Java API: Math, Wrapper Classes, StringBuffer Random Data Quiz2

Transcription

1 Java API: Math, Wrapper Classes, StringBuffer Random Data Quiz2 163

2 Recap Lessons learned: We can generate documentation from Javadoc We can call any method in a program from any other method in that program Method definitions only get executed when the method is called, regardless of where it appears in the file Understanding parameter passing & variable scoping is tricky sometimes! 164

3 Variable scoping & lifetimes Lifetime of a variable is the duration in which it exists in memory. This is from the declaration to the end of execution of its enclosing block. Scope of a variable is where it can be used. This is in statements from the declaration to the end of the block in which it was declared. Lifetime persists if control is temporarily transferred to another method, but scope is only within the block in which is was declared. Variables with the same name can be declared and used in separate blocks/methods, as long as they are not nested. 165

4 Common Java API Classes Libraries Math notice in the API that methods are "static" System Data Types notice the lack of "static" for methods String Scanner Character, Integer, other wrapper classes mix of object instance methods and class methods Random more I/O: PrintWriter, FileReader 166

5 Review Math class In java.lang (no need to import) Contains lots of useful methods We call methods directly with class name (no object, because "static") Most methods return double data values Examples: Math.pow(12, 3.5) => Math.sqrt(243) => Math.ceil(24.234) => 25.0 Math.floor(52.958) => 52.0 Math.round(24.345) => 24.0 Useful constants (declared public final in the class): Math.PI, Math.E 167

6 Wrapper Classes Character, Integer, Long, Float, Double (etc) These provide ways to create an object from a primitive type (wrapping the primitive data) Integer iobj = new Integer(7); They also provide useful methods, particularly for conversion to/from Strings Double dobj = new Double(23.40); String s = dobj.tostring(); // "23.40" int i = Integer.parseInt("23"); // 23 char ch = Character.toLowerCase('P'); // 'p' These are a mix of static (class-based) and instance (object-based) methods. 168

7 More Useful JAVA Classes StringBuffer Random Start looking at some of these in the Java API on-line documentation 169

8 StringBuffer class similar to String, but allows character changes, insertions, deletions not all String methods work directly on StringBuffer objects initialize with new StringBuffer(String) setcharat(index, value) to change a character at a particular index to a new value length() the actual number of characters append(*) - add any primitive, or String, to the end of the current value see also insert & delete methods tostring() to get the simple String out of the object 170

9 StringBuffer Examples StringBuffer sb = new StringBuffer("hello Student"); sb.setcharat(0,'h'); sb.setcharat(5,'-'); System.out.println(sb); // Hello-Student // NOT: sb.touppercase(); String s = sb.tostring().touppercase(); // new String // sb not changed remember to check the JAVA API to see what else you can do with this class see also hangman program from lab (week2) 171

10 Random class used to generate "random" data based on pseudorandom sequence in java.util, must import must create random object: Random rand = new Random(); uses current time as the seed = start of sequence instance methods these are all the options! nextint() - random integer in the range of (-,+) ints nextint(n) - random int in range [0,N) nextfloat() - 0 <= float < 1 nextboolean() - returns true or false nextdouble() - like nextfloat() returning double 173

11 Review: Static Methods A statement in any method of a class can call any static method in the same class just use the method name & expected parameters. A statement in one class can also call a static method from another class: Both classes need to be in the same package (file folder/directory) Use the class name and the method name, along with the parameters: ClassName.methodName(params) Example (next): RandomStuff.java and RandomStuffUsage.java 174

12 Random examples We need to massage data from the instance methods into customized forms with transformations, for example: char ch = (char) (rand.nextint(26) + 'A'); Download RandomStarter.java from schedule, save as RandomStuff.java Also download RandomStuffUsage.java to see how to call the methods in another class Upload your RandomStuff.java solution to c09 classwork on Blackboard 175

13 Quiz2 Please spread out! No phones, calculators, laptops, etc. We will pass out and collect. Do ALL work on the quiz paper. Write your JHED LOGIN where indicated. Start & stop on our mark. You have 10 minutes. 176