2 基础语法.ppt

1、注意命名规则 2、每行一条一句 3、适当的注释 4、适当的缩进和空行(indentation and spacing) 现在的IDE基本都具有代码风格化的功能了，所以照章执行即可，不要另外改动代码格式。 NOTE（浮点数运算不精确、但整数运算精确） Calculations involving floating-point numbers are approximated because these numbers are not stored with complete accuracy. For example, System.out.println(1.0 - 0.1 - 0.1 - 0.1 - 0.1 - 0.1); displays 0.5000000000000001, not 0.5, and System.out.println(1.0 - 0.9); displays 0.09999999999999998, not 0.1. Integers are stored precisely. Therefore, calculations with integers yield a precise integer result. Increment andDecrement Operators, cont. Numeric Type Conversion (数值类型转换) Consider the following statements: byte i = 100; long k = i * 3 + 4; double d = i * 3.1 + k / 2; Type Casting(类型转换)(注意casting这个词) 1) Implicit casting(隐式转换) double d = 3; (type widening) 2) Explicit casting(显式转换) int i = (int)3.0; (type narrowing) int i = (int)3.9; (Fraction part is truncated 截断) What is wrong? int x = 5 / 2.0; Casting between char and Numeric Types（字母与数值类型之间的转换） JDK1.5中for 循环的优化 将一个集合作为一个整体放入for循环中，在for循环中可将集合中的元素进行逐个处理。 String[ ] names ={”Wang”,”Zhang”,”Li”,”Wu”}; 　　for(String option: names) { 　　 System.out.println(option); } Note （下面2例写法不常见，但都是正确的，不推荐这样写代码） Caution（不要用浮点数比较大小来作为循环控制的条件） Don’t use floating-point values for equality checking in a loop control. Since floating-point values are approximations for some values, using them could result in imprecise counter values and inaccurate results. Consider the following code for computing 1 + 0.9 + 0.8 + ... + 0.1: double item = 1; double sum = 0; while (item != 0) { // No guarantee item will be 0 sum += item; item -= 0.1; } System.out.println(sum); Variable item starts with 1 and is reduced by 0.1 every time the loop body is executed. The loop should terminate when item becomes 0. However, there is no guarantee that item will be exactly 0, because the floating-point arithmetic is approximated. This loop seems OK on the surface, but it is actually an infi