ch1 Flashcards
What is an object?
It is an instance of a class (in memory)
What are the two primary elements of a class?
fields/variables
methods
What are variables?
hold the state of the program
what are methods?
Block of code that performs it’s duty when called
What are the three types of comments?
// single
/*
* multi
*/
/**
*javadoc
*/
Can you have two classes in the same file?
yes
Given the following,
public class Animal{
}
class Animal2{
}
What should the filename be named? Why?
Animal because the public class need to match the filename.
Hence the name of the file needs to be Animal.java and not Animal2.java
What the main method in java
public static void main(String[] args)
{
log.info(“Hello”);
}
How do you compile a hello.java?
javac hello.java
java hello (execute it)
What happens when the hello.java is compiled?
It converts to bytecode, i.e hello.class which is something that the JVM care read and exectue
Name the rules of a file name?
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. As you might imagine, a
reserved word is a keyword that Java has reserved so that you are not allowed to use it.
Remember that Java is case sensitive, so you can use versions of the keywords that only
differ in case. Please don’t, though
What package is automatically imported?
java.lang
contains System.
How do you create an instance of a class?
simply write new
Random r = new Random();
Make a constructor for the following class :
public class Hello{
}
public Hello(){
System.out.println(“Constructor”);
}
What is a constructor?
It is a method that matches the class name and does not have a return type.
Is this a contructor?
public void Hello(){}
no because it has a return type = void
What is the purpose of a constructor?
Constructors create Java objects.
Make an instance variable
public class hello{
int number = 0; //instance variable
}
What is a code block
The code between braces is called a code block
How to count an instance initializer?
code that are outside the method
What are the rules of order or initialization?
Fields and instance initializer blocks are run in the order in which they appear in the file
The constructor runs after all fields and instance intializers blocks have run
List the 8 primities data types
int
double
float
char
boolean
long
short
byte
how do you initialize a variable?
int q = 1;
Are the following declaration legal?
boolean b1, b3;
int i, int i=1;
double d1, double d2;
int i; int w;
int num; number;
int i, double number;
1 fine
2. No
NO for double. you need to do double d1, d2;
int i and int w fine
int num; number; no
last one no because it needs to be of the same data type
Why do local variable need to be initialized?
Cause they do not have default values.
What is a local variable?
A variable defined within a method. They need to be initialized. They do not have defaut values.
What is the default value of initialization of the following:
boolean
byte,short, int, long
double, float
All object references
false
0
0.0
null
State the scope of the local variable
from the declaration to the end of the block
State the instance variable
in scope from declaration until object garbage collected
public class Hell{
int hello = 0; //instance variable
}
Class variables
In scope from declaration until the program ends
static int MAX_LENGHT = 0;
Order the element in a class
P - package
I - import
C - class declaration
Methods and fields go anywhere
Where are all the objects stored in java
My program’s memory called the heap
What is garbage collection?
The process of automatically freeing memory on the heap by deleting objects that are no longer reachable in your program
System.gc()
not guaranteed to run
Name the range of the octal.
0-7
Hexadecimal
0-9 x X A-F a-f
Binary
0-1 b B
Which takes precedence?
import aquarium.*;
import aquarium.Water;
Always the one with the class name take the precedence
Can you import methods?
no only classes
Can you do this?
import aquarium.Water;
import aquarium.jellies.Water;
You cannot cannot specify the same classname in two imports
What makes java secure?
it runs inside JVM. This creates a sandbox that makes it hard for Java
code to do evil things to the computer it is running on.
What makes 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
What makes java robust?
ne 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. Bad memory
management in C++ is a big source of errors in programs.
Why java called platform independent language?
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.
Why is java OOP?
Java is an object-oriented language, which means all code is defi ned in
classes and most of those classes can be instantiated into objects
When is finalize() run?
when the object is eligible for gc
Can finalize() be called twice?
Never. Might or might not be called once. but never twice.
Why do local variable need to be initialized?
Cause they do not have default values.
When does finalize() gets called?
If the garbage collector tries to collect the object.
If the garbage collector doesnt run, the method doesnt get called
Can finalize get called twice?
NEVER
What’s the output?
public class orderPractise {
int birds=10;
public static void main(String[] args) {
int count =1, birds=5;
System.out.println(count+birds);
}
}
6
What’s the output?
public class orderPractise {
int birds=10;
public static void main(String[] args) {
int count =1;
System.out.println(count+birds);
}
}
it wont compile
What is the output?
public class orderPractise {
private static int yesterday =1;
int tomorrow = 10;
public static void main(String[] args) {
orderPractise tolls = new orderPractise();
int today =20, tomorrow=40;
System.out.println(orderPractise.yesterday+ tolls.tomorrow+ today);
}
}
31
Can Java bytecode can often be easily decoded/decompiled.?
no
What are the property of JVM?
