Mason Vail

 A data type definition – “blueprint for objects”  Includes properties and/or methods ◦ “instance” data / methods – specific to one object (instance) of the class ◦ “static” or “class” data / methods – belongs to the class itself, shared by all objects of the type

 One instance of a data type  Looks and acts like other objects of the same type, but has its own identity and state independent of other objects

 One of the central pillars of object-oriented design  Idea that each object is in control of its own state (instance data) and outsiders are required to interact with the object through its limited public interface (set of methods)

 Visibility regulates the exposure of data and methods ◦ “public” – accessible to any outside entity with a reference to the object ◦ “protected” – default visibility – accessible to child objects or any other entity defined in the same package ◦ “private” – accessible only within the class/object  All data / methods should be private until a good reason is given to relax visibility

 A named set of statements within a class that carry out a specific task  Public methods make up the public interface of an object  Protected/private methods generally serve as supporting utility methods for carrying out intermediate tasks for the object

 The class containing the main() method, where a program begins  The class invoked by name when launching the Java Runtime Environment (JRE)

 The most basic linear data structure for storing multiple values of the same type  In some languages, they are a primitive data type, but in Java arrays are objects, though they have special syntax that makes them look a little different than other objects

 As objects, arrays must be instantiated with a constructor call before they can be used ◦ int[] a = new int[10]; //creates an array with room for 10 ints  The size of an array, once instantiated, is immutable

 Arrays can be multidimensional – each dimension can be of a different size  2D arrays (“tables”) are common  The first dimension of a 2D array is commonly considered to be the row reference  The second dimension of a 2D array is commonly considered to be the column

 Any multidimensional array can be thought of as an “array of arrays” until the last dimension is accessed and an individual value of the array type is reached  int[][] someArray = new int[5][10]; ◦ someArray is an array of int[]s of length 5 ◦ someArray[0] is in int[] of length 10

 A primitive variable is a named memory location containing a value of the primitive type  An object reference variable contains the address of an object somewhere else in memory

 Making a copy of a primitive variable duplicates the value, after which the two variables have no effect on the other  Making a copy of an object reference creates an alias – a duplicate reference to the same object – so changes can be made to the object via any of its aliased references  Copying an object requires instantiating a new, independent object and explicitly duplicating the state of the original object in the new object

 A collection of bodiless, abstract method signatures defined in a.java file  May be implemented by one or more classes  Not a class – may not be instantiated

 Accessor (“getter”) methods – methods with the special purpose of returning object information without violating encapsulation ◦ Returned values are typically already known – the method only needs to return an independent copy of instance data  Mutator (“setter”) methods – methods with the special purpose of updating object state without violating encapsulation ◦ Changing data takes the form of a request, rather than a demand – the object has the opportunity to validate input and enforce its own rules about updating its state