I want an explanation about below java code-CodePudding

this is the code .

int x=99;
if(x  ==x){
System.out.println("x  ==x : " x);//line1
}
x=99;
if(  x==x){
System.out.println("  x==x : " x); //line2
}
x=99;
if(x==x  ){
System.out.println("x==x   : " x); //line3
}
x=99;
if(x==  x){
System.out.println("x==  x : " x); //line4
}
x=99;
if(  x==  x){
System.out.println("  x==  x : " x);  //line5
}
x=99;
if(x  ==x  ){
System.out.println("x  ==x   : " x); //line 6
}
x=99;
if(  x==x  ){
System.out.println("  x==x   : " x); // line7
}
x=99;
if(x  ==  x){
System.out.println("x  ==  x : " x); // line 8

when i compile and run this code i got below out put.

  x==x  : 100
x==x    :100
  x==x    :101

this output belongs to line 2,3 and 7.I want to know what happen to other lines, why they haven't output.

CodePudding user response：

This question is an excellent example depicting the differences between post and pre increment. In short, when you write before a variable (pre-increment), the variable is first increased by 1 and then, it is used in the operation. If you write after the variable then it is post increment and first, the variable will be used in the operation and then it will be incremented. You may refer this : How do the post increment (i ) and pre increment ( i) operators work in Java? for knowing about it in detail.

CodePudding user response：

You must know about Operator priority in Java. I translate your sentences based Java grammar to understand better. It is helpful to read about parsers in compilers and grammars which used to parse them.

/*------------------------------*/
x  ==x; 
//line1 equals to 
xl = x; x  ; xr = x; xl == xr;  //xl=99; xr=100
/*------------------------------*/
  x==x; 
//line2 equals to 
  x; xl = x; xr = x; xl == xr;  //xl=100; xr=100
/*------------------------------*/
x==x  ; 
//line3 equals to 
xl = x; xr = x;  x  ; xl == xr;  //xl=99; xr=99;
/*------------------------------*/
x==  x; 
//line4 equals to 
xl = x;   x; xr = x; xl == xr; //xl=99; xr=100;
/*------------------------------*/
  x==  x; 
//line5 equals to 
  x; xl = x;   x; xr = x; xl == xr; //xl=100; xr=101;
/*------------------------------*/
x  ==x  ; 
//line6 equals to 
xl = x; x  ; xr = x; x  ; xl == xr; //xl = 99; xr = 100;
/*------------------------------*/
  x==x  ; 
//line7 equals to 
  x; xl = x; xr = x; x  ; xl == xr; //xl = 100; xr = 100;
/*------------------------------*/
x  ==  x; 
//line7 equals to 
xl = x; x  ;   x; xl == xr; //xl = 99; xr = 101;
/*------------------------------*/

The following is an example grammar and you can see in equalityExpression which left expression and right expression is calculated before comparison.


primaryExpression
    :   Identifier
    |   Constant
    |   StringLiteral 
    |   '(' expression ')'
    ;


postfixExpression
    :
    (   primaryExpression )
    ('[' expression ']'        
    | ('  ' | '--')
    )*
    ;

unaryExpression
    :
    ('  ' |  '--' )*
    (postfixExpression
    |   unaryOperator castExpression        
    )
    ;
    
unaryOperator
    :   '&' | '*' | ' ' | '-' | '~' | '!'
    ;

castExpression
    :   unaryExpression    
    ;

multiplicativeExpression
    :   castExpression (('*'|'/'|'%') castExpression)*
    ;

additiveExpression
    :   multiplicativeExpression ((' '|'-') multiplicativeExpression)*
    ;

shiftExpression
    :   additiveExpression (('<<'|'>>') additiveExpression)*
    ;

relationalExpression
    :   shiftExpression (('<'|'>'|'<='|'>=') shiftExpression)*
    ;

equalityExpression
    :   relationalExpression (('=='| '!=') relationalExpression)*
    ;