04b: Recap

ICS211, Fall 2012
Dr. Zach

(switch view)

Status check

Discussion 01: Algorithm Selection Criteria

What we've learned so far...

"Let me explain... No, there is too much. Let me sum up."

      --Inigo Montoya, The Princess Bride

What you should know for the exam... Java

What you should know for the exam... concepts

What you should know for the exam... processes

The exam itself

Looking ahead...

Java Review

Odds and Ends

Arrays: Useful Tricks

int[] nums = {1, 2, 3, 4};
nums = {2, 4, 6, 8}; // NO!  Can only initialize this way
nums = new int[]{2, 4, 6, 8};  // works

java.util.Arrays has many useful methods, including copyOf, equals, and toString. 

String asStr = java.util.Arrays.toString(nums);
System.out.println(asStr);  //prints: [2, 4, 6, 8]
System.out.println(java.util.Arrays.toString(new int[]{5, 8, 13, 21}));

Some people also like System.arraycopy method. (Very versatile. Check out the API.)

Arrays and the Heap (1/2)

int[] evens = {2, 4, 6};
int[] odds = {1, 3, 5};
odds = evens;
odds[1] = 7;
System.out.println(evens[1]);

What does this print?

Arrays and the Heap (2/2)

public class Example {
  public static void main(String[] args) {
    int[] nums = {1, 3, 5, 8};
    int a = 1;
    int b = 2;
    swap(nums, a, b);
    System.out.println("nums[a] = " + nums[a] + ", nums[b] = " + nums[b]);
  }

  public static void swap(int[] array, int left, int right) {
    int temp = array[left];
    array[left] = array[right];
    array[right] = temp;
  }
}

Wrapper classes

Wrapper classes: Autoboxing example

//old manual way
int num = 5;
Integer number = new Integer(num);  //wrapping up value
//...could now pass number to some method that only takes an object
int x = number.intValue()           //unwrapping 

//now, with autoboxing
Integer five = num;   //autobox: assign int to Integer and it worked!
Integer f = 5;        //same
int n = five;         //unboxing: assigned Integer value to int

java.util.ArrayList

  java.util.ArrayList<Integer> nums = new java.util.ArrayList<Integer>();
  nums.add(3);   //autoboxing at work
  nums.add(6);
  nums.add(1, 4);
  int x = nums.get(0);  //x == 3
  nums.set(2, 5);  //changes 6 to 5
  System.out.println(nums);  //prints: [3, 4, 5]
  System.out.println("size=" + nums.size());  //prints: size=3

Footnote: Later in the term we'll learn why it's better to use List<Integer> nums = new ArrayList<Integer>(); here and to write methods to take and return List rather than ArrayList.

Java Review

Objects

Classes

public class Simple {
}

public static void main(String[] args) {
  Simple s = new Simple();
}

Like arrays, objects are in the heap.

Object state = Instance variables

public class Cat {
  String name;
  int lives;
}

Constructor

public class Cat {
  String name;
  int lives;
  
  public Cat(String name, int lives) {
    this.name = name;
    this.lives = (lives < 1) ? 1 : lives;
  }
}

this

public class Cat {
  String name;
  int lives;

  //preferred version  
  public Cat(String name, int lives) {
    this.name = name;
    this.lives = (lives < 1) ? 1 : lives;
  }

  //alt: works (but is not as good)
  public Cat(String n, int l) {
    name = n;
    lives = (l < 1) ? 1 : l;  //Tip: NEVER use l as a variable name!
  }

  //alt: broken!
  public Cat(String name, int lives) {
    name = name;
    lives = (lives < 1) ? 1 : lives;
  }
}

Overloading constructors

public class Cat {
  String name;
  int lives;

  public Cat(String name, int lives) {
    this.name = name;
    this.lives = (lives < 1) ? 1 : lives;
  }
  
  public Cat(String name) {
    this(name, 9);  //invoke other constructor rather than repeat code here
  }
}

Object Behavior = Instance Methods

public class Cat {
  String name;
  int lives;

  //(constructors omitted)
  
  public void die() {
    this.lives--;
  }
  
  public boolean isReallyDead() {
    return lives <= 0;
  }
}

Encapsulation: Good practice

public class Cat {
  private String name;
  private int lives;

  public Cat(String name, int lives) {
    this.name = name;
    this.lives = (lives < 1) ? 1 : lives;
  }
  
  public String getName() {
    return this.name;
  }

  public void setName(String name) {  //can rename a cat?
    this.name = name;  
  }

  public int getLives() {
    return this.lives;
  }  
}

Enums

(might be review)

Defining

public enum Suit {
  SPADES, HEARTS, DIAMONDS, CLUBS;
}

public enum Direction {
  N, NE, E, SE, S, SW, W, NW, HERE;
}

Example Use

// a constructor in class PlayingCard
public PlayingCard(int value, Suit suit) {
  //...
}

PlayingCard card = new PlayingCard(4, Suit.CLUBS);

Other advantages

Important methods

Every enum can has the following:

More details

Before next time...