Java包装类、File类、枚举类举例

具体类方法查api文档

                                                 包装类对应表

基本数据类型

包装类

byte

Byte

boolean

Boolean

short

Short

char

Character

int

Integer

long

Long

float

Float

double

Double

/***包装类*/public  class Test{public static void main (String[] args){Integer i = new Integer(100);//分配在堆空间里面Double d = new Double("123.456");int j = i.intValue() + d.intValue();float f = i.floatValue() + d.floatValue();System.out.println(j);System.out.println(f);double pi = Double.parseDouble("3.1415926");double r = Double.valueOf("2.0").doubleValue();double s = pi*r*r;System.out.println(s);try{int k = Integer.parseInt("1.25");}catch (NumberFormatException e){System.out.println("数据格式不对");}System.out.println(Integer.toBinaryString(123)+"B");System.out.println(Integer.toHexString(123)+"H");System.out.println(Integer.toOctalString(123)+"O");}}

/***File类*/import java.io.File;import java.io.IOException;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";//也可以,用正/，windows和linux都可以File f = new File(directory,fileName);if(f.exists()){System.out.println("文件名"+f.getAbsolutePath());System.out.println("文件名大小"+f.length());}else{f.getParentFile().mkdirs();try{f.createNewFile();}catch(IOException e){e.printStackTrace();}}}}

/***枚举类型*/public class TestEnum{public enum MyColor{red,green,blue};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;case blue:System.out.println("blue");break;default:System.out.println("default");}System.out.println(m);}}