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java.util.ArrayList
(Later we will see another way to accomplish this that requires less typing effort.)
The exception
The one exception to the rule is the use of classes in the java.lang package, (such as Boolean , Class , and Double ) . Your source code can refer to classes in the java.lang package without the requirement to provide a fully-qualified class name.
An ArrayList object
Now back to the use of the object previously instantiated from the class named ArrayList . This is the kind of object that is often referred to as a container.
(A container in this sense is an object that is used to store references to other objects.)
Many methods available
An object of the ArrayList class provides a variety of methods that can be used to store object references and to fetch the references that it contains.
The add method
One of those methods is the method named add .
The following code fragment instantiates three objects of the String class, and stores them in the ArrayList object instantiated earlier.
(Note that since the initial capacity of the ArrayList object was adequate to store only a single reference, the following code causes the object toautomatically increase its capacity to at least three.)
ref.add("ABC ");
ref.add("DEF ");ref.add("GHI"); The get() method
The references stored in an object of the ArrayList class can be fetched by calling the get method on a reference to the object passing a parameter of type int .
The code in the following fragment calls the get method to fetch the references stored in index locations 0, 1, and 2. These references are passed to the println method, where the contents of the String objects referred to by those references are concatenated and displayed on the computerscreen.
System.out.println(
(String)ref.get(0) +(String)ref.get(1) +
(String)ref.get(2)); The output
This results in the following being displayed:
ABC DEF GHI
Summary
The above discussion gave you a preview into the use of containers in general, and the ArrayList container in particular.
However, the primary purpose of this program was to help you to understand the use of packages in Java.
The ArrayList class was simply used as an example of a class file that is stored in a standard Java package.
D. Joe Joe true
Two String objects with identical contents
As in Question 1 , the program instantiates two String objects containing identical character strings, as shown in the following codefragment.
char[] anArray = {'J','o','e'};String Str1 = new String(anArray);
String Str2 = new String(anArray); Compare objects for equality
Also, as in Question 1 , this program compares the two objects for equality and displays the result as shown by the call to the equals method in the following fragment.
System.out.println(
Str1 + " " + Str2 + " " +Str1.equals(Str2)); Compare using overridden equals method
The == operator is not used to compare the two objects in this program. Instead, the objects are compared using an overridden version of the equals method. In this case, the equals method returns true, indicating that the objects are of the same type and contain thesame data values.
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