Java: OOP Summary

Objects

• All objects are allocated on the heap.
• Objects consist of instance fields for the class of the object, plus instance fields for its superclasses.
• Every object is created by a class constructor. Interfaces don't have constructors.

Reference variables

• All non-primitive variables contain only references to objects on the heap. They never contain the object. C++ object variables may be either, but Java supports only object references.

Assignment

• Reference variable assignment is fast because only the reference is copied (currently 4 bytes).
• Making a copy of an object requires using clone().
• Assigning up the inheritance hierarchy is always possible without a cast. (is-a is obvious)
• Assigning down the inheritance hierarchy is only possible with a cast. Downcasting makes a run-time check to be sure it's legal. (is-a is possible)
• Assignment to an interface type variable from a reference type implements that interface is possible without a cast. (is-a is obvious).
• Assignment to an interface type variable from a reference type that does not implement that interface requires an explicit cast which causes a run-time check to make sure that the object's class really does implement this interface. (is-a is possible)
• null can be assigned to any reference type.

Inheritance and interfaces

• Every class has one superclass, except Object, which is the root class for all other classes.
• A class may implement many interfaces, but extend only one class.

Overriding and polymorphism

• Methods (non-final) may be redefined in a subclass.
• The @Override annotation provides compiler checking.
• Polymorphism: When a method is called, the overridden method corresponding to the actual object type is called, regardless of reference variable type.

Generics

• Generics add compile-time checking and documentation.
• Java uses type erasure so generic types are actually implmented as Object.

Terminology

• Superclass - subclass. Parent class - child class. Base class - derived class.
• Overloading is reusing method names with different parameters.

Interfaces

• Interfaces list methods that implementing classes must define.
• Interfaces have no constructors.

Static variables

• There is only one copy of a static variable, not one per object.

Constructors

• A default no-parameter constructor is created automatically when no constructor is defined.
• If you define any constructor, no default constructor is created.
• Constructor chaining. The first statement of a constructor is either a call to another constructor in the same class using this, or a call to the super class constructor using super. If not called explicitly, a call to the no-parameter super constructor is inserted by the compiler.

Inner classes

• A class that is defined inside another class is called an inner class. There are four kinds of inner classes, but only two are commonly used: named inner classes, which are members of the outer class, and anonymous inner classes which are created in a code block (eg, in a method).
• Typical use: An inner class is commonly used for listeners, comparators, and other interface implementations as an easy way to provide an object with the required methods.
• Advantage: An inner class can access the outer class's instance variable. For example, this lets an inner class button listener refer to GUI text fields, etc.
• Two common kinds: named inner classes, and anonymous inner classes.
• Named inner classes
  • Defined within the outer class.
  • May extend any class.
  • May implement many interfaces.
  • May have constructors like any normal class.
  • May reference the enclosing classes instance variables.
• Anonymous inner classes
  • Defined inside a method.
  • Subclass of Object.
  • Implement exactly one interface.
  • Are instantiated at the point of definition.
  • No constructor.
  • Have names created by the compiler. Usually have a $ in them.

Casting

Casting an object reference is necessary when the compiler can't tell that the actual object is of the needed type. The cast creates a runtime check to make sure. There are two cases where a cast is required:

• To cast down the inheritance hierarchy. For example,

    Object ob = "abc";      // No cast needed to assign up hierarchy.
    String s = ob;          // BAD. Requires a cast
    String s = (String)ob;  // Required.

  All Strings are Objects, but not all Objects are Strings.

• [an error occurred while processing this directive] To cast from an interface type to a class type. (String implements Comparable)

    Comparable com = "def"; // No cast to assign to implemented interface
    String s = com;         // BAD. Requires a cast.
    String s = (String)com; // Required.

  All Strings are Comparables, but not all Comparables are Strings.