CSE 1020:Using Objects

Mark Shtern

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Summary

• Read API• Method binding and overloading• Development process• Input validation• Assert

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Utility Class

• Features are static• No Instantiation • Memory Diagram (Fig 3.12 and Fig 3.14)• Advantages of Utility Class– Simplicity– Suitability

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Objects

• An Abstraction View• An API View

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The Life of an Object

• The Birth of an Object– Locate the Class– Declare a Reference– Instantiate the Class– Assign the Reference

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Object at Work

• Write a code fragment that creates a fraction object that corresponds to 8/6 and then outputs the results of invoking getNumerator and toProperString on it

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AnswerFraction f;f = new Fraction(8,6)output.println(f.getNumerator());output.println(f.toProperString())

Object at Work

• Write a code fragment that creates fraction object that corresponds to 8/6 and then outputs the results of invoking toString method on it, once with the default separator and once with a different separator.

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Answerfraction f = new Fraction (8,6);output.println(f.toString());f.Separator = ‘\u00f7’;output.println(f.toString());

The Object and its Reference

Fraction f1;f1 = new Fraction(6,3);Fraction f2;f2 = f1;f2 = new Fraction(5,8);f2 = null;output.println(f2.getNumerator()) ;

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Objects’ Equality

Fraction f1 = new Fraction(3,5);Fraction f2 = f1;Fraction f3 = new Fraction(2,7);Fraction f4 = new Fraction(6,10);Fraction f5 = f4;output.println(f1 == f2);output.println(f4 == f5);

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Equality of References

output.println(f1 == f2);output.println(f4 == f5);

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Equality of Objects

output.println(f1.equals(f4));output.println(f4.equals(f2));

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Obligatory Methods

• toString() returns a string that represents the current object

• equals() indicates whether some other object is "equal to" this one

• hashCode() returns an integer (hash code value) for the object. This method is supported for the benefit of containers

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The Death of an Object

Fraction x = new Fraction(3,5);Fraction y=x;y = new Fraction(4,7);x=null;

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Object’s state

• State– Determine the state of objects– Change the state of objects

• Accessors– public typeX getX();– public boolean isVisible();

• Mutators– public void setX(typeX value);

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Predict its output

Fraction f = new Fraction(5,3);f.setDenominator(25);output.println(f.getNumerator());

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Answer1

Attribute Privacy

• Neither an accessor nor a mutator: zero visibility

• An accessor and no mutator: read only access• A mutator and no accessor: write only access• An accessor and a mutator:read/write access

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Summary

• Object (attributes, methods, states and identity)

• Object Life Time• Reference vs Object• Attribute Privacy

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Example 4.5• Consider the following fragment Fraction f = new Fraction(1,2); Fraction g = f; // Place mystery statement here output.println(f.getNumerator()); output.println(f==g); output.println(e.equals(g));• Replace the comment with one statement such that output

is 3 true true

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Example 4.5• Consider the following fragment Fraction f = new Fraction(1,2); Fraction g = f; // Place mystery statement here output.println(f.getNumerator()); output.println(f==g); output.println(e.equals(g));• Replace the comment with one statement such that output

is 3 false false

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Example 4.5• Consider the following fragment Fraction f = new Fraction(1,2); Fraction g = f; // Place mystery statement here output.println(f.getNumerator()); output.println(f==g); output.println(e.equals(g));• Replace the comment with one statement such that output

is 3 false true

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Example 4.5• Consider the following fragment Fraction f = new Fraction(1,2); Fraction g = f; // Place mystery statement here output.println(f.getNumerator()); output.println(f==g); output.println(e.equals(g));• If possible, replace the comment with one statement such

that output is 3 true false

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Example 4.6

• Examine the following fragment, and predict its output for various user inputs:

Fraction f = new Fraction(7,10); output.print(“Enter a separator...”); boolean ok =

f.setSeparator(input.nextLine().charAt(0)); output.println(ok); output.println(f);

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Object with static Features (predict its output)

Fraction f = new Fraction(3,2);output.println(f.toProperString());f.isQuoted = false;output.println(f.toProperString());

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Answer “1 1/2”1 1/2

Object with static Features (predict its output)

Fraction f = new Fraction(3,2);f.isQuoted = true;Fraction g = new Fraction(5,2);g.isQuoted = true;output.println(f.toProperString());output.println(g.toProperString());

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Answer1 1/22 1/2

Object with final Feature

• Final fields are made static:– Saves memory– Access without having to create instance first

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Ex4.15

• Create an instance of the Random class, a member of the java.util package. Generate two random integers and output the them.

• Why was nextInt method overloaded?• Modify your program so that it also generates

two double and boolean values.

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Ex 4.20• Create an instance, soap, of type.lib.Item with the

following state:– Item Number: “001”– Item Name: “The ABC Chicken Soup”– Sale Price: $9.75 per unit

• Assume store order two batches of these soup items:– 100 units and has overall cost of $500– 50 units and has overall cost of $400

• Simulate these two orders in your program by using purchase method

• Output the following information for this item:– Number, Name, Stock (quantity on hand) and cost price per unit

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Ex 4.21

• Continue previous program by processing two more transactions:– Sells 20 units of soup at the posted sale price

using sell method– Sells 10 units for a total of $80

• You program must generate two output as following:– output.println(soup.getUnitPrice());– output.println(soup.getSales()/soup.getSoldQy());

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