How do the post increment (i++) and pre increment (++i) operators work in Java?

0 votes
asked Mar 3, 2010 by ankit-sachan

Can you explain to me the output of this Java code?

int a=5,i;

i=++a + ++a + a++;
i=a++ + ++a + ++a;
a=++a + ++a + a++;

System.out.println(a);
System.out.println(i);

The output is 20 in both cases

11 Answers

0 votes
answered Jan 3, 2010 by aurril

++a increments a before it is evaluated. a++ evaluates a and then increments it.

Related to your expression given:

i = ((++a) + (++a) + (a++)) == ((6) + (7) + (7)); // a is 8 at the end
i = ((a++) + (++a) + (++a)) == ((5) + (7) + (8)); // a is 8 at the end

The parenteses I used above are implicitly used by Java. If you look at the terms this way you can easily see, that they are both the same as they are commutative.

0 votes
answered Jan 3, 2010 by codaddict
i=++a + ++a + a++;

is

i = 6 + 7 + 7

Working: increment a to 6 (current value 6) + increment a to 7 (current value 7). Sum is 13 now add it to current value of a (=7) and then increment a to 8. Sum is 20 and value of a after the assignment completes is 8.

i=a++ + ++a + ++a;

is

i=5 + 7 + 8

Working: At the start value of a is 5. Use it in the addition and then increment it to 6 (current value 6). Increment a from current value 6 to 7 to get other operand of +. Sum is 12 and current value of a is 7. Next increment a from 7 to 8 (current value = 8) and add it to previous sum 12 to get 20.

0 votes
answered Mar 3, 2010 by kgiannakakis

Does this help?

a = 5;
i=++a + ++a + a++; =>
i=6 + 7 + 7; (a=8)

a = 5;
i=a++ + ++a + ++a; =>
i=5 + 7 + 8; (a=8)
0 votes
answered Mar 3, 2010 by lombo

++a increments and then uses the variable.
a++ uses and then increments the variable.

If you have

a = 1;

and you do

System.out.println(a++); //You will see 1

//Now a is 2

System.out.println(++a); //You will see 3

@codaddict explains your particular snippet.

0 votes
answered Jan 5, 2014 by vinod

In the above example

int a = 5,i;

i=++a + ++a + a++;        //Ans: i = 6 + 7 + 7 = 20 then a = 8 

i=a++ + ++a + ++a;        //Ans: i = 8 + 10 + 11 = 29 then a = 11

a=++a + ++a + a++;        //Ans: a = 12 + 13 + 13 = 38

System.out.println(a);    //Ans: a = 38

System.out.println(i);    //Ans: i = 29
0 votes
answered Jan 30, 2015 by vineet-sahu
a=5; i=++a + ++a + a++;

is

i = 7 + 6 + 7

Working: pre/post increment has "right to left" Associativity , and pre has precedence over post , so first of all pre increment will be solve as (++a + ++a) => 7 + 6 . then a=7 is provided to post increment => 7 + 6 + 7 =20 and a =8.

a=5; i=a++ + ++a + ++a;

is

i=7 + 7 + 6

Working: pre/post increment has "right to left" Associativity , and pre has precedence over post , so first of all pre increment will be solve as (++a + ++a) => 7 + 6.then a=7 is provided to post increment => 7 + 7 + 6 =20 and a =8.

0 votes
answered Jan 12, 2016 by java-main

pre-increment and post increment are equivalent if not in an expression

int j =0;
int r=0         
for(int v = 0; v<10; ++v) { 
          ++r;
          j++;
          System.out.println(j+" "+r);
  }  
 1 1  
 2 2  
 3 3       
 4 4
 5 5
 6 6
 7 7
 8 8
 9 9
10 10
0 votes
answered Jan 15, 2016 by rishabh-vashishtha

I believe however if you combine all of your statements and run it in Java 8.1 you will get a different answer, at least that's what my experience says.

The code will work like this:

int a=5,i;

i=++a + ++a + a++;            /*a = 5;
                                i=++a + ++a + a++; =>
                                i=6 + 7 + 7; (a=8); i=20;*/

i=a++ + ++a + ++a;           /*a = 5;
                                i=a++ + ++a + ++a; =>
                                i=8 + 10 + 11; (a=11); i=29;*/

a=++a + ++a + a++;            /*a=5;
                                a=++a + ++a + a++; =>
                                a=12 + 13 + 13;  a=38;*/

System.out.println(a);        //output: 38
System.out.println(i);         //output: 29
0 votes
answered Sep 15, 2017 by hi-buddy

Just refer these principles and that image and you would be pro in a matter of minutes.

Principle:-

1.) When the unary operators ++ and -- are used in prefix notation, the value of the variable increments/decrements just before the variable is used in an expression.

2.) When the unary operators ++ and -- are used in postfix notation, the value of the variable increments/decrements just after the variable is used in an expression.

Example:- The evaluation of the expression in image starts from left to right. Assume a to be 10.

Evaluation of Postfix and Prefix Expression

Your Question:-

int a = 5, i;

i = ++a + ++a + a++;
i = a++ + ++a + ++a;
a = ++a + ++a + a++;

System.out.println(a);
System.out.println(i);


Answer:-

i = 6 + 7 + 7; [a = 8]
i = 8 + 10 + 11;
a = 12 + 13 + 13; [a = 14]

O/P :- 38
       29
0 votes
answered Sep 15, 2017 by xinyi-liu

++a is prefix increment operator:

  • the result is calculated and stored first,
  • then the variable is used.

a++ is postfix increment operator:

  • the variable is used first,
  • then the result is calculated and stored.

Once you remember the rules, EZ for ya to calculate everything!

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