Home > Articles

Working with Objects

  • Print
  • + Share This
This chapter is from the book

Casting and Converting Objects and Primitive Types

One thing you discover quickly about Java is how finicky it is about the information it will handle. Like Morris, the perpetually hard-to-please cat in the old 9Lives Cat Food commercials, Java methods and constructors require things to take a specific form and won't accept alternatives.

When you are sending arguments to methods or using variables in expressions, you must use variables of the right data types. If a method requires an int, the Java compiler responds with an error if you try to send a float value to the method. Likewise, if you're setting up one variable with the value of another, they must be of the same type.

NOTE

There is one area where Java's compiler is decidedly flexible: Strings. String handling in println() methods, assignment statements, and method arguments is simplified by the concatenation operator (+). If any variable in a group of concatenated variables is a string, Java treats the whole thing as a String. This makes the following possible:

float gpa = 2.25F;
System.out.println("Honest, dad, my GPA is a " + (gpa+1.5));

Using the concatenation operator, a single string can hold the text representation of multiple objects and primitive data in Java.

Sometimes you'll have a value in your Java class that isn't the right type for what you need. It might be the wrong class or the wrong data type, such as a float when you need an int.

You use casting to convert a value from one type to another.

Casting is the process of producing a new value that has a different type than its source.

Although the concept of casting is reasonably simple, the usage is complicated by the fact that Java has both primitive types (such as int, float, and boolean) and object types (String, Point, ZipFile, and the like). This section discusses three forms of casts and conversions:

  • Casting between primitive types, such as int to float or float to double

  • Casting from an instance of a class to an instance of another class, such as Object to String

  • Casting primitive types to objects and then extracting primitive values from those objects

When discussing casting, it can be easier to think in terms of sources and destinations. The source is the variable being cast into another type. The destination is the result.

Casting Primitive Types

Casting between primitive types enables you to convert the value of one type to another primitive type. It most commonly occurs with the numeric types, and there's one primitive type that can never be used in a cast. Boolean values must be either true or false and cannot be used in a casting operation.

In many casts between primitive types, the destination can hold larger values than the source, so the value is converted easily. An example would be casting a byte into an int. Because a byte holds values from –128 to 127 and an int holds from –2,100,000 to 2,100,000, there's more than enough room to cast a byte into an int.

You often can automatically use a byte or a char as an int; you can use an int as a long, an int as a float, or anything as a double. In most cases, because the larger type provides more precision than the smaller, no loss of information occurs as a result. The exception is casting integers to floating-point values; casting an int or a long to a float, or a long to a double, can cause some loss of precision.

NOTE

A character can be used as an int because each character has a corresponding numeric code that represents its position in the character set. If the variable i has the value 65, the cast (char)i produces the character value "A." The numeric code associated with a capital A is 65, according to the ASCII character set, and Java adopted this as part of its character support.

You must use an explicit cast to convert a value in a large type to a smaller type, or else converting that value might result in a loss of precision. Explicit casts take the following form:

(typename)value

In the preceding example, typename is the name of the data type to which you're converting, such as short, int, or float. value is an expression that results in the value of the source type. For example, in the following statement, the value of x is divided by the value of y, and the result is cast into an int in the following expression:

int result = (int)(x / y);

Note that because the precedence of casting is higher than that of arithmetic, you have to use parentheses here; otherwise, the value of x would be cast into an int first and then divided by y, which could easily produce a different result.

Casting Objects

Instances of classes also can be cast into instances of other classes, with one restriction: The source and destination classes must be related by inheritance; one class must be a subclass of the other.

Some objects might not need to be cast explicitly. In particular, because a subclass contains all the same information as its superclass, you can use an instance of a subclass anywhere a superclass is expected.

For example, consider a method that takes two arguments, one of type Object and another of type Component. You can pass an instance of any class for the Object argument because all Java classes are subclasses of Object. For the Component argument, you can pass in its subclasses, such as Button, Container, and Label.

This is true anywhere in a program, not just inside method calls. If you had a variable defined as class Component, you could assign objects of that class or any of its subclasses to that variable without casting.

This is true in the reverse, so you can use a superclass when a subclass is expected. There is a catch, however: Because subclasses contain more behavior than their superclasses, there's a loss in precision involved. Those superclass objects might not have all the behavior needed to act in place of a subclass object. For example, if you have an operation that calls methods in objects of the class Integer, using an object of class Number won't include many methods specified in Integer. Errors occur if you try to call methods that the destination object doesn't have.

