Examples of knowledge points in java

java annotation document

Some commonly usedjavadoctags

##{@inheritDoc}Inherit relevant documents from the most direct base class of the current class into the current document comment@version version-information @author author-information#

Class related

Composition of classes:

Use an object of existing class A as The member variable ( of the new class B usually limits the object of the class A to private)

class A {}public class B {

    Private A a;

}

A,BThe relationship can be determined as ”has-a”(有 )Relationship, means B owns A

Inheritance from class

Existing base classA, ClassBInherited from classA, then class A also becomes the parent class of class B

class A {}class B extends A {}

The relationship between class A and class B can be determined as ”is-a”(是A ) relationship , means B is a A

Override(overwrite)and overload

Overwriting refers to the relationship between the parent class and the subclass. ,The subclass overrides a method of the parent class,and the method name,Return value,Parameter type,Number of parameters,Parameters,are in the same order.

Overloading is in a class,There are multiple methods with the same method name,Their return values,Parameter types,Number of parameters,Parameters,The order must be at least one different .

class A {public void helloWorld() {
    
        System.out.println(“hello world”);
    }
}class B extends A {//重写(覆盖)父类方法public void helloWorld() {
    
        System.out.println(“hello world”);
    
    } 
    public void hi() {
    
        System.out.println(“hi”);
    
    }    //重载上一个方法public int hi() {
    
        System.out.println(“hi”);        return 1;
    }
}

 

 

java.lang类默认会被导入java文件中

static修饰的方法或变量属于类,不属于类的对象.static对每个类来说只有一个存储空间,而每个对象都有一个存储空间.可通过staticMethod, 类名.staticMethod, 对象变量.staticMethod等来访问.

 

创建对象时注意

String s;    //创建引用(自动初始化为null),并不是创建对象new String(“hello world”);    //用new创建对象String s = new String(“hello world”);    //先用new创建对象,再创建引用s,并指向创建的对象(=具有右结合性)

 

 

操作符注意

==和!=用于比较基本类型的值,对象的引用是否相同

equals()比较对象的值是否相同, 但是不适用于基本类型

注意:equals默认行为是比较引用

 

class Value {int i;int j = 10;
}public class EqualMethod {public static void main(String[] args) {//==和!=比较基本类型的值System.out.println(47 == 47.00);    //trueInteger n1 = new Integer(47);
        Integer n2 = new Integer(47);        //==和!=比较对象的引用System.out.println(n1 == n2);        //false//equals()比较对象的值System.out.println(n1.equals(n2));    //true        
        
        Value v1 = new Value();
        Value v2 = new Value();
        v1.i = v2.i = 100;//equals()默认比较引用,这里v1,v2都为类Value的对象的引用System.out.println(v1.equals(v2));    //false    }

}

逻辑操作符(&&, ||, !) 产生一个布尔值,存在短路现象

按位操作符(&, |, ~, ^) 不存在短路现象

移位操作符:(<<, >>, >>>):左移<<时低位补0;有符号右移>>时,若符号为正则高位补0,为负则高位补1;无符号右移>>>时,无论正负高位补1(byte和short类型进行无符号右移时,先会被转换为int型, 再右移,然后截断赋值给原来类型)

 

直接常量

long: L, l(后缀)

float:F, f(后缀)

double: D, d(后缀)

十六进制: 0X, 0x(前缀)

八进制: 0(前缀)

 

long a = 1024L;float b = 40f;double c = 2048d;int d = 0x21;    //十进制33int e = 021;    //十进制17

 

Commonly used javadoc tags

##@see: other-class	Quoteother-class The generated html document will have a See Alse as a hyperlink The only entries are
##{@link package.class#member label}	Same as @see, but used for inline . generatedhtml The document will have a label as a hyperlink only to the entry
{@docRoot}	Generates a relative path to the document root directory , is used for displaying the document tree page Hyperlink
	Used to generate customized version information (version-information)
	Used to generate customized author information(author-information)
@since	Specify the earliest version of the program code used
@param parameter-name description	##parameter-name is the identifier of the method parameter list, description means it can be extended to multiple lines
##@return return-value description	return-value Describe the meaning of the return valuedescriptionIndicates that multiple lines can be continued
@throws fully-qualified-class-name description	fully-qualified -class-name gives an exception class name defined elsewhere ( cannot be ambiguous), description means it can Continue multiple lines
@Deprecated	Old features that users are advised not to use

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