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Object Oriented Programming II 7.2. Standard Classes There are certain predefined classes that provide useful services. Among these are the String class, the Math class, the NumberFormat class and the DecimalFormat class. The String class Every character string is an object in Java, defined by the String class. Every literal string, delimited by double quotation marks, represents a String object. String concatenation (+) is used to append one string to the end of another. It can also be used to append a number to a string. The plus operator (+) is also used for arithmetic addition. The function that the + operator performs depends on the type of the information on which it operates. If both operands are strings, or if one is a string and one is a number, it performs string concatenation. If both operands are numeric, it adds them. The + operator is evaluated left to right. Parentheses can be used to force the operation order. The String class has several methods that are useful for manipulating strings. These methods return a value, such as an integer or a new String object. Since strings are objects, we cannot use the relational operators to compare strings. The equals method can be called on a string to determine if two strings contain exactly the same characters in the same order. The String class also contains a method called compareTo to determine if one string comes before another alphabetically (as determined by the Unicode character set). Class Libraries A class library is a collection of classes that we can use when developing programs. A Java standard class library comes with any Java Development Environment. These classes are not part of the Java language per se, but we rely on them heavily. The System class and the String class are part of the Java standard class library. Other class libraries can be obtained through third party vendors, or you can create them yourself. 1 Object Oriented Programming II The classes of the Java standard class library are organized into packages. The packages in the standard class library include: Package is used for java.lang General support java.applet Creating applets for the web java.awt Graphics and graphical user interfaces javax.swing Additional graphics capabilities and components java.net Network communication java.util Utilities In order to use a class from a package, you first need to import it using the import statement. For example, the statement import java.util.Random; gives you access to a random generator. To import all classes in a particular package, you can use the * wildcard character. For example, import java.util.*; All classes of the java.lang package are automatically imported into all programs. Oracle’s API specification for Java provides a description of all classes in java.lang, which includes the services provided by these classes. The System and String classes belong to java.lang, and so they do not need to be imported into classes that use them. The Random class is part of the java.util package. It provides methods that generate pseudo-random numbers. We often have to scale and shift a number into an appropriate range for a particular purpose. The Math Class The Math class contains many static methods that provide various mathematical functions, including, absolute value, trigonometry functions and square root. These methods can be invoked through the class name, instead of through an object of the class. 2 Object Oriented Programming II For example, double discriminant = Math.pow(b, 2) - (4 * a * c); You should look these methods up in the Oracle API. The NumberFormat Class The NumberFormat class has static methods that return a formatter object. For example, getCurrencyInstance() //returns a number formatted as currency getPercentInstance() // returns a number formatted as a percentage Each formatter object has a method called format that returns a string with the specified information in the appropriate format. The video demonstrates how the NumberFormat class is used. The DecimalFormat class The DecimalFormat class can be used to format a floating point value in generic ways. For example, you can specify that the number be printed to three decimal places. The constructor of the DecimalFormat class takes a string that represents a pattern for the formatted number. The video demonstrates how the DecimalFormat class is used. 3