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
    }
}