java语言程序设计基础篇原书第八版课件PPT(第十七章).ppt

服务理念中的“点点” ◆理解多一点 真情浓一点 ◆学习勤一点 品质高一点 ◆理由少一点 效率高一点 ◆处理问题灵活点 工作过程用心点 ◆对待同事宽容点 互相协作快乐点 * Copying Variables of Primitive Data Types and Object Types * Garbage Collection As shown in the previous figure, after the assignment statement c1 = c2, c1 points to the same object referenced by c2. The object previously referenced by c1 is no longer referenced. This object is known as garbage. Garbage is automatically collected by JVM. * Garbage Collection, cont TIP: If you know that an object is no longer needed, you can explicitly assign null to a reference variable for the object. The JVM will automatically collect the space if the object is not referenced by any variable. * The Date Class Java provides a system-independent encapsulation of date and time in the java.util.Date class. You can use the Date class to create an instance for the current date and time and use its toString method to return the date and time as a string. * The Date Class Example For example, the following code ? java.util.Date date = new java.util.Date(); System.out.println(date.toString()); displays a string like Sun Mar 09 13:50:19 EST 2003. * The Random Class You have used Math.random() to obtain a random double value between 0.0 and 1.0 (excluding 1.0). A more useful random number generator is provided in the java.util.Random class. * The Random Class Example If two Random objects have the same seed, they will generate identical sequences of numbers. For example, the following code creates two Random objects with the same seed 3. Random random1 = new Random(3); System.out.print(From random1: ); for (int i = 0; i 10; i++) System.out.print(random1.nextInt(1000) + ); Random random2 = new Random(3); System.out.print(\nFrom random2: ); for (int i = 0; i 10; i++) System.out.print(random2.nextInt(1000) + ); From random1: 734 660 210 581 128 202 549 564 459 961 From random2: 734 660 210 581 128 202 549 564 459 961 * Displaying GUI Components When you develop programs