1)
Object-Oriented Programming

2)
Help reduce redundancy with customized types.
Can create more complicated program with virtual objects.

3)
A class type is a type for a class, that is,
a type for objects with both data and methods.

A primitive type is a simpler type.
Values of a primitive type are not complex items but simple,
indecomposable values.


4)
// Search for student with highest gpa and return name:
public static String maxGPA(String s1, double g1,
                            String s2, double g2,
                            String s3, double g3,
                            String s4, double g4,
                            String s5, double g5) {
    
  double max = g5;  // assume max gpa is 3rd gpa
  String s = s5; // name of student with max gpa
    
  if (g1 > max) { max = g1; s = s1; }
  if (g2 > max) { max = g2; s = s2; }
  if (g3 > max) { max = g3; s = s3; }
  if (g4 > max) { max = g2; s = s4; }
    
  return s+": "+max;    
}

5)
//returns the average GPA of all students in array s
public static double avgGPA(Student[] s) {
  double sumGPA = 0.0;
  for (int i=0; i<s.length; i++)
    sumGPA += s[i].gpa;
  return (sumGPA/s.length);
}

6)
Java uses the defualt toString, which returns the address
of the object in memory.
Java output:
Complex@8530b8

public class TestComplex {
  public static void main(String[] args) {
    Complex c1 = new Complex(5,6);
    Complex c2 = new Complex(7,8);
    System.out.println(c1.add(c2));
    System.out.println(c1.sub(c2));
    System.out.println(c1.mul(c2));
  }
}

class Complex {
  
  // fields
  private double real; // real component
  private double imag; // imaginary component
  
  // constructor
  public Complex(double r, double i) { real = r; imag = i; }
  
  // methods
  public Complex add(Complex other) {
    return new Complex(real+other.real, imag+other.imag);
  }
  
  public Complex sub(Complex other){
    return new Complex(real-other.real, imag-other.imag);
  }
  
  public Complex mul(Complex other){
    double newReal = real*other.real - imag*other.imag;
    double newImag = real*other.imag + imag*other.real;
    return new Complex(newReal, newImag);
  }
  
  public String toString() {
    if(imag>=0)
      return real+"+"+imag+"i";
    else
      return real+""+imag+"i";
  }
}

7)
public class TestPerson {
  public static void main(String[] args) {
    Person p1 = new Person("Adam",145.6);
    Person p2 = new Person("Bob",156.7);
    p1.compare(p2);
  }
}

class Person { 
  // fields
  private String name;
  private double weight;
  
  // constructor
  public Person(String n, double w) { name = n; weight = w; }
  
  // methods
  public void compare(Person other) {
    double diff = weight - other.weight;
    if(diff > 0)
      System.out.println(name+" is heavier.");
    else if(diff < 0)
      System.out.println(other.name+" is heavier.");
    else
      System.out.println("They have the same weight.");
  }
  
  public String toString() {
    return name+":"+weight;
  }
}