1 Class structure

Question 1

What is the sequence to compile and run a java program from command line?

Answer 1

• open the command prompt
• navigate to the directory where the .java file with main method is located
• type & enter: javac filename.java –> the file will get compiled and a .class file will be generated
• type & enter: java filename –> program should start running

Example:
$ javac Zoo.java
$ java Zoo (* NOT Zoo.java)

Question 2

How can the arguments be written in the main method?

Answer 2

In practice, you can write String[] args, String args[] or String…
args; the compiler accepts any of these.

Question 3

What is typed at the command line when arguments need to be passed into main?

Answer 3

compile as normal
add the parameters at the end of the run line
$ javac Zoo.java
$ java Zoo Bronx Zoo (Bronx and Zoo are parameters)

Question 4

What java elements are necessary to compile? to run code?

Answer 4

you need to have a JDK to compile because it includes a compiler.

You do not
need to have a JDK to run the code—a JRE is enough

Java class files run on the JVM and
therefore run on any machine with Java rather than just the machine or operating system
they happened to have been compiled on.

Question 5

Which package is automatically imported to all java classes and doesn’t show at the top?

Answer 5

java.lang.

It is automatically imported. You can still type this package in an import statement, but you don’t have to.

Question 6

What are the rules for wildcards?

Answer 6

has to replace a class name
can only have one wildcard in the statement

Question 7

What is a code block that is outside a method?

Answer 7

instance initializers

Question 8

What must happen first before an object reference (non primitive variable) can be referred to?

Answer 8

You can’t refer to a object’s reference before it has
been initialized:
{ System.out.println(name); } // DOES NOT COMPILE
private String name = “Fluffy”;

Question 9

What is the order of initialization?

Answer 9

Fields and blocks are run first in order, setting

Then the constructor runs.

Question 10

What are the Java primitive types and their sizes?

Answer 10

boolean  true or false true
byte     8-bit integral value 123
short   16-bit integral value 123
int        32-bit integral value 123
long     64-bit integral value 123L
float     32-bit floating-point value 123.45f
double 64-bit floating-point value 123.456
char     16-bit Unicode value 'a'

Question 11

How is a long value written?

Answer 11

the integers followed by “l” or “L” Uppercase L is preferred.

Question 12

Can a primitive be assigned “null”?

Answer 12

No, only object references can be assigned null

Question 13

Is this a legal declaration?

int num, String value;

Answer 13

int num, String value; // DOES NOT COMPILE

You can’t declare two different types on same line

Question 14

What are the rules for identifiers?

Answer 14

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.
Remember that Java is case sensitive, so you can use versions of the keywords that only
differ in case. Please don’t, though

Question 15

What is a local variable?

Answer 15

A local variable is a variable defi ned within a method

Question 16

Do local variables have a default value?

Answer 16

No, local variables have to be initialized.

Question 17

What happens if you try to run code that has an uninitialized local variable?

Answer 17

It depends if you try to use it. It won’t compile if you try to use a local variable that hasn’t been initialized. If you declare it but don’t use it, then it will just be ignored.

Question 18

Does this compile?

4: public int notValid() {
5: int y = 10;
6: int x;
7: int reply = x + y;
8: return reply;
9: }

Answer 18

No, the local variable X is not initialized at the time it is used. The code won’t compile.

Question 19

What does finalize() do?

Answer 19

finalize() may or may not run. It is only called when the object is eligible for garbage collection so don’t add it while there is still a variable referring to the object (like a static variable that stays in scope until the program ends.

The finalize() method will run once for each object if/when it is first garbage collected. finalize() call could run zero or one time. It will NEVER run a second time

Question 20

Can a local variable be used outside the method?

Answer 20

No.

Question 21

How are local variables affected by code blocks (Braces { } )?

Answer 21

Remember that blocks can contain other blocks. These smaller contained blocks can reference
variables defined in the larger scoped blocks, but not vice versa.

Question 22

What is the order of variable initialization?

Answer 22

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 initializer blocks have run.

Question 23

What are the default initialization values by type?

Answer 23

boolean false
byte, short, int, long 0 (in the type’s bit-length)
float, double 0.0 (in the type’s bit-length)
char ‘\u0000’ (NUL)

All object references (everything else) null

Question 24

What is the order of initialization?

Answer 24

1. If there is a superclass, initialize it first (we’ll cover this rule in the next chapter. For
   now, just say “no superclass” and go on to the next rule.)
2. Static variable declarations and static initializers in the order they appear in the file.
3. Instance variable declarations and instance initializers in the order they appear in the file.
4. The constructor.

Question 25

What are the JavaBeans naming conventions (getters & setters)?

Answer 25

``` Properties are private. private int numEggs; Getter methods begin with is if the property is a boolean. public boolean isHappy() { return happy; } Getter methods begin with get if the property is not a boolean. public int getNumEggs() { return numEggs; } Setter methods begin with set. public void setHappy(boolean happy) { this.happy = happy; } The method name must have a prefix of set/get/is, followed by the first letter of the property in uppercase, followed by the rest of the property name. public void setNumEggs(int num) { numEggs = num; } ```

Question 26

What does a main method look like that takes parameters?

Answer 26

? ask Terril

Question 27

How can you write a main method that takes parameters?

Answer 27

``` public class Zoo { public static void main(String[] args) { System.out.println(args[0]); System.out.println(args[1]); } } ```

Question 28

When the main method takes parameters, how can the info be supplied?

Answer 28

In the command line, or ? other ways

Question 29

What does the wildcard import?

Answer 29

It only imports classes in the indicated package. It doesn't import child packages, fields or methods.

Question 30

What package is automatically imported (you don't see the import)?

Answer 30

java.lang

Question 31

import java.nio.file.Files; import java.nio.file.Paths; Explain why these imports don't work 1) import java.nio.*; 2) import java.nio.*.*; 3) import java.nio.files.Paths.*;

Answer 31

``` import java.nio.*; // NO GOOD – a wildcard only matches //class names, not "file.*Files" import java.nio.*.*; // NO GOOD – you can only have one wildcard //and it must be at the end import java.nio.files.Paths.*; // NO GOOD – you cannot import methods //only class names ```