It supports platform independence.
It manages memory for the application.
It translates Java instructions to machine instructions
public class InputImports {
public void read(Files files) {
Paths.get(“name”);
}
}
Can you do this?
import java.nio.*;
import java.nio.file.Files;
import java.nio.file.Paths;
No, wildcard should be at the end.
Given,
import java.nio.file.Files;
import java.nio.file.Paths;
Can you do the following?
import java.nio..;
Only one wildcard is needed and must be at the end
Given,
import java.nio.file.Files;
import java.nio.file.Paths;
Can you do the following?
import java.nio.files.Paths.*?
You can only import class names not methods.
What is default package used for?
unnamed package and should be used for throwaway code.
The fully qualified name for String is?
java.lang.String
Can comments appear before a package?
How about after a package?
Yes
Can a multiline comment contain special characters?
~~~
/*
comments can exist
003423/12323##$%^^&y&y!**
>.< ^^
*/
~~~
yes
What is the output?
String name = “/*Harry */ Paul”;
System.out.println(name);
/*Harry */ Paul
When making a class in java, what are the mandatory fields?
class keyword
name of the class
class body
class hello{
}
Can the main method be written this way?
static public void main(String[] args) {}
yes
JVM will run which method?
public class Practiceee {
static public void main(String args) {
System.out.println("hello");
}
public static void main(int num) {
System.out.println("this");
}
static public final void main(String... args) {
System.out.println("yo");
}
}
yo
Why wont this compile?
import java.util.Date;
import java.sql.Date;
Does not compile because code to import with the same name from different package wont compile.
Can you do this?
import java.util.Date;
public class Practiceee {
static public final void main(String... args) {
System.out.println("yo");
Date date1;
java.sql.Date date3;
}
}
yes
Can access modifiers be applied to all types of java entities?
Only to classes, interfaces and their members( instance and class variables and methods)
NOT Local variables and method parameters
True or false?
The javac command compiles directly into native machine code.
false
first bytecode then native machine code.
Is bytecode human readable?
No
Will it compile?
import java.util.*;
import java.sql.*;
public class Birthday {
private Date rob = new Date();
java.sql.Date Nelly;
}
No because it will complain about ambigious at private Date rob = new Date();
import java.util.*;
import java.sql.*;
public class Birthday {
java.sql.Date Nelly = new java.sql.Date(10L);
java.sql.Date Nelly2 = new java.sql.Date(10);
java.sql.Date Nelly3 = new java.sql.Date();
}
Nelly, Nelly2 is fine
Nelly3 is not because it requires some params
import java.util.*;
import java.sql.*;
public class Birthday {
java.sql.Date Nelly = new java.sql.Date(10L);
java.sql.Date Nelly2 = new java.sql.Date(10);
java.sql.Date Ann = new Date(10);
}
ann doesnt compile bc ambigious
Will this compile?
import java.util.*;
public class Birthday {
java.sql.Date Nelly = new java.sql.Date(10L);
java.sql.Date Nelly2 = new java.sql.Date(10);
java.util.Date Ann = new Date();
}
Yes
Will this compile?
import java.util.*;
public class Birthday {
java.sql.Date Nelly = new java.sql.Date(10L);
java.sql.Date Nelly2 = new java.sql.Date(10);
Date Ann = new Date();
Date Anny = new Date(3);
}
Yes
Will this compile?
import java.util.*;
import java.sql.*;
public class Birthday {
private Date rob = new Date();
private java.util.Date sharon = new java.util.Date();
}
No ambigious
Will this compile?
import java.util.Date;
import java.sql.*;
public class Birthday {
private Date rob = new Date();
private java.util.Date sharon = new java.util.Date();
}
Yes
Will it compile?
import java.util.*;
import java.sql.Date;
public class Birthday {
private Date rob = new Date();
private java.util.Date sharon = new java.util.Date();
}
No
becaue sqlDate need an int or long in the params
import java.util.*;
import java.sql.*;
public class Birthday {
private java.util.Date rob = new Date();
private java.util.Date sharon = new java.util.Date();
}
No still ambigius.
fix it by
private java.util.Date rob = new java.util.Date();
What is the technique of structuring programming data as a unit consisting of
attributes, with actions defined on the unit.
Object orientation
Java takes the name of the class as a parameter.
true
javac Hello.java
so Hello is the parameter
What is this called?
public int numberVisitors(int month)
method signature
What is a method signature?
The full declaration of a method is called a method signature.
why does the main method have a void return type?
Bc it is considered a good practise to return void if the method changes the state of the object. .the main() method changes the program state from started to finished.
