常用类
StringBuffer类：代表可变的字符序列，可以对字符序列任意增加、删改；String代表不可变的字符序列public class ArrayParser {public static void main(String[] args) {double[][] d;String s = "1,2;3,4,5;6,7,8";String[] sFirst = s.split(";");d = new double[];for (int i = 0; i < sFirst.length; i++) {String[] sSecond = sFirst.split(",");d = new double;for (int j = 0; j < sSecond.length; j++) {d = Double.parseDouble(sSecond);}}for (int i = 0; i < d.length; i++) {for (int j = 0; j < d.length; j++) {System.out.print(d + " ");}System.out.println();}}}
基本数据类型包装类：。。。。。
file类：代表系统文件名（路径和文件名）
import java.io.*;public class TestFile {public static void main(String[] args) {//String separator = File.separator;String filename = "myfile.txt";//String directory = "mydir1" + separator + "mydir2"; String directory = "mydir1/mydir2";// String directory = "mydir1\\mydir2";File f = new File(directory, filename);if (f.exists()) {System.out.println("文件名：" + f.getAbsolutePath());System.out.println("文件大小：" + f.length());} else {f.getParentFile().mkdirs();try {f.createNewFile();} catch (IOException e) {e.printStackTrace();}}}}
Enum枚举类型：只能够取特定值中的一个、使用enum关键字、是java.lang.Enum类型
public class TestEnum {public enum MyColor {red, green, blue};public enum MyDoorOpener {me, mywife};public static void main(String[] args) {MyColor m = MyColor.red;switch (m) {case red:System.out.println("red");break;case green:System.out.println("green");break;default:System.out.println("default");}System.out.println(m);}}
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