Java - Trang ď Chap02 tài liệu, giáo án, bài giảng , luận văn, luận án, đồ án, bài tập lớn về tất cả các lĩnh vực kinh t...
Programming Java Classes & Methods Incheon Paik Java Computer Industry lab Contents ? ? ? ? ? ? ? ? ? ? ? Java The Structure of Method Static Methods & Static Variables Instance Methods & Variables The Integer Class The New Operator Garbage Collection Other Wrapper Classes The StringBuffer Class Arrays of Objects Command-Line Arguments The System Class Computer Industry lab The Structure of Method Method retType mthName(paramList) { } void displayInts (int array[]) { } Exception Definition of Method retType mthName(paramList) throws exceptionList { } int parse(String s) throws Exc eptionX { } Method Invocation mthName(args); Java Computer Industry lab Static Methods & Variables Invocation of Static Method Clsname.mthName(args); double x = Math.sin(3.12); Access to Static Variable clsName.varName Class Aclass { Object x static int a; int b; … … Java Object y a same a b different b } x.a = 10; x.b = 20; Class Aclass x = new Aclass(); y.a = 30; y.b = 40; Class Aclass y = new Aclass(); Now, x.a = ? x.b = ? Computer Industry lab Static Methods & Variables class AreaCircumference { public static void main(String args[]) { System.out.println(“Max of –8 and –4 is “ + Math.max(-8, -4)); System.out.println(“Min of –8 and –4 is “ + Math.min(-8, -4)); double radius = 5; double area = Math.PI * radius * radius; double circumference = * Math.PI * radius; System.out.println(“Radius is “ + radius); System.out.println(“Area is “ + area); System.out.println(“Circumference is “ + circumference); } } Java Computer Industry lab Instance Methods & Variables Invocation of Instance Method double x = myObjA.somemth(); objRef.mthName.mthName(args); class DollarAmount { public static void main(String args[]) { String s1 = “The total cost is $45.67”; int i1 = s1.indexOf(‘$’); String s2 = s1.substring(i1); Find out this position Get substring after ‘$’ character System.out.println(s2); } // end of main } // end of class Java Computer Industry lab The Integer Wrapper Class Wrapper Class : Corresponding class to the basic type: Number, Integer, Float, Double,Boolean, Character, Byte, Short, Long class StringToInt { public static void main(String args[]) { String s = “125”; Integer obj = Integer.valueOf(s); int i = obj.intValue(); i += 10; System.out.println(i); } // end of main 135 } // end of class Java Computer Industry lab The New Operator new Operator clsName objRef = new clsName(args); MyClass x = new MyClass(); class IntegerTest { public static void main(String args[]) { Integer iobj1 = new Integer(5); int i1 = iobj1.intValue(); i1 = System.out.println(“i1 = “ + i1); System.out.println(iobj1); } // end of main } // end of class Java Computer Industry lab Garbage Collection ? Garbage Collector ? Automatic Memory Management ? finalize Method ? Call the finalize method before the garbage collector reclaim the memory Provide the method to release the resources ? Programmer can remove the resources(ex:open files) directly using finalize method which garbage collector cannot reclaim ? Java Computer Industry lab Garbage Collection protected protectedvoid voidfinalize() finalize()throws throwsThrowable Throwable{{ //// }} public class GcTest{ static { System.out.println("Initialize"); } protected void finalize() throws Throwable { System.out.println("Clean Up"); } How can we display Clean Up public static void main(String args[]) { message? => In Exercise System.out.println("In main "); GcTest x = new GcTest(); x = null; System.gc(); // We need to call this point to see the "Clean Up" } } Java 10 Computer Industry lab The Double Wrapper Class class DoubleDemo { public static void main(String args[]) { // variable initialize double d1 = 0; double d2 = 0; double d3 = d1 / d2; Result : System.out.println(d3); NaN System.out.println(Double.isInfinite(d3)); False System.out.println(Double.isNaN(d3)); True System.out.println(Double.MAX_VALUE); 1.797E308 System.out.println(Double.MIN_VALUE); 4.9E-324 System.out.println(Double.POSITIVE_INFINITY); System.out.println(Double.NEGATIVE_INFINITY); Infinity -Infinity } } Java 11 Computer Industry lab The StringBuffer Class StringBuffer Constructor StringBuffer() StringBuffer(int size) StringBuffer(String s) class StringBufferDemo { public static void main(String args[]) { StringBuffer sb1 = new StringBuffer(); StringBuffer sb2 = new StringBuffer(30); StringBuffer sb3 = new StringBuffer(“abcde”); System.out.println(“sb1.capacity = “ + sb1.capacity()); 16 System.out.println(“sb1.length = “ + sb1 length()); System.out.println(“sb2.capacity = “ + sb1.capacity()); 30 System.out.println(“sb2.length = “ + sb1 length()); System.out.println(“sb3.capacity = “ + sb1.capacity()); System.out.println(“sb3.length = “ + sb1 length()); 21 What will be append() and reverse() ? } // end of main } // end of class Java 12 Computer Industry lab Arrays of Objects Declaration of Array Make the Space for Array Element class StringArray { public static void main(String args[]) { String array[] = new String[5]; array[0] = “String 0”; array[1] = “String 1”; System.out.println(array.length); System.out.println(array[0]); System.out.println(array[1]); } // end of main } // end of class Java 13 Computer Industry lab Command-Line Argument Declaration of Array Make the Space for Array Element class CommandLineArguments { java CommandLineArguments abcde public static void main(String args[]) { System.out.println(args[0]); System.out.println(args[1]); System.out.println(args[2]); } // end of main } // end of class Java 14 Computer Industry lab The System Class Runtime Environment System.out.println( ****) Static variable of PrintStream Exit() Method void exit(int code) currentTimeMillis() Method long currentTimeMillis() Method Invocation Void arraycopy(Object source, int sourceIndex, Object destination, int destinationIndex, int size) Java 15 Computer Industry lab Exercise ? Step (Read Input) ? C Language scanf(“%s”, name); ? Java BufferedReader charStream = New BufferedReader(new InputStreamReader(System.in)); String data = charStream.readLine(); ? Step (Read Input Double) import java.io.*; ? You need to write this statement at top of your program double d = Double.valueOf(charStream.readLine().trim()).doubleValue(); ? Step Using System.in.read() and StringBuffer StringBuffer buf = new StringBuffer(); String while ((c = (char) System.in.read()) != '\r') buf.append(c); RandomAccessFile file = new RandomAccessFile(buf.toString(), "rw"); Java 16 Computer Industry lab Exercise ? Step (OptionPane of Swing) ? ? Step (Garbage Collection) ? Java Show Dialog and Get Input from GUI Refer to this slide #9,10 17 Computer Industry lab ... args[]) { System.out.println(“Max of –8 and –4 is “ + Math.max (-8 , -4 )); System.out.println(“Min of –8 and –4 is “ + Math.min (-8 , -4 )); double radius = 5; double area = Math.PI * radius * radius;... System.out.println(Double.MIN_VALUE); 4.9E-324 System.out.println(Double.POSITIVE_INFINITY); System.out.println(Double.NEGATIVE_INFINITY); Infinity -Infinity } } Java 11 Computer Industry lab The StringBuffer... end of main } // end of class Java 13 Computer Industry lab Command-Line Argument Declaration of Array Make the Space for Array Element class CommandLineArguments { java CommandLineArguments abcde