To use superclass objects where subclass objects are expected, you must cast them explicitly. You won't lose any information in the cast, but you gain all the methods and variables that the subclass defines. To cast an object to another class, you use the same operation as for primitive types:

(classname)object

In this case, classname is the name of the destination class, and object is a reference to the source object. Note that casting creates a reference to the old object of the type classname; the old object continues to exist as it did before.

The following example casts an instance of the class VicePresident to an instance of the class Employee; VicePresident is a subclass of Employee with more information:

Employee emp = new Employee();
VicePresident veep = new VicePresident();
emp = veep; // no cast needed for upward use
veep = (VicePresident)emp; // must cast explicitly

As you'll see when you begin working with graphical user interfaces during Week 2, "The Java Class Library," casting one object is necessary whenever you use Java2D graphics operations. You must cast a Graphics object to a Graphics2D object before you can draw text or graphics onscreen. The following example uses a Graphics object called screen to create a new Graphics2D object called screen2D:

Graphics2D screen2D = (Graphics2D)screen;

Graphics2D is a subclass of Graphics, and both are in the java.awt package. You explore the subject fully during Day 13, "Using Color, Fonts, and Graphics."

In addition to casting objects to classes, you also can cast objects to interfaces, but only if an object's class or one of its superclasses actually implements the interface. Casting an object to an interface means that you can call one of that interface's methods even if that object's class does not actually implement that interface.

Converting Primitive Types to Objects and Vice Versa

One thing you can't do under any circumstance is cast from an object to a primitive data type, or vice versa.

Primitive types and objects are very different things in Java, and you can't automatically cast between the two.

As an alternative, the java.lang package includes classes that correspond to each primitive data type: Float, Boolean, Byte, and so on. Most of these classes have the same names as the data types, except that the class names begin with a capital letter (Short instead of short, Double instead of double, and the like). Also, two classes have names that differ from the corresponding data type: Character is used for char variables and Integer for int variables.

Using the classes that correspond to each primitive type, you can create an object that holds the same value. The following statement creates an instance of the Integer class with the integer value 7801:

Integer dataCount = new Integer(7801);

After you have an object created in this manner, you can use it as you would any object (although you cannot change its value). When you want to use that value again as a primitive value, there are methods for that, as well. For example, if you wanted to get an int value from a dataCount object, the following statement would be apt:

int newCount = dataCount.intValue(); // returns 7801

A common translation you need in programs is converting a String to a numeric type, such as an integer. When you need an int as the result, this can be done by using the parseInt() class method of the Integer class. The String to convert is the only argument sent to the method, as in the following example:

String pennsylvania = "65000";
int penn = Integer.parseInt(pennsylvania);

The following classes can be used to work with objects instead of primitive data types: Boolean, Byte, Character, Double, Float, Integer, Long, Short, and Void. These classes are commonly referred to as object wrappers because they provide an object representation that contains a primitive value.

CAUTION

If you try to use the preceding example in a program, your program won't compile. The parseInt() method is designed to fail with a NumberFormatException error if the argument to the method is not a valid numeric value. To deal with errors of this kind, you must use special error-handling statements, which are introduced during Day 7, "Threads, Exceptions, and Assertions."

Java 2 version 1.5 introduces autoboxing and unboxing, a new language feature that makes it much easier to work with primitive types and the objects that represent the same kind of value.

Autoboxing automatically converts a primitive type to an object, and unboxing converts in the other direction.

If you write a statement that uses an object where a primitive type is expected, or vice versa, the value will be converted so that the statement executes successfully.

This is a marked departure from all preceding versions of the language.

As a demonstration, the following statements can't be compiled by the rules of Java 2 version 1.4:

Float f1 = new Float(12.5F);
Float f2 = new Float(27.2F);
System.out.println("Lower number: " + Math.min(f1, f2)); 

When you attempt to compile a class containing these statements, the compiler stops with an error message stating that the Math.min() method requires two float primitive values as arguments, rather than Float objects.

If you enable Java's version 1.5 support, the statements compile successfully. The Float objects are unboxed into primitive values automatically when the Math.min() method is called.

CAUTION

Unboxing an object works only if the object has a value. If no constructor has been called to set up the object, compilation fails with a NullPointerException error.

By default, the SDK compiler does not support autoboxing, unboxing, or the other new features of Java 2 version 1.5. You can enable this support by running the compiler with the flag -source 1.5, as in the following command:

javac -source 1.5 Outliner.java 
  • + Share This
  • 🔖 Save To Your Account

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.

Overview


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information


To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.

Surveys

Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.

Newsletters

If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information


Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.

Security


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.

Children


This site is not directed to children under the age of 13.

Marketing


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information


If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.

Choice/Opt-out


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information


Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents


California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure


Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.

Links


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact


Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice


We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020