IC211 6 Week Exam Practice

How does java attempt to provide the following advantages?
- portability
  By compiling to byte code that is executed on the virtual machine, compiled java programs can run on any machine with a virtual machine installed.
- safety/robustness
  There are a number of ways java does this. They include, cleaning up the systax rules so only certain types are allowed in certain conexts, such as booleans in 'if's, result in fewer run-time errors. When run-time errors do occor, because the program is running in the virtual machine, damage to the running system is minimized. The virtual machine can also prevent malicious programs from damaging your system.
What is a virtual machine?

A virtual machine is a program that simulates a computer. It can be programmed as if it were a real computer, and those programs do not know they are not running directly on a computer. Examples include both the Java Virtual Machine, and VirtualBox.
Given the following code:
```
         Scanner in = new Scanner(System.in);
         System.out.print("Please input an integer ");
         int n = in.nextInt();
	 System.out.println("you input : " + n++);
```
- What would be the output if the user entered 7?
  you input : 7
- what would be the output if the user entered seven?
  Exception in thread "main" java.util.InputMismatchException
What does it mean when you declare a method static?
Circle the correct option in each sentence:
- "An intended benefit of encapsulation and information hiding is to prevent the users of a class from
  modifying data / discovering itellectual property / invalidating algorithms
  that cause bugs later."
- "An intended benefit of encapsulation and information hiding is to enable a class writer to change the representations and algorithms inside a class so that the change has
  no / slight / moderate / great
  effect on the users of the class."

Assume you're given the following file:

Ex1.java

public class Ex1
{
  public static int cat;
  public int pig;
  public static int rat() { return 0; }
  public int dog() { return 0; }
}

Circle the valid calls of the function rat in the code below:
Valid calls bold below

public class App1 {
  public static void 
  main(String[] args) {
    Ex1 a = new Ex1();
    a.rat();
    Ex1.rat();
    Ex1.a.rat();
    a.Ex1.rat();
  }
}

Circle the valid calls of the function dog in the code below:
Valid calls in bold

public class App1 {
  public static void
  main(String[] args) {
    Ex1 a = new Ex1();
    a.dog();
    Ex1.dog();
    Ex1.a.dog();
    a.Ex1.dog();
 }
}

Circle the valid assignments to pig code below:
Valid assignments in bold

public class App1
{
  public static void 
  main(String[] args) {
    a.pig = 3;
    Ex1.pig = 3;
    Ex1.a.pig = 3;
    a.Ex1.pig = 3;
  }
}

Of the following, which variables are base types, and which are pointers to objects. Some variables might refer to objects defined in lecture. Label base for base type and object for object.
```
 int i; base

 String s;;object

 Angle a;;object

 double d; base

 boolean b; base

 Scanner in;;object

 char c; base

 Node n;;object

 Player p;object
```

Find and fix the errors in this code:

import java.util.*

class Main {

  public void main(String args) {

    int myArray[10];

    for (i=0;i<=10;i++) {

      myArray[i] = i;

  }

};

import java.util.*;

class Main {

  public void main(String [] args) {

    int myArray[] =new int[10];

    for (i=0;i<10;i++) {

      myArray[i] = i;

  }

}

In the typical case, where do we define new classes?
We define new classes in a new file with the same name as the class.

There are 3 versions of 'swap' below. One works, the other two do not. Identify the correct version and describe why the broken versions are broken:

public class Foo 
{
int i;
public static void swap(int i, int j)
{
  int tmp = i;
  i = j;
  j = tmp;
}
}

Assuming a and b are Foo's, call as:

Foo.swap(a.i,b.j);

public class Foo 
{
int i;
public static void swap(Foo i, Foo j)
{
  Foo tmp = i;
  i = j;
  j = tmp;
}
}

Assuming a and b are Foo's, call as:

Foo.swap(a,b);


public class Foo
{
int i;
public static void swap(Foo i, Foo j)
{
  int tmp;
  tmp = i.i;
  i.i = j.i;
  j.i = tmp;
}
}

Assuming a and b are Foo's, call as:

Foo.swap(a,b);

Only the last version of swap works. Java is always pass by value, so if ints are passed as arguments, you cannot swap them. If pointers to objects are passed as arguments, you cannot swap them either (the pointers are still passed by value), but the contents of an object can be swaped, because we are given the pointers to the locations of the contents.

Assuming a 40 array of characters named charlie, circle code segment correctly prevents an ArrayIndexOutOfBoundsException.
if (i < 40 || charlie[i] == 'x') {
//do something...

if (i <= 40 || charlie[i] == 'x') {
   //do something...

if (charlie[i] = 'x' || i <= 40 ) {
   //do something...

if (charlie[i] = 'x' || i < 40 ) {
   //do something...

};

import java.util.*;

class Main {

  public void main(String [] args) {

    int myArray[] =new int[10];

    for (i=0;i<10;i++) {

      myArray[i] = i;

  }

}

In the typical case, where do we define new classes?
We define new classes in a new file with the same name as the class.

There are 3 versions of 'swap' below. One works, the other two do not. Identify the correct version and describe why the broken versions are broken:

public class Foo 
{
int i;
public static void swap(int i, int j)
{
  int tmp = i;
  i = j;
  j = tmp;
}
}

Assuming a and b are Foo's, call as:

Foo.swap(a.i,b.j);

public class Foo 
{
int i;
public static void swap(Foo i, Foo j)
{
  Foo tmp = i;
  i = j;
  j = tmp;
}
}

Assuming a and b are Foo's, call as:

Foo.swap(a,b);


public class Foo
{
int i;
public static void swap(Foo i, Foo j)
{
  int tmp;
  tmp = i.i;
  i.i = j.i;
  j.i = tmp;
}
}

Assuming a and b are Foo's, call as:

Foo.swap(a,b);

Assuming a 40 array of characters named charlie, circle code segment correctly prevents an ArrayIndexOutOfBoundsException.
if (i < 40 || charlie[i] == 'x') {
//do something...

if (i <= 40 || charlie[i] == 'x') {
   //do something...

if (charlie[i] = 'x' || i <= 40 ) {
   //do something...

if (charlie[i] = 'x' || i < 40 ) {
   //do something...

What is the output of the program Main?

Main.java

Independency.java

public class Main {
  public static void main(String []args) {
    int i = -1;
    Independency j = new Independency(1,2,3);
    int k = j.i(i,5);
    System.out.println("result: " + i + " " + j + " " + k);
  }
}

public class Independency {
  int i;
  int j;
  int k;

  public Independency(int i,int j, int k) {
    this.i = i;
    this.j = j;
    this.k = k;
  }    
    public int i(int i,int k) {
    int j =4;
    return i + j;
  }
}

result: -1 Independency@e53108 3

Add appropriate javadoc comments to this code:

/** Class representing a book
 * @author Prof Crabbe
 * @version 0.1
*/
public class Book {
  private int year;
  private int length;
  private Page []pages;
/** year and length constructor
 * @param y year of publication
 * @param l length of book
 */ 
  public Book(int y,int l) {
    year= y;
    length = l;
    pages = new Page[MAXLENGTH];
  }
/** Adds a new page to the book
 * @param the page object
 */ 
  public void addPage(Page p) {
    pages[size++] = p;
  }   
/** Finds the page number a string occurs on to help make an index.
 * @param s the sring to find
 * @return the page number
 */  
  public int index(String s) {
    for(int i = 0; i<size; i++) {
      if(pages[i].contains(s))
        return i;
    }
    return MAXLENGTH;
  }
}