabbit();
11: System.gc();
12: } }
A. The Rabbit object from line 3 is first eligible for garbage collection immediately
following line 6.
B. The Rabbit object from line 3 is first eligible for garbage collection immediately
following line 8.
C. The Rabbit object from line 3 is first eligible for garbage collection immediately
following line 12.
D. The Rabbit object from line 4 is first eligible for garbage collection immediately
following line 9.
E. The Rabbit object from line 4 is first eligible for garbage collection immediately
following line 11.
F. The Rabbit object from line 4 is first eligible for garbage collection immediately
following line 12.
B, D. The Rabbit object from line 3 has two references to it: one and three. The references
are nulled out on lines 6 and 8, respectively. Option B is correct because this
makes the object eligible for garbage collection after line 8. Line 7 sets the reference
four to the now null one, which means it has no effect on garbage collection. The Rabbit
object from line 4 only has a single reference to it: two. Option D is correct because
this single reference becomes null on line 9. The Rabbit object declared on line 10
becomes eligible for garbage collection at the end of the method on line 12. Calling
System.gc() has no effect on eligibility for garbage collection.
What is true about the following code? (Choose all that apply)
public class Bear {
protected void finalize() {
System.out.println(“Roar!”);
}
public static void main(String[] args) {
Bear bear = new Bear();
bear = null;
System.gc();
} }
A. finalize() is guaranteed to be called.
B. finalize() might or might not be called
C. finalize() is guaranteed not to be called.
D. Garbage collection is guaranteed to run.
E. Garbage collection might or might not run.
F. Garbage collection is guaranteed not to run.
G. The code does not compile.
B, E. Calling System.gc() suggests that Java might wish to run the garbage collector.
Java is free to ignore the request, making option E correct. finalize() runs if an object
attempts to be garbage collected, making option B correct.
What does the following code output?
1: public class Salmon {
2: int count;
3: public void Salmon() {
4: count = 4;
5: }
6: public static void main(String[] args) {
7: Salmon s = new Salmon();
8: System.out.println(s.count);
9: } }
A. 0
B. 4
C. Compilation fails on line 3.
D. Compilation fails on line 4.
E. Compilation fails on line 7.
F. Compilation fails on line 8.A. While the code on line 3 does compile, it is not a constructor because it has a return
type. It is a method that happens to have the same name as the class. When the code
runs, the default constructor is called and count has the default value (0) for an int.
Which of the following are true statements? (Choose all that apply)
A. Java allows operator overloading.
B. Java code compiled on Windows can run on Linux.
C. Java has pointers to specific locations in memory.
D. Java is a procedural language.
E. Java is an object-oriented language.
F. Java is a functional programming language.
B, E. C++ has operator overloading and pointers. Java made a point of not having
either. Java does have references to objects, but these are pointing to an object that can
move around in memory. Option B is correct because Java is platform independent.
Option E is correct because Java is object oriented. While it does support some parts of
functional programming, these occur within a class.
Which of the following are true? (Choose all that apply) A. javac compiles a .class file into a .java file. B. javac compiles a .java file into a .bytecode file. C. javac compiles a .java file into a .class file. D. Java takes the name of the class as a parameter. E. Java takes the name of the .bytecode file as a parameter. F. Java takes the name of the .class file as a parameter.
C, D. Java puts source code in .java files and bytecode in .class files. It does not use a .bytecode file. When running a Java program, you pass just the name of the class without the .class extension.
