java常用类

String构造方法举例

//用数组构造一个字符串public class StringTest {    public static void main(String[] args) {        char[] c = {'s', 'u', 'n', ' ', 'j', 'a', 'v', 'a'};        String s1 = new String(c);        //从第4个下标处开始截取字符串，截取4个字符        String s2 = new String(c, 4, 4);        System.out.println(s1);        System.out.println(s2);    }}

String类常用方法

//valueOf方法和split方法public class StringTest1 {    public static void main(String[] args) {        int j = 1234567;        String str = String.valueOf(j);        System.out.println("j是"+str.length()+"位数");        String s = "my,love";        String[] split = s.split(",");        for(int i = 0; i < split.length; i++) {            System.out.println(split[i]);        }    }}

String和StringBuffer的区别

//String类和StringBuffer类的区别public class StringTest2 {    public static void main(String[] args) {        //String是不可变的字符序列        String s1 = "hello";        String s2 = " world";        s1 += s2;        System.out.println(s1);        //StringBuffer是可变的字符序列        StringBuffer s3 = new StringBuffer();        s3.append("hello");        s3.append(" world");        System.out.println(s3);    }}

以上程序的内存情况：

如果是StringBuffer就可以直接在s1后面分配内存并将s2复制过来，实现字符串的连接

StringBuffer常用的方法

常用包装类

//包装类的应用public class ArrayPaser {    public static void main(String[] args) {         double[][] arr;         String s = "1, 2; 3, 4, 5; 6, 7, 8";         String[] sFirst = s.split(";");         arr = new double[sFirst.length][];         for(int i = 0; i < sFirst.length; i++) {             String[] sSecond = sFirst[i].split(",");             arr[i] = new double[sSecond.length];             for(int j = 0; j < sSecond.length; j++) {                 arr[i][j] = Double.parseDouble(sSecond[j]);             }         }         for(int i = 0; i < arr.length; i++) {             for(int j = 0; j < arr[i].length; j++) {                 System.out.print(arr[i][j] + " ");             }             System.out.println();         }    }}

File类

//File类public class FileTest {    public static void main(String[] args) {        //分隔符，windows和linux系统下通用        String separator = File.separator;        //文件名        String fileName = "myfile.txt";        //路径        String directory = "F:"+separator+"DB"+separator+"mydir";        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{            try {                //创建路径                f.getParentFile().mkdirs();                //创建文件                f.createNewFile();            } catch (IOException e) {                e.printStackTrace();            }        }    }}

//递归列出指定目录下的所有子目录和子文件public class FileList {    public static void main(String[] args) {        File f = new File("F:/DB/mydir");        System.out.println(f.getName());        tree(f, 1);    }    private static void tree(File f, int level) {        String preStr = "";        for(int i = 0; i < level; i++) {            preStr += "  ";        }        //拿到所有的孩子目录和文件        File[] childs = f.listFiles();        for(int i = 0; i < childs.length; i++) {            System.out.println(preStr + childs[i].getName());            //如果是目录，进行递归            if(childs[i].isDirectory()) {                //递归                tree(childs[i], level + 1);            }        }    }}

枚举类型

//枚举public class EnumTest {    public enum MyColor{        red, green, blue    }    public static void main(String[] args) {        MyColor m = MyColor.red;        switch(m) {        case red:            System.out.println("red"); //red            break;        case green:            System.out.println("green");            break;        default:            System.out.println("default");        }        System.out.println(m); //red    }}