Chapter 4: operators

Chapter four ： The operator

4.1、 Introduction to operators

Operators can be used to manipulate data values , Including arithmetic operators 、 Bit operators 、 Relational operators and equality operators .

4.2、 Integer operator

brief introduction ： It can be done to byte、short、int、long Operators that operate .

classification ： Unary operators 、 Binary operators

4.2.1、 Unary integer operator

brief introduction ： Only one operand can be operated （ Generally speaking, only one value can be operated ）

The operator	Practical effect	Case study
-	Change the sign of an integer , Take the opposite	-x amount to -1*x
~	Reverse bit by bit , It belongs to bit operation	~x
++	Add 1	x++
–	reduce 1	x–

Be careful ：++ and – Will change its value and - and ~ Does not change the value of the variable itself .

++ It can also be divided into pre ++ And post ++

x++  // Indicates that after a program is executed x Add the value of itself 1
++x  // First execute a program , And then x The value of the add 1

– It can also be divided into pre – And post –

x--  // Indicates that after a program is executed x Its own value minus 1
--x  // First execute a program , And then x The value of the reduction 1

4.2.2、 Binary integer operator

brief introduction ： Operators that can operate on two operands .

The operator	Practical effect	Case study
+	Add operation	a+b
-	Subtraction operation	a-b
*	Multiplication	a*b
/	In addition to the operation	a/b
%	Take the remainder operation	a%b
&	And operation	a&b
|	Or operations	a|b
^	Exclusive or operation	a^b
<<	Move arithmetic left	a<<b
>>	Count right	a>>b
>>>	Logical shift right	a>>>b

4.3、 Floating point operators

brief introduction ： Most integer operators are also applicable to floating-point operators , The usage is similar to that of integer operators . The type of the operation result is consistent with the type that represents the largest range in the operand .

Be careful ：

• Floating point numbers do not support bit operations
• Floating point data types have two special values -Infinity and Infinity, They can be used to represent the operation results of floating-point numbers , for example ： An integer divided by 0 It's infinite , Negative divided by 0 Get negative infinity .

4.4、 Comparison operators and logical operators

brief introduction ： The results are all boolean type , except ！ Other than binary operators

The operator	The actual meaning
<	Less than
>	Greater than
<=	Less than or equal to
>=	Greater than or equal to
==	be equal to
!=	It's not equal to
&&	Short circuit and
&	Non short circuit and
||	Short circuit or
|	No short circuit or
!	Not

The difference between short circuit and non short circuit ：

Non short circuiting always executes Boolean expressions on both sides of the operator .

Short circuit as long as the Boolean expression on one side of the operator is executed, the value of the expression can be determined, and the other side will not be executed .

4.5、 Special operators “?:”

brief introduction ：Java There is a special ternary operator in “?:”

grammar ： Boolean expression ？ expression 1： expression 2

explain ： If the Boolean expression holds , Execute expression 1, Execution expression does not hold 2

4.6、 String join operator “+”

** brief introduction ：** The operator + Can connect strings , And generate a new string .

matters needing attention ：

• One of the two operands is String Type of , The other is any type of use + The operator must return String Type of .
• If both operands are not String Type of , Then the ratio of two operands must be division boolean Types other than .
• For multiple + Operator expression ,Java Will be based on + Operators carry out left combination characteristics , Evaluate the expression from the left . Determine according to the type of operands + Properties of operators .
• except + Operators can connect outside the string , No other operator can connect strings .

4.7、 The operator “==” With the object equals() Method

4.7.1、 The operator “==”

brief introduction ： Compare whether the two operands are equal , The two operands can be basic data types , It can also be a reference data type .

When both sides of the operator are reference data types , These two variables must both reference the same object , The result is true.

Integer int1=new Integer(1);
Integer int2=new Integer(1);
Integer int3=int1; //int3 and int1 From the same object , So use int3==int1 The output is zero true, however int1 and int2 Not the same object ,int2==int1 The return value is false

• The operator “==” And polymorphism

  For variables of reference type ,Java The compiler will compile according to the type of variable being declared .“==” When comparing reference type variables , The types declared on both sides of the operator must be the same type or have inheritance relationship （ That is, they are all on the same branch of the inheritance tree ）.

• The operator “==” Used for array types

  boolean b1=new int[4]==new long[5];   // Compilation error , The reason is that the types on both sides are inconsistent 
  boolean b2=new int[4]==new int[4];    // legal   return false
  int[]array1=new int[4];
  int[]array2=array1;
  boolean b3=array1=array2; // return true
  

4.7.2、 Object's equals() Method

brief introduction ：equals() The method is in Object Methods defined in class , The format of the statement is as follows ：

public boolean equals(Object obj)

Compare the rules ： When parameters obj When the referenced object is the same as the current object, it returns true, Otherwise return to false

public boolean equals(Object obj){
    
    if(this==obj){
    
        return true;
    }else{
    
        return fasle;
    }
}

4.7.3、 Simple summary

• “==” And objects equals() Methods are used to compare whether two operands are equal .
• “==” The actual comparison is whether their memory addresses are equal .
• Object's equals() Methods actually compare whether their values are equal .

4.8、instanceof The operator

brief introduction ：instanceof The operator is used to determine whether the referenced object is an instance of a class . To the left of the operator is an object , On the right is often a class or interface .

grammar ：

obj instanceof  Class name 、 Interface 

An instance of a class contains content ： The class itself 、 Instances of all direct or indirect subclasses .

• instanceof And polymorphism

  For variables of reference type ,Java The compiler will compile according to the type of variable being declared .instanceof The types declared on both sides of the operator must be the same type or have inheritance relationship （ That is, they are all on the same branch of the inheritance tree ）.

• instanceof Used for array types

  boolean b1=new int[4] instanceof long[];  // Because the types on both sides are inconsistent , The return value is false
  boolean b2=new int[4] instanceof int[];   // return true
  

4.9、 Assignment and type conversion of variables

brief introduction ：“=” The use of operators in the development process is the most mundane , The meaning expressed is to assign the value on the right side of the operator to the variable on the left side of the operator .

The rules ：

• A direct number can be assigned directly to a variable of the same type .
• The assignment between variables of the same type can be carried out directly
• Assignment between different types of variables , Type conversion required

4.9.1、 Basic data type conversion

• Automatic type conversion

Conversion rules ：

(byte、int、short、long、char、float) option( For operation ) (double)——double

(byte、int、short、long、char) option( For operation ) (float)——float

(byte、int、short、char) option( For operation ) (long)——long

(byte、short、char) option( For operation ) (int)——int

(byte、short、char) option( For operation ) (byte、short、char)——int

• Cast

Assigning a high-order type to a low-order type must be cast , Otherwise, it will compile and report errors .

Forced type conversion will lead to the loss of data precision or data overflow .

4.9.2、 Type conversion of reference type

brief introduction ： When assigning values between variables of reference type , Subclasses are assigned to direct or indirect parent classes , Automatic type conversion . When a parent class assigns a value to a child class, it needs to be cast .