基础部分之System、Object、Calendar、StringBuffer

public static void main(String[] args) {
        Object o = new Object();
        System.out.println(o);//java.lang.Object@1540e19d
        System.out.println(o.toString());//java.lang.Object@1540e19d
    }

System：

public final static PrintStream out = null;

public void println(Object x) {
        String s = String.valueOf(x);
        synchronized (this) {
            print(s);
            newLine();
        }
    }

String：

public static String valueOf(Object obj) {
    return (obj == null) ? "null" : obj.toString();
}

public static void main(String[] args) {
        Object o1 = new Object();
        Object o2 = new Object();
        System.out.println(o1==o2);//false
        System.out.println(o1.equals(o2));//false
    }

public class TestOne {
    public static void main(String[] args) {
        Person p1 = new Person("liuyan", ‘女‘);
        Person p2 = new Person("yangmi", ‘女‘);
        Person p3 = new Person("成龙", ‘男‘);
        System.out.println(p1.equals(p2));//true
        System.out.println(p1==p2);//false
        System.out.println(p1.equals(p3));//false
    }
}
class Person{
    private String name;
    private char gender;
    public Person(String name,char gender){
        this.name = name;
        this.gender = gender;
    }

    //比较Person对象重写后的的性别
    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (!(o instanceof Person)) return false;
        Person person = (Person) o;
        if (gender != person.gender) {
            return false;
        }else {
            return true;
        }
    }

}
实际意义也就是如果有多个对象，地址值肯定不同，但只要属性值相同，用重写后的equals方法比较后就应该相等。

 public static void main(String[] args) {
        /**
         * Calendar是一个抽象类，不能创建实例
         * Calendar.getInstance()得到的是Calendar的子类对象
         * 这个方法可以根据本地信息（时区/语言）来获取相应版本的日历
         * 如果在日本，获取的就是JapaneseImperialCalendar 日本日历
         * 如果在中国，获取的就是GregorianCalendar ,格里高利历 俗称‘公历’
         */
        Calendar ca = Calendar.getInstance();//2019-08-16
        System.out.println("ca = " + ca);
        //给月份向后平移两个月
        ca.add(Calendar.MARCH,2);
        int year = ca.get(Calendar.YEAR);
        //国外时间是（0-11）所以需要加1
        int month = ca.get(Calendar.MARCH) + 1;
        int day = ca.get(Calendar.DAY_OF_MONTH);
        System.out.println(year+"-"+month+"-"+day);//2019-10-16
    }

Calendar ca2 = Calendar.getInstance();//2019-08-25
ca2.set(Calendar.MARCH, 4-1);
System.out.println(ca2.get(Calendar.MARCH)+1);
System.out.println(new SimpleDateFormat("yyyy-MM-dd").format(ca2.getTime()));
Calendar ca3 = Calendar.getInstance();//2019-08-25
ca3.set(Calendar.MARCH, 4);
System.out.println(ca3.get(Calendar.MARCH));
System.out.println(new SimpleDateFormat("yyyy-MM-dd").format(ca3.getTime()));

//结果

4
2019-04-25
4
2019-05-25

    private static void arrayCopy() {
        /**
         * 前提是目标数组要又对应的值，不能造成数组越界异常
         */
        Integer[] arr1 = {1, 2, 3, 4, 5};
        Integer[] arr2 = {6,7,8,9};
        System.arraycopy(arr1,2,arr2,0,3);
        System.out.println(Arrays.toString(arr2));
    }