要从标准输入读取数字，必须将其读取为字符串并将其解析为数字。java.util包中的Scanner类根据模式读取并解析基本类型和字符串中的文本。文本源可以是InputStream，文件，String对象或可读对象。
可以使用Scanner对象从标准输入System.in中读取原始类型值。以下代码说明了如何使用Scanner类构建一个小型计算器来执行加，减，乘和除。
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mport java.util.Scanner;public class Calculator { public static void main(String[] args) { System.out.println("type something like: 1+3"); Scanner scanner = new Scanner(System.in); double n1 = Double.NaN; double n2 = Double.NaN; String operation = null; try { n1 = scanner.nextDouble(); operation = scanner.next(); n2 = scanner.nextDouble(); double result = calculate(n1, n2, operation); System.out.printf("%s %s %s = %.2f%n", n1, operation, n2, result); } catch (Exception e) { System.out.println("An invalid expression."); } } public static double calculate(double op1, double op2, String operation) { switch (operation) { case "+": return op1 + op2; case "-": return op1 - op2; case "*": return op1 * op2; case "/": return op1 / op2; } return Double.NaN; }} |