Java 常用类库

public class testString {
    public static void main(String [] args)
    {
        String s = "Michael";
        s += ", ok";
        System.out.println(s);
        StringBuffer x = new StringBuffer("Ming");
        x.append("ming").append(", good!");
        System.out.println(x.toString());
        // 频繁修改字符串，使用 StringBuffer 性能更好
    }
}

Michael, ok
Mingming, good!

String s1 = "10";
// Integer.parseInt 字符串转整数
int n = Integer.parseInt(s1,16);// x 进制, 不写默认10进制
System.out.println(++n); // 17

// System 类, 如打印系统信息
Properties sp = System.getProperties();
Enumeration e = sp.propertyNames();
while(e.hasMoreElements()) {
    String key = (String) e.nextElement();
    System.out.println(key + " = " + sp.getProperty(key));
}
System.exit(-1);

Runtime run = Runtime.getRuntime();
try{
    run.exec("notepad.exe");// 打开记事本，exec返回子进程Process类对象
}
catch (Exception ep){
    ep.printStackTrace();
}

// 时间类
Calendar c1 = Calendar.getInstance();
System.out.println(c1.get(c1.YEAR)+"年"+(c1.get(c1.MONTH)+1)+"月"+
        c1.get(c1.DAY_OF_MONTH)+"日"+c1.get(c1.HOUR_OF_DAY)+":"+
        c1.get(c1.MINUTE)+":"+c1.get(c1.SECOND));

c1.add(c1.DAY_OF_MONTH, 3); // 加3天
System.out.println(c1.get(c1.YEAR)+"年"+(c1.get(c1.MONTH)+1)+"月"+
        c1.get(c1.DAY_OF_MONTH)+"日"+c1.get(c1.HOUR_OF_DAY)+":"+
        c1.get(c1.MINUTE)+":"+c1.get(c1.SECOND));

2021年2月26日23:5:26
2021年3月1日23:5:26

SimpleDateFormat sp1 = new SimpleDateFormat("yyyy-MM-dd hh:mm:ss");
SimpleDateFormat sp2 = new SimpleDateFormat("yyyy年MM月dd日 hh点mm分ss秒");
try{
    Date d = sp1.parse("2021-2-26 23:02:03");
    System.out.println(sp2.format(d));
    // 2021年02月26日 11点02分03秒
}
catch (ParseException e2){
    e2.printStackTrace();
}

Random r = new Random();//以时间为种子的
for(int i = 0; i < 5; ++i)
    System.out.print(r.nextInt(100)+"\t");

class Employee implements Cloneable {
    private String name;
    private int age;
    public Employee(String name, int age)
    {
        this.name = name;
        this.age = age;
    }

	// 克隆方法 覆写
    public Object clone() throws CloneNotSupportedException{
        return super.clone();
    }
    public String toString(){
        return "name: "+this.name+" age: "+this.age;
    }
    public int getAge(){
        return age;
    }
    public void setAge(int age){
        this.age = age;
    }
    public String getName(){
        return name;
    }
    public void setName(String name){
        this.name = name;
    }
}

class test5{
    public static void main(String[] args){
        Employee e1 = new Employee("Michael", 18);
        Employee e2 = null;
        try{
            e2 = (Employee) e1.clone();
        }
        catch (CloneNotSupportedException e){
            e.printStackTrace();
        }
        e2.setName("Ming");
        e2.setAge(19);
        System.out.println("两个对象的地址比较："+(e1 == e2));
        System.out.println(e1);
        System.out.println(e2);
    }
}

两个对象的地址比较：false
name: Michael age: 18
name: Ming age: 19