If statement with a variable and no expression

Question

I came across these if statements while watching a Java pong game tutorial video:

boolean upAccel, downAccel;
double y, velY;

public HumanPaddle() {
   upAccel = false; downAccel = false;
}

public void setUpAccel(boolean input) {
   upAccel = input;
}
public void setDownAccel(boolean input) {
   downAccel = input;
}

// moves the paddle
public void move() {

   /* What does the 'if(upAccel){ }' expression do..? */
   if(upAccel) {
      velY -= 1; 
   }
   if(downAccel) {
      velY += 1;
   }
   y = y + velY;
}

So I understand that the setUpAccel and setDownAccel methods accept a boolean input which can either be true or false. However, I experimented with the if statements - and changed if(upAccel) to if(upAccel = true). Java didn't see the expression as the same thing, so I realized that the two expressions were different!

My question is, what does the if(upAccel) expression test out?

`if (x == true)` is equivalent to `if (x)` when `x` is a boolean. — hnefatl, Apr 15 '18 at 15:07
If statements evaluate a boolean expression. Boolean variables can be used as boolean expressions. — Andy Turner, Apr 15 '18 at 15:07
`=` is assignment. `==` is comparison. So `if (upAccel = true)...` is the equivalent of `upAccel = true; if (upAccel)...` — T.J. Crowder, Apr 15 '18 at 15:13

score 1 · Accepted Answer · answered Apr 15 '18 at 15:14

1

/* What does the 'if(upAccel){ }' expression do..? */
   if(upAccel) {
      velY -= 1; 
   }

it will evaluate to true can be rewritten as

/* What does the 'if(upAccel){ }' expression do..? */
   if(upAccel==true) {
      velY -= 1; 
   }

answered Apr 15 '18 at 15:14

Roushan

4,074
3
21
38

score 0 · Answer 2 · answered Apr 15 '18 at 15:08

0

The statement (upAccel) checks if the condition is true then it continues to perform velY -=1. (upAccel) is the same as (upAccel == true). The same goes with downAccel.

answered Apr 15 '18 at 15:08

amk

70
7

My bad, I was using one equal sign - which is used to ASSIGN variables, not set them. Thanks for the clarification. – MattGotJava Apr 15 '18 at 15:12
You're welcome, good luck. – amk Apr 15 '18 at 15:14

If statement with a variable and no expression

2 Answers2