3 classes and objects

8/2/2019 3 Classes and Objects

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Classes and Objects


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Topics

Object oriented programming

Defining Classes

Using Classes

References vs Values Static types and methods


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Represent the real world


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Objects group together Primitives (int, double, char, etc..)

Objects (String, etc)


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Why use classes?

Why not just primitives?


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Why use classes?


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Why use classes?


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Why use classes?


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Why use classes?


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Why use classes?


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Why use classes?


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Defining classes


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Class overview


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Class -overview


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Lets declare a baby!


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Note

Class names are Capitalized

1 Class = 1 file

Having a main method means the class can be run


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Baby fields


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Baby fields


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Baby Siblings?


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Ok, lets make this baby!


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Constructors


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Constructors

Constructor name == the class name No return typenever returns anything

Usually initialize fields

All classes need at least one constructor If you dont write one, defaults to

CLASSNAME () {

}


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Baby constructor


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Baby methods


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Baby methods


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Baby class


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Using classes


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Classes and Instances


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Accessing fields

Object.FIELDNAME


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Calling Methods

Object.METHODNAME([ARGUMENTS])


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References vs Values


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Primitives vs References

Primitive types are basic java types int, long, double, boolean, char, short, byte, float

The actual values are stored in the variable

Reference types are arrays and objects

String, int[], Baby,


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How java stores primitives

Variables are like fixed size cups Primitives are small enough that they just fit into the cup


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How java stores objects

Objects are too big to fit in a variable Stored somewhere else

Variable stores a number that locates the object


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References

The objects location is called a reference == compares the references

Does shiloh1 == shiloh2?


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References


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References


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References


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References

Using = updates the reference


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References


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References

using [ ] or Follows the reference to the object

May modify the object, but never the reference

Imagine

Following directions to a house

Moving the furniture around

Analogous to

Following the reference to an object Changing fields in the object


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static types and methods


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static

Applies to fields and methods

Means the field/method

Is defined for the class declaration,

Is not unique for each instance


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static


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static example

Keep track of the number of babies that have been made


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static field

Keep track of the number of babies that have been made


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static method


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static notes

Non-static methods can reference static methods, but not theother way around. Why?


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main

Why is main static?


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Reference/Source

MIT OpenCourseWareIntroduction to Programming in Java

January (IAP) 2010

3 classes and objects

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