References Aliases Garbage Collection and Packages Packages and

References, Aliases, Garbage Collection and Packages • Packages and Importing Classes • Reading for this Lecture: L&L, 3. 2 - 3. 3 • Familiarize yourself with Appendix M as a reference for the Java Class Library

References • A primitive variable contains the value itself, but an object variable contains an object reference • An object reference can be thought of as a pointer to the location of the object in memory • Rather than dealing with arbitrary address values, we often depict a reference graphically num 1 title 38 “Moby Dick"

Assignment Revisited • The act of assignment takes a copy of a value and stores it in a variable • For primitive types: Before: num 1 38 num 2 96 num 2 = num 1; After: num 1 38 num 2 38

Reference Assignment • For object references, assignment copies the address: “Moby Dick" Before: title 2 “Silas Marner" title 2 = title; After: title 2 “Moby Dick" Garbage: See later slide “Silas Marner"

Aliases • Two or more references that refer to the same object are called aliases of each other • That creates an interesting situation: one object can be accessed using more than one reference variable • Aliases can be useful, but should be managed carefully • Changing an object via one reference variable changes it for all of its aliases, because there is really one object

Garbage Collection • When there are no longer any variables containing a reference to an object (e. g. “Silas Marner” on the earlier slide), the program can no longer access it • The object is useless and is considered garbage • Periodically, Java performs automatic garbage collection and returns an object's memory to the system for future use • In other languages such as C/C++, the programmer must write explicit code to do the garbage collection

Garbage Collection • Setting object variable’s value equal to null, makes the object garbage (unavailable): Before: After: “Moby Dick" title = null; title null No object Garbage now “Moby Dick"

Garbage Collection • If an object variable’s value is equal to null, any reference to an attribute or method of that object will cause your program to fail. String title = new String(“Moby Dick”); System. out. println(title. length()); // prints 9 title = null; System. out. println(title. length()); // fails

The String Class • Because strings are so common, we don't have to use the new operator to create a String object title = “Moby Dick"; • This special syntax works only for strings • Each string literal (enclosed in double quotes) represents a String object

String Methods • Once a String object has been created, neither its value nor its length can be changed • Thus we say that an object of the String class is immutable • However, several methods of the String class return new String objects that are modified versions of the original • See the list of String methods on page 119 and in Appendix M

String Indexes • It is occasionally helpful to refer to a particular character within a string • This can be done by specifying the character's numeric index • The indexes begin at zero in each string • In the string “Moby Dick", the character ‘M' is at index 0 and the ‘D' is at index 5 • See String. Mutation. java (page 120)

Class Libraries • A class library is a collection of classes that we can use when developing programs • The Java standard class library is part of any Java development environment • Its classes are not part of the Java language per se, but we rely on them heavily • Various classes we've already used (System, Scanner, String) are part of the Java standard class library (Look them up in Appendix M!) • Other class libraries can be obtained through third party vendors, or you can create them yourself

Packages • The classes of the Java standard class library are organized into packages • Some packages in the standard class library are: Package Purpose java. lang java. applet java. awt javax. swing java. net java. util javax. xml. parsers General support Creating applets for the web Graphics and graphical user interfaces Additional graphics capabilities Network communication Utilities XML document processing

The import Declaration • When you want to use a class contained in a package, you can use its fully qualified name java. util. Scanner scan =. . . • Or you can import the package containing the class and just use the class name Scanner import java. util. Scanner; Scanner scan =. . . • To import all classes in a particular package, you can use the * wildcard character import java. util. *;

The import Declaration • All classes of the java. lang package are imported automatically into all programs • It's as if all programs contain the following line: import java. lang. *; • That's why we didn't have to import the System or String classes explicitly in earlier programs • The Scanner class, on the other hand, is part of the java. util package, so that class must be imported as part of its package