Computer Science - Using Objects

primitive type

Most basic data type in Java. These do no have associated methods.

reference type

Instantiable classes made by programmers that often use primitive types. Reference variables store the address of the value. These do have associated methods.

State of an object

Data about that object.

Behavior of an object

Actions that can be performed on the object.

Class

A blueprint that defines an object's variables and methods.

Class skeleton

public class ClassName
{
}

Object

An instance of a class that contains both state and behavior.

Instance Variables

Storage for data in an instance of a class.

Constructor, signature

Allows us to create a new instance of a class, usually initializing instance variables.

Using constructor

Rectangle rect1 = new Rectangle(14,16);

Formal vs Actual Parameters

Formal parameters are found in the constructor of a class and used to link actual parameters to an object.

Overloading constructors

Addition of constructors to take different number, or type of parameters, allowing the user to set the values of different combinations of the instance variables when the object is created.

Methods

Procedures that allow us to control and define the behavior of an object.

Return type

The type of data a method gives back.

Instance variables in a class

Can be used across different methods.

Call a method

objectName.method()

Procedural abstraction

Making a procedure available for general use so that programmers do not have to worry about the details within the procedure's algorithm.

Instance method

Piece of code called on a specific instance (an object) of the class. Need an object to call.

Parameters

The formal names given to the data that gets passed into a method.

Parameter list

A list of the types and variable names for the arguments being passed to a method or constructor.

Overloading methods

Overloading methods enable a method to accept different parameters.

Void method

A method that does not return a value.

name.concat("value")

Returns a new String with "value" appended to the end of name.

name.replace("a", "p")

Returns a new String with all instances of "a" in name replaced with "p".

name.toUpperCase();

Returns name as Upper Case Letters.

\"

Double-quote (")

\\

Backslash (\)

\n

new line

\t

tab

name.length()

Returns the number of characters in a String object.

name.substring(2,6)

Returns the substring beginning at index 2 and ending at index 5.

name.indexOf(d)

Returns the index of the first occurrence of d; returns -1 if not found.

name.equals("karel")

Returns true if name is equal to Karel; returns false otherwise.

name.compareTo("karel")

Returns a value < 0 if name is less than Karel.
Returns zero if name is equal to Karel.
Returns a value > 0 if name is greater than Karel.

Printing an object without toString method

Will return the object's hashcode, not the values associated with it.

Library

A collection of packages and classes that are available for use in other programs.

Package

Generally made up of alike classes that address specific programming problems.

toString

Returns the value of a String object.

Wrapper class

A class which contains or "wraps" primitive data types as an object. Data types must match the Wrapper Class type. Characterize by starting with an UpperCase letter

Instantiating an Object of a Wrapper Class

Integer y = new Integer(17);
Double z = new Double(3.14);

How do Wrapper Classes Help?

Wrapper classes contain special values and methods that are useful for converting between types.

Integer.MIN_VALUE

Returns the minimum possible int or Integer value

Integer.MAX_VALUE

Returns the maximum possible int or Integer value

objectName.intValue()

Allows us to extract the primitive value from the object Integer value.

objectName.doubleValue()

Allows us to extract the primitive value from the object Double value.

Autoboxing

Java automatically converts a primitive value into an object of the corresponding wrapper class.
Integer myInt = 53;

Unboxing

Java automatically converts an object of a wrapper type to its corresponding primitive value.
int myInt2 = myInt;

Autoboxing and Unboxing

Allows us to use primitive types and Wrapper class objects interchangeably without having to perform casting explicitly.

Math.abs(x)

Returns the absolute value of x.

Math.pow(base, exponent)

Returns the value of base raised to the power of exponent.

Math.sqrt(x)

Returns the positive square root of x.

Math.random()

Returns a double value greater than or equal to 0.0 and less than 1.0

How would you generate a random number between 0 and 9?

(int)(Math.random() * 10);

Random number from 0 - integer

(int)(Math.random() * (integer + 1));

The general formula for generating any range from (start to start+range).

(int)(Math.random() * (range + 1) + start);

Static method

Static methods are defined in an object but do not require instance data. Therefore, they can be called directly with the class name, such as Math.random().

How do you round to the nearest 2 decimal places?

(Math.round(x * 100) / 100)

Math.PI

value of pi