lecture-notes.md

CSE 142

Lecture Notes

Week 9

Definitions

Implicit: Something that is called or returned automatically without you having to state it

Explicit: Something that you call or return by specifically stating it

toString()

• By convention, when you print an object Java will always call the toString() method of that class
  • This is because every single object inherits from the Object class in Java, and within that Object class there is a toString() method
• By default, Java includes a toString() method that returns a String containing the memory address of that object
• You can override the toString() method to customize the String that it returns

Example Code

public class Point {
  private int x;
  private int y;

  pubic Point(int initialX, int initialY) {
    x = initialX;
    y = initialY;
  }

  public String toString() {
    return "(" + x + ", " + y + ")";
  }
}

// same as above, but uses this
public class Point {
  private int x;
  private int y;

  pubic Point(int x, int y) {
    this.x = x;
    this.y = y;
  }

  public String toString() {
    return "(" + x + ", " + y + ")";
  }
}

Initializing Objects

Last week, if we wanted to create an object it required 3 lines

Point p = new Point();
p.x = 3;
p.y = 8;

Instead, we can define a constructor that allows you to pass in values as parameters when creating an object.

Point p = new Point(3, 8);

Constructors

• The constructor's name needs to be the same as the name of the class itself
• Initializes the state of new objects
• Runs when the client uses the new keyword
• Does not specify a return type
• Implicitly returns the new object that was created
• Every class needs a constructor, but if you do not define a constructor then Java gives it a default constructor with no parameters and sets all fields to zero
• A class can have multiple constructors, but they each have to have a unique set of parameters
  • If you do have multiple constructors it is possible for one constructor to call another constructor

Structure

public type(parameters) {
  statements;
}

Example Code

public class Point {
  int x;
  int y;

  // constructor
  pubic Point(int initialX, int initialY) {
    x = initialX;
    y = initialY;
  }
}

Encapsulation

• Hiding implementation details of an object from its clients
• Implemented with the private keyword before fields; which stops code outside of the class from being able to access it
• If you then want to give access, you add an accessor and mutator method
• Encapsulation provides abstraction between an object and its clients
• Encapsulation protects objects from unwanted access by clients
• Every field for the final two assignments in CSE 142 should be declared to be private as shown in the example below.

Example Code

public class Point {
  private int x;
  private int y;

  pubic Point(int initialX, int initialY) {
    x = initialX;
    y = initialY;
  }

  public int getX() {
    return x;
  }

  public void setX(int newX) {
    x = newX;
  }
}

Functional Programming

You are not required to know this .. just some extra fun. Note: You are not allowed to use any of this on the homeworks or the final exam.

for each loop

// prints the numbers in the array each on its own line
int[] numbers = {3, 4, 7};
for (int n : numbers) {
  System.out.println(n);
}

Anonymous Functions

// print all the numbers
int[] numbers = {3, 4, 7};
Arrays.stream(numbers)
  .forEach(n -> System.out.print(" " + n));
System.out.println();

// print the even numbers
int[] numbers = {3, 4, 7, 8};
Arrays.stream(numbers)
  .filter(n -> n % 2 == 0)
  .forEach(n -> System.out.print(" " + n));
System.out.println();

// print the sorted even numbers
int[] numbers = {10, 3, 4, 7, 8};
Arrays.stream(numbers)
  .filter(n -> n % 2 == 0)
  .sorted()
  .forEach(n -> System.out.print(" " + n));
System.out.println();

// print the distinct sorted even numbers
int[] numbers = {10, 3, 2, 2, 4, 7, 8};
Arrays.stream(numbers)
  .filter(n -> n % 2 == 0)
  .sorted()
  .distinct()
  .forEach(n -> System.out.print(" " + n));
System.out.println();

// print the squares of the distinct sorted even numbers
int[] numbers = {10, 3, 2, 2, 4, 7, 8};
Arrays.stream(numbers)
  .filter(n -> n % 2 == 0)
  .sorted()
  .distinct()
  .map(n -> n * n)
  .forEach(n -> System.out.print(" " + n));
System.out.println();

// store the sum of the squares of the distinct sorted even numbers
int[] numbers = {10, 3, 2, 2, 4, 7, 8};
int sum = Arrays.stream(numbers)
  .filter(n -> n % 2 == 0)
  .sorted()
  .distinct()
  .map(n -> n * n)
  .reduce(0, (a, b) -> a + b);
System.out.printf("sum %d", sum);

// same as directly above, but uses sum()
int[] numbers = {10, 3, 2, 2, 4, 7, 8};
int sum = Arrays.stream(numbers)
  .filter(n -> n % 2 == 0)
  .sorted()
  .distinct()
  .map(n -> n * n)
  .sum();
System.out.printf("sum %d", sum);

public static boolean isPrime(int n) {
  return IntStream.range(1, n + 1)
    .filter(x -> n % x == 0)
    .count() == 2
}

// note that printSum adds all prime numbers in the range
printSum(IntStream.range(1, 20001).parallel());