Some commonly used classes in java introduce Object Date Date Format Calendar System StringBuilder

Object class

java.lang.object

langThe package does not need to be guided, it can be used directly

Is the parent of all classes

java.util.Objects JDK7 adds an Object tool class

It provides some methods to manipulate objects. It consists of some static practical methods. These methods are null-save (null pointer safe) or null-tolerant (tolerant of null pointers), used to calculate objects hashcodeand return objects String representation, compare two objects.

When comparing two objects, the Object equalsmethod is prone to throw a null pointer exception.

null.equals null cannot call method

public boolean equals(Object obj) {
    
    
        return (this == obj);
    }

The equals method in the Objects class optimizes this problem.

public static boolean equals(Object a, Object b) {
    
      
    return (a == b) || (a != null && a.equals(b));  
}

toString()

default

Do not rewrite, return address value

toStringThe method returns the string representation of the object. In fact, the content of the string is the type of the object + @ + memory address value.

Rewrite

@Override
public String toString(){
    
    
    return "";
}

equals()

###default

Object ==address comparison of operators is performed by default in the Object class . As long as they are not the same object, the result must be false.

Rewrite

equals under the lang package

@Override
public boolean equals(object o){
    
    
    if(this == o)
        return ture;
    if(o==null||getclass()!=o.getclass())
        return false;
    Person person =(person) o;
    return age == person.age &&Object.equals(name,person.name);
}

Date class

java.util.DateThe class represents a specific moment, accurate to the millisecond.

Standard reference time 00:00:00 January 1, 1970 (UK)

public long getTime() Convert the date object into the corresponding time millisecond value.

DateFormat class

Time/date formatting subclassAbstract class

java.text.SimpleDateFormat

• public SimpleDateFormat(String pattern)

Identification letter (case sensitive)	meaning
Y	year
M	month
d	day
H	Time
m	Minute
s	second

The commonly used methods of the DateFormat class are:

• public String format(Date date): Format the Date object as a string.
• public Date parse(String source): Parse the string into a Date object.

Calendar class

java.util.CalendarIt is a calendar class, which appears after Date, replacing many Date methods. This class encapsulates all possible time information as static member variables for easy access. The calendar class is convenient for obtaining various time attributes.

method of obtaining

Calendar is an abstract class. Due to language sensitivity, the Calendar class is not created directly when creating objects, but is created through static methods.Return subclass object,as follows:

Calendar static method

• public static Calendar getInstance(): Get a calendar using the default time zone and locale

Common method

According to the API documentation of the Calendar class, the commonly used methods are:

• public int get(int field): Returns the value of a given calendar field.
• public void set(int field, int value): Set the given calendar field to the given value.
• public abstract void add(int field, int amount): According to the rules of the calendar, add or subtract a specified amount of time for a given calendar field.
• public Date getTime(): Returns a Date object representing the time value of this Calendar (the millisecond offset from the epoch to the present).

Many member constants are provided in the Calendar class, which represent a given calendar field:

Field value	meaning
YEAR	year
MONTH	Month (start from 0, can be used with +1)
DAY_OF_MONTH	Day of the month (what day)
HOUR	Hour (12-hour clock)
HOUR_OF_DAY	Hour (24 hour clock)
MINUTE	Minute
SECOND	second
DAY_OF_WEEK	Day of the week (day of the week, Sunday is 1, and -1 can be used)

Tips:

The Western Week starts on Sunday, and China on Monday.

In the Calendar class, the month is represented by 0-11 representing January to December.

The date is related to the size, the later the time, the greater the time.

System class

java.lang.System

A large number of static methods are provided in the class to obtain system-related information or system-level operations. In the API documentation of the System class, the commonly used methods are:

• public static long currentTimeMillis(): Returns the current time in milliseconds. Equivalent to Date's getTime()
• public static void arraycopy(Object src, int srcPos, Object dest, int destPos, int length): Copy the specified data in the array to another array.

##Array Copy (arraycopy)

The copy action of the array is system level,High performance. The System.arraycopy method has 5 parameters, the meanings are:

Parameter number	parameter name	Parameter Type	Parameter meaning
1	src	Object	Source array
2	srcPos	int	Source array index start position
3	dest	Object	Target array
4	destPos	int	The starting position of the target array index
5	length	int	Number of copied elements

StringBuilder class

Difference with String

Common method

There are 2 commonly used methods in StringBuilder:

• public StringBuilder append(...): Add the string form of any type of data,And return the current object itself。
• public String toString(): Convert the current StringBuilder object to String object.Has been rewrittenObject's toString();

public class Demo02StringBuilder {
    
    
	public static void main(String[] args) {
    
    
		//创建对象
		StringBuilder builder = new StringBuilder();
		//public StringBuilder append(任意类型)
		StringBuilder builder2 = builder.append("hello");
		//对比一下
		System.out.println("builder:"+builder);
		System.out.println("builder2:"+builder2);
		System.out.println(builder == builder2); //true
	    // 可以添加 任何类型
		builder.append("hello");
		builder.append("world");
		builder.append(true);
		builder.append(100);
		// 在我们开发中，会遇到调用一个方法后，返回一个对象的情况。然后使用返回的对象继续调用方法。
        // 这种时候，我们就可以把代码现在一起，如append方法一样，代码如下
		//链式编程
		builder.append("hello").append("world").append(true).append(100);
		System.out.println("builder:"+builder);
	}
}
回一个对象的情况。然后使用返回的对象继续调用方法。
        // 这种时候，我们就可以把代码现在一起，如append方法一样，代码如下
		//链式编程
		builder.append("hello").append("world").append(true).append(100);
		System.out.println("builder:"+builder);
	}
}