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Lesson 5 Working with Objects

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Objectives After completing this lesson, you should be able to: Declare, instantiate, and initialize object reference variables Compare how object reference variables are stored in relation to primitive variables Access fields on objects Call methods on objects Create a String object Manipulate data by using the String class and its methods Manipulate data by using the StringBuilder class and its methods Use the Java API documentation to explore the methods of a foundation class

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Topics Declaring, instantiating, and initializing objects Working with object references Using the String class Using the Java API documentation Using the StringBuilder class

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Working with Objects: Introduction Objects are accessed via references. Objects are instantiated versions of their class. Objects consist of attributes and operations: In Java, these are fields and methods.

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Accessing Objects by Using a Reference

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Shirt Class

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Topics Declaring, instantiating, and initializing objects Working with object references Using the String class Using the Java API documentation Using the StringBuilder class

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Working with Object Reference Variables Declaration: Classname identifier;

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Declaring and Initializing: Example

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Working with Object References

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Working with Object References

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Working with Object References

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References to Different Objects

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References to Different Object Types

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References and Objects In Memory

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Assigning a Reference to Another Reference

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Two References, One Object

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Assigning a Reference to Another Reference

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Quiz Which of the following lines of code instantiates a Boat object and assigns it to a sailBoat object reference? Boat sailBoat = new Boat(); Boat sailBoat; Boat = new Boat() Boat sailBoat = Boat();

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Topics Declaring, instantiating, and initializing objects Working with object references Using the String class Using the Java API documentation Using the StringBuilder class

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String Class The String class supports some non-standard syntax A String object can be instantiated without using the new keyword; this is preferred: String hisName = "Fred Smith"; The new keyword can be used, but it is not best practice: String herName = new String("Anne Smith"); A String object is immutable; its value cannot be changed. A String object can be used with the string concatenation operator symbol (+) for concatenation.

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Concatenating Strings When you use a string literal in Java code, it is instantiated and becomes a String reference Concatenate strings: String name1 = "Fred" theirNames = name1 + " and " + "Anne Smith"; The concatenation creates a new string, and the String reference theirNames now points to this new string.

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Concatenating Strings

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Concatenating Strings

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Concatenating Strings

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String Method Calls with Primitive Return Values A method call can return a single value of any type. An example of a method of primitive type int: String hello = "Hello World"; int stringLength = hello.length();

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String Method Calls with Object Return Values Method calls returning objects: String greet = " HOW ".trim(); String lc = greet + "DY".toLowerCase(); Or String lc = (greet + "DY").toLowerCase();

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Method Calls Requiring Arguments Method calls may require passing one or more arguments: Pass a primitive String theString = "Hello World"; String partString = theString.substring(6); Pass an object boolean endWorld = "Hello World".endsWith("World");

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Topics Declaring, instantiating, and initializing objects Working with object references Using the String class Using the Java API documentation Using the StringBuilder class

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Java API Documentation Consists of a set of webpages; Lists all the classes in the API Descriptions of what the class does List of constructors, methods, and fields for the class Highly hyperlinked to show the interconnections between classes and to facilitate lookup Available on the Oracle website at:

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Java Platform SE 7 Documentation

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Java Platform SE 7 Documentation

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Java Platform SE 7: Method Summary

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Java Platform SE 7: Method Detail

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System.out Methods To find details for System.out.println(), consider the following: System is a class (in java.lang). out is a field of System. out is a reference type that allows calling println() on the object type it references. To find the documentation: Go to System class and find the type of the out field. Go to the documentation for that field. Review the methods available.

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Documentation on System.out.println()

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Using the print() and println() Methods The println method: System.out.println(data_to_print); Example: System.out.print("Carpe diem "); System.out.println("Seize the day"); This method prints the following: Carpe diem Seize the day

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Topics Declaring, instantiating, and initializing objects Working with object references Using the String class Using the Java API documentation Using the StringBuilder class

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StringBuilder Class StringBuilder provides a mutable alternative to String. StringBuilder: Is a normal class. Use new to instantiate. Has an extensive set of methods for append, insert, delete Has many methods to return reference to current object. There is no instantiation cost. Can be created with an initial capacity best suited to need String is still needed because: It may be safer to use an immutable object A class in the API may require a string It has many more methods not available on StringBuilder

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StringBuilder Advantages over String for Concatenation (or Appending) String concatenation Costly in terms of creating new objects

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StringBuilder: Declare and Instantiate

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StringBuilder Append

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Quiz Which of the following statements are true? (Choose all that apply.) The dot (.) operator creates a new object instance. The String class provides you with the ability to store a sequence of characters. The Java API specification contains documentation for all of the classes in a Java technology product. String objects cannot be modified.

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Summary Objects are accessed via references: Objects are instantiated versions of their class. Objects consist of attributes and operations: In Java, these are fields and methods. To access the fields and methods of an object, get a reference variable to the object: The same object may have more than one reference. An existing object’s reference can be reassigned to a new reference variable. The new keyword instantiates a new object and returns a reference.

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Practice for Lesson 5 Overview: In this practice, you create instances of a class and manipulate these instances in several ways. During the practice, you: Create and initialize object instances Manipulate object references In this practice, you create, initialize, and manipulate StringBuilder objects In this practice, you examine the Java API specification to become familiar with the documentation and with looking up classes and methods. You are not expected to understand everything you see. But as you progress through this course, you will understand more and more of the Java API documentation.