Abstract methods do not provide implementations.

A class that contains abstract methods must be an abstract class even if that class contains some non-abstract (i.e. concrete) methods.

Constructors and static methods can be declared as abstract.

A class can be made abstract by declaring it with the keyword abstract.

An abstract class normally contains one or more abstract methods.

The declaration of an abstract method in Java can be like this Java statement, public abstract void goulin();

Each concrete subclass of an abstract superclass must also provide concrete implementations of each of the superclass's abstract methods.

Invoking a method on a subclass object via a superclass reference invokes the subclass functionality.

In polymorphism, the type of the variable, but not the type of the referenced object, determines which method is called.

A program can create an array of superclass variables that refer to objects of many subclass types.

In polymorphism, an object of a subclass can be treated as an object of its superclass, enabling various interesting manipulations.

In polymorphism, an object of a superclass is not an object of any of its subclasses.

In polymorphism, the is-a relationship applies only up the hierarchy from a subclass to its direct (and indirect) superclasses, and not down the hierarchy.

In polymorphism, a technique known as downcasting enables a program to invoke subclass methods that are not in the superclass.

A multidimensional array with the same number of columns in every row can be created with an array-creation expression, such as        int[ ][ ] mwee =  new int [2] [5];

A multidimensional array in which each row has a different number of columns can be created as follows: int[ ][ ] jet =  new int [5] [8];

The lengths of the rows in a two-dimensional array are not required to be the same.

The elements of a multidimensional array are initialized when the array object is created.

int[ ][ ] mwee =  new int [2][ ]; // a multidimensional array named mwee is created with 2 //rows in which each row has a different number  of columns

calm[0] =  new int [3]; // a multidimensional array named calm is created with row 0 having 3 //columns

yee[2] =  new int [9]; // a multidimensional array named yee is created with row 2 having 9 //columns

Array objects are created with the keyword new.

An array-creation expression and declaration for an array of five integer elements and named mwee can be  int[ ] mwee = new int[5];

When an array is created, each element of the array will receive a default value.

When an array for the numeric primitive-type element is created, each element will receive a default value of zero.

When an array for the reference-type of element is created, each element will receive a default value of null.

When an array for the boolean-type of element is created, each element will receive a default value of true.

The first element in every array has an index of zero.

The expression of an array is the name of the array followed by the index of the particular element in the square bracket.

An indexed array name is an array-access expression.

Every array object knows its own length and the length of an array can always be changed.

The highest index in an array is one less than the number of elements in the array.

Elements of the arrays can be either primitive or reference types.

'Enhanced for statement' iterates through the elements of an array without using a counter.

In Enhancement for Statement, the parameter type does not have to be consistent with the array's element type.

The Enhancement for Statement simplifies the code for iterating through an array.

The syntax for 'Enhanced for statement' is  for (parameter : arrayName) statement

In Enhancement for Statement, it avoids the possibility of "stepping outside" the array.

In the syntax for 'Enhanced for statement', i.e., for (parameter : arrayName) statement, parameter has a type and an identifier and arrayName is the array through which to iterate.

Student and Alumnus is each an indirect superclass of Faculty, Staff, Administrator and Teacher.

Employee is a direct superclass of Faculty and Staff.

Staff is an indirect superclass of Administrator and Teacher.

Staff inherits directly from Student which extends CommunityMember.

Faculty inherits from Employee who is a CommunityMember.

Teacher extends Faculty which inherits from Employee who is a CommunityMember.

A Circle is a TwoDimensionalShape and is a Shape.

A Cube is a ThreeDimensionalShape and is a Shape.

A Shape inherits from TwoDimensionalShape and ThreeDimensionalShape.

Several is-a relationships follow the arrows from the bottom of the diagram to the topmost superclass in this class hierarchy.

Triangle inherits from TwoDimensionalShape which extends Shape.

Tetrahedron extends ThreeDimensionalShape which inherits from Shape.