Outline

• Overriding methods, overloading methods, and polymorphism
• casting and instanceof/getClass
• Abstract classes
• class Object
• Exception class hierarchy

Overriding methods

• A class overrides a method when it provides another implementation of a method its superclass provides
• examples:
  • public String toString()
  • public boolean equals(Object obj)

Overloading methods

• A class overloads a method when it provides another method of the same name but with a different signature:
• examples:
  • public String toString(int numberOfElements)
  • public String toString(double maxValue)
  • public String toString(int numberOfElements, double maxValue)

Wrapper classes

• each of the 8 basic types has a wrapper class: Boolean, Byte, Char, Double, Float, Integer, Long, Short.
• like other regular classes, wrapper classes can be instantiated as objects
• each wrapper class has a constructor that takes as argument the corresponding basic type, e.g.

    public Integer (int value) { ...
  

  as well as other methods
• Byte, Double, Float, Integer, Long, and Short are subclasses of Number
• Java automatically boxes a primitive type if it is in a context where an object is needed, and conversely unboxes the wrapper type to its primitive value in the context where that primitive type is needed:

    Integer x = new Integer(15);
    Integer y = x + 2;
  

  x is unboxed to compute 15+2, then the result (17) is autoboxed to produce a value that can be stored in the variable y.

Polymorphism

• A variable of a superclass type can always refer to an object of a subclass type:
  Object o = new String("hello world");
• assigning an object of a superclass type to a variable of a subclass type requires a cast, and may throw a ClassCastException:
  String s = (String) o;
• polymorphism allows operations to be implemented on the most general possible type (the highest superclass) that supports the operation
• For example:

    private double sum(Number a, Number b) {
      return a.doubleValue() + b.doubleValue();
    }
    double x = sum(new Double(1.3), new Integer(2));
  

• with autoboxing, the last line can be written as:

   
    double x = sum(1.3, 2);
  

Abstract Classes

• Reminder: an interface can be used to specify that a class provides certain methods
• a class can implement any number of interfaces
• a class can have only one superclass
• if that superclass is abstract, that means that the superclass itself cannot be instantiated
• if an abstract class has abstract methods, those methods must be implemented by the subclass
• Number is an abstract class

Abstract Class Example

public abstract class AgriculturalPlant {
  protected double plantSize;

  // cannot be instantiated, so no constructor needed or possible

  // non-abstract (i.e., implemented) methods may appear in an abstract class
  public void grow(double fraction)
  {
    plantSize = plantSize * (1.0 + fraction);
  }

  // abstract methods must be implemented by the subclass(es)
  public abstract double harvest();  // returns weight harvested
  public abstract void fertilize(double fertilizerAmount);
}

class Object

• class Object is the superclass of every object class in Java, even if not extended explicitly
• useful methods include:

  boolean equals(Object obj);
  String toString();
  int hashCode();
  Class<?> getClass();
  

• hashCode() will be used later in this course, when hash tables are discussed
• getClass() can be used wih equals to test whether two objects are in the same class. For example,

  public class Foo {
    public boolean equals(Object obj) {
      if (this.getClass() != obj.getClass()) {
        return false;
      }
      ...
  

• instanceof can also be used to test class membership, e.g.

    Integer x = new Integer (99);
    if (x instanceof Number) {
      ...
  

Exception class Hierarchy

• Throwable is a subclass of Object
• Exception is a subclass of Throwable
• many exceptions are subclasses of Exception
• RuntimeException is also a subclass of Exception
• many common exceptions are subclasses of RuntimeException
• exceptions that are subclasses of Exception (but not of RuntimeException) are checked exceptions: if a method may throw such an exception, the method's declaration must say so
• runtime exceptions need not be declared, e.g.

  public String readFoo() throws java.io.FileNotFoundException
  {
    // this constructor may throw java.io.FileNotFoundException
    java.io.FileInputStream foo = new java.io.FileInputStream("foo");
    java.util.Scanner s = new java.util.Scanner (foo);
    // s.nextLine may throw NoSuchElementException or IllegalStateException,
    // both of which are subclasses of RuntimeException, so need not be declared
    return s.nextLine();
  }