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COS 260 DAY 19

Tony Gauvin

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Agenda• Questions?• 8th Mini Quiz not corrected yet• 9Th Mini Quiz next class

– Due November 19

• Finish Discussion on More on Inheritance

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Final Countdown

• Nov 16– Finish Chap 9

• Nov 19– 9th Mini Quiz– Assignment 5 due – Begin Chap 10

• Nov 23 – Finish Chapter 10

• Nov 30 – 10th Mini quiz– Begin Chap 11

• Dec 3– Finish Chapter 11– Assignment 6 due

• Dec 7– 11th mini quiz– Begin Chapter 12

• Dec 10– Finish Chapter 12 – Assignment 7 Due

• Dec 14 @ 10 AM– 12th mini quiz– Capstone Presentations

Copyright © 2014 Pearson Education, Inc. Slide 1-3

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Change to Grade Calculations

• 12 instead of 11 mini quizzes– Still best 10 @ 3% each

• 7 instead of 8 assignments– AVG(7) = 40% of final grade– Was 8 @ 5% each

Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

More about inheritance

Exploring polymorphism

5.0

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Question

• What is the difference between static type and dynamic type in JAVA variable assignments?

• What is method polymorphism>

Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

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The instanceof operator

• Used to determine the dynamic type.

• Recovers ‘lost’ type information.• Usually precedes assignment with

a cast to the dynamic type:if(post instanceof MessagePost) { MessagePost msg = (MessagePost) post; … access MessagePost methods via msg …}

Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

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The Object class’s methods

• Methods in Object are inherited by all classes.

• Any of these may be overridden.• The toString method is

commonly overridden:– public String toString()– Returns a string representation of

the object.

Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

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Overriding toString in Post

Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

public String toString(){ String text = username + "\n" + timeString(timestamp); if(likes > 0) { text += " - " + likes + " people like this.\n"; } else { text += "\n"; } if(comments.isEmpty()) { return text + " No comments.\n"; } else { return text + " " + comments.size() + " comment(s). Click here to view.\n"; }}

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Overriding toString

• Explicit print methods can often be omitted from a class:System.out.println(post.toString());

• Calls to println with just an object automatically result in toString being called:System.out.println(post);

Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

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StringBuilder

• Consider using StringBuilder as an alternative to concatenation:

StringBuilder builder = new StringBuilder();builder.append(username);builder.append('\n');builder.append(timeString(timestamp));…return builder.toString();

Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

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Object equality

• What does it mean for two objects to be ‘the same’?– Reference equality.– Content equality.

• Compare the use of == with equals() between strings.

Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

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Overriding equals

Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

public boolean equals(Object obj){ if(this == obj) { return true; } if(!(obj instanceof ThisType)) { return false; } ThisType other = (ThisType) obj; … compare fields of this and other}

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Overriding equals in Student

Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

public boolean equals(Object obj){ if(this == obj) { return true; } if(!(obj instanceof Student)) { return false; } Student other = (Student) obj; return name.equals(other.name) && id.equals(other.id) && credits == other.credits;}

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Overriding hashCode in Student

Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

/** * Hashcode technique taken from * Effective Java by Joshua Bloch. */public int hashCode(){ int result = 17; result = 37 * result + name.hashCode(); result = 37 * result + id.hashCode(); result = 37 * result + credits; return result;}

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Protected access

• Private access in the superclass may be too restrictive for a subclass.

• The closer inheritance relationship is supported by protected access.

• Protected access is more restricted than public access.

• We still recommend keeping fields private.– Define protected accessors and mutators.

Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

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Access levels

Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

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Review

• The declared type of a variable is its static type.– Compilers check static types.

• The type of an object is its dynamic type.– Dynamic types are used at runtime.

• Methods may be overridden in a subclass.

• Method lookup starts with the dynamic type.

• Protected access supports inheritance.

Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling