• Class is a programmer-defined blueprint from which objects are created

• An object is an instance of a class

• Instance variable — variable defined in a class that represents an attribute of an object

  • Declaring an Instance Variable

  • private int xLocation;

    • Private — can’t be accessed outside of the class

    • Int — data type

    • xLocation — name for the instance variable

• When an object is instantiated, it gets its own copy of the instance variable

• Inheritance — when a subclass inherits the attributes and behaviors of a superclass

• Access modifier — set the visibility of classes, variables, constructors, and methods

  • public — visible to all classes in a program

  • private — visible to only inside the class

• Encapsulation — instance variables of a class are hidden from other classes and can be accessed only through the methods of the classes

• Inheritance helps in making our code reusable and keeping it DRY

  • DRY — “Don’t repeat yourself” —> reduce repetition

  • Refactor code — improve the readability, reusability, or structure of a program without altering its functionality

• No-argument constructor — constructor w/ no parameters

• Constructor is a block of code that has the same name as the class and tells the computer how to create a new object

• How to write a constructor:

  • public name(){

    • Constructor signature

    • Inside the curly braces we write the body to the constructor by assigning values to the instance variables

    • No-argument constructor often assigns default values to the instance variables

      • A default value is a predefined value that is used by a program when the user does not provide a value (based on the data type, Java will automatically provide one)

• When a constructor is called, it sets the state of the object by assigning values to the instance variables

  • State — attributes of an object that are represented by its instance variables

• Parameterized Constructor — has specific # of arguments to be passed to assign values to an object’s instance variables

  • public Painter(int x, int y, String dir, int paint)

• Overloading — defining 2 or more constructors w/ the same name but w/ different signatures

  

Formal variables are local variables. Local variables are used first. The instant variables are then assigned the default values based on their data types

• Scope — where a variable can be used

• We can use the this keyword to refer to the instance variables instead of the local variables (refer to current object)

• Subclasses inherit the attributes & methods of the superclass but they don’t inherit its constructors!

• Subclasses inherit the private instance variables of a superclass, but they cannot access them because they are private

• Super — refers to the superclass

  • Used to call superclass methods & to access the superclass constructor

• If a subclass doesn’t include super as the first line in its constructor then Java will automatically add a super() call w/o parameters as the first line in a constructor

• int score = 125;

  • Declaring the variable: giving a name & data type to a variable

  • = Assignment operator —> initialize or change the value assigned to a variable

  • 125 —> starting value —> initializing

    • Literal —> source code representation of a value, such as a # or text

• Primitive type: double, int, boolean

• Reference type: data type that contains a pointer to the memory location of an object

• At the top of your program, if you want user input, you put : import java.util.Scanner;

• Accessor method — gives the value that is currently assigned to an instance variable

• How to write an Accessor method:

  • public int getX(){

  • return xLocation;

  • }

    • Public so it can be accessed from outside the class

    • Return type is the same as the data type of the instance variable

    • getX() —> name of the method is typically get followed by the name of the instance variable

    • Return keyword exits the method and provides the value to where the method is called

    • Then we specify the instance variable we want to return

• Expression — combination of data & operators that evaluates to a single value

• Operand — data that is operated on

• Operators can be used to create compound expressions, or a combination of expressions

• Dividing two int values will result in an int value

  • The decimal portion is truncated, or cut off

• Dividing an int by 0 will result in an Arithmetic Exception

• If you try to divide a double by 0, Java assigns the result to infinity

• Compound assignment operator: shortcut syntax to perform an operation on both operands & assign the result to the variable on the left

  

• Java evaluates expressions according to its order of operations

Scanner objects, user input ^^^

• Don’t forget to input.close();

• Mutator method: method that changes the value of an instance variable

• Public so it can be accessed from outside of the class

• The return type is void since it doesn’t return a value

• The name of the method is typically set followed by the name of the instance variable

• Specify a parameter that matches the type of the instance variable

• Then we assign the value passed to the parameter to the instance variable

• Calling a Mutator Method:

  • lisa.setAge(17);

  • When we call the method, we pass the new value to assign to the age instance variable

• Using relational operators, we can write a Boolean expression, which is a logical statement that gives either a true or false

• public String toString() — called whenever an object is printed

• The toString() method in the Object class returns a String that consists of the name of the class, the @ symbol, and the hash code of the object

• The toString() method is automatically called when an object reference is passed to a print statement or when an object reference is concatenated with a String

• To override is to define a method in a subclass with the same method signature as a method inherited from a superclass

• In the body of the toString() method, we return a String that contains information about the object

• We can use the super keyword to call the superclass version of a method in a subclass