Java的基础知识3——常用类

1、String类（不可变的字符序列）

public static String valueOf(…)可以将基本类型数据转换为字符串。
public String[] split(…)可以将一个字符串按照指定的分隔符分隔，返回分隔后的字符数组。

public class Test{    public static void main(String args[]){        String s = "we:are:good:friends:!";        String array[] = s.split(":");        for(int i=0;i<array.length;i++){            System.out.print(array[i] + " ");        }        int i = 1234567;        String si = String.valueOf(i);        int len = si.length();        System.out.println("\n整数i是" + len + "位数。");    }}

查找子串出现的次数

public class Test{    public static void main(String args[]){        String s = "java adninjava timan javajavainiajava ()77java";        String find = "java";        int counter = 0;        int index = s.indexOf(find);        if(index == -1){            System.out.println("没有找到该字符串。");            System.exit(0);        }        while(index != -1){            counter++;            s = s.substring(index + find.length());            index = s.indexOf(find);        }        System.out.println(counter);    }}

2、StringBuffer类（可变的字符序列）

public class Test {    public static void main(String[] args) {        String s1 = "color";        StringBuffer sb1 = new StringBuffer(s1);        sb1.append(':').append("red,").append("green,").append("blue").append('.');        System.out.println(sb1);        String s2 = "number:";        StringBuffer sb2 = new StringBuffer(s2);        char c[] = {'g','o','o','d',' '};        for(int i=0;i<=9;i++){            sb2 = sb2.append(i);        }        System.out.println(sb2);        sb2 = sb2.delete(13, sb2.length());        System.out.println(sb2);        sb2 = sb2.insert(7, c);        System.out.println(sb2);    }}

3、基本类型的包装类

编写一个方法，返回一个double型二维数组，数组中的元素通过解析字符串参数获得，如：“1,2；3,4,5；6,7,8”对应的数组：
d[0,0] = 1.0 d[0,1] = 2.0
d[1,0] = 3.0 d[1,1] = 4.0 d[1,2] = 5.0
d[2,0] = 6.0 d[2,1] = 7.0 d[2,2] = 8.0

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

4、Math类

public class Test {    public static void main(String[] args) {        double a = Math.random();        double b = Math.random();        System.out.println(a);        System.out.println(b);        System.out.println(Math.pow(2.0, 3.0));        System.out.println(Math.exp(3.0));        System.out.println(Math.log(Math.E));    }}

5、File类

编写一个程序，在命令行中以树状结构展现特定的文件夹及其子文件

import java.io.File;public class Test {    public static void main(String[] args) {        File f = new File("E:/a");        System.out.println(f.getName());        listFiles(f,1);    }    private static void listFiles(File f,int level){        File files[] = f.listFiles();        String preString = "";        for(int j=0;j<level;j++){            preString  = preString + "    ";        }        for(int i=0;i<files.length;i++){            System.out.println(preString + files[i].getName());            if(files[i].isDirectory()){                listFiles(files[i],level+1);            }        }    }}