The self-increment operator and the intermediate cache variable mechanism in Java

Source: Internet
Author: User

Transferred from: http://blog.csdn.net/maggiedorami/article/details/7986098

Let's look at a program like this:

  1. Public static void Main (string[] args) {
  2. int i, SUM1, sum2;
  3. i=0;
  4. Sum1 = (i++) + (i++);
  5. System.out.println ("sum1=" +sum1);
  6. i = 0;
  7. sum2 = (++i) + (++i);
  8. System.out.println ("sum2=" +sum2);
  9. }

The result of this operation is:

[Java]View Plaincopy
    1. sum1=1
    2. sum2=3


And I write the same logic code in C, and I get different results:

[CPP]View Plaincopy
    1. void Main ()
    2. {
    3. int i,sum1,sum2;
    4. i=0;
    5. Sum1= (i++) + (i++);
    6. printf ("sum1=%d\n", sum1);
    7. i=0;
    8. Sum2= (++i) + (++i);
    9. printf ("sum2=%d\n", sum2);
    10. GetChar ();
    11. }

The result of this operation is:

[CPP]View Plaincopy
    1. Sum1=0
    2. Sum2=4

This differs because in C, each variable can have only one unique value at each point in its life cycle. As a result, the contents of the corresponding memory area of the variable are rewritten at each increment operation.

In Java, sum1= (i++) + (i++) is executed, and 2 temporary integer variable objects are created to store the results of each self-increment operation.

Java uses the mechanism of this intermediate cache variable.

Then look at the programmer interview a very classic example:

[Java]View Plaincopy
    1. Public static void Main (string[] args) {
    2. Int J = 0;
    3. For (int i = 0; i < ; i++)
    4. J = j + +;
    5. System.out.println (j);
    6. }

For Java, the output value of j is 0.

Because the middle cache variable mechanism of Java causes the j=j++ statement to be decomposed into the following actions:

[Java]View Plaincopy
    1. temp = j;
    2. j = j + 1;
    3. j = temp;

Individuals feel that this use of self-growth is not good, should be in complex statements to avoid the use of post-increment (self-subtraction).

In addition, it is worth noting that some languages that use the mechanism of intermediate cache variables, the output is not necessarily 0. For example, when C + + has a post-increment operation on some basic types and pointer types, the compiler will omit intermediate cache variables.

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