if-elseif-else statements stop doing comparisons as soon as it finds one that's true. if-if-if does every comparison. The first is more efficient.

Edit: It's been pointed out in comments that you do a return within each if block. In these cases, or in cases where control will leave the method (exceptions), there is no difference between doing multiple if statements and doing if-elseif-else statements.

However, it's best practice to use if-elseif-else anyhow. Suppose you change your code such that you don't do a return in every if block. Then, to remain efficient, you'd also have to change to an if-elseif-else idiom. Having it be if-elseif-else from the beginning saves you edits in the future, and is clearer to people reading your code (witness the misinterpretation I just gave you by doing a skim-over of your code!).

When you write multiple if statements, it's possible that more than one of them will be evaluated to true, since the statements are independent of each other.

When you write a single if else-if else-if ... else statement, only one condition can be evaluated to true (once the first condition that evaluates to true is found, the next else-if conditions are skipped).

You can make multiple if statements behave like a single if else-if .. else statement if each of the condition blocks breaks out of the block that contains the if statements (for example, by returning from the method or breaking from a loop).

For example :

Copypublic void foo (int x)
{
    if (x>7) {
        ...
        return;
    }
    if (x>5) {
        ...
        return;
    }    
}

Will have the same behavior as :

Copypublic void foo (int x)
{
    if (x>7) {
        ...
    }
    else if (x>5) {
        ...
    }
}

But without the return statements it will have different behavior when x>5 and x>7 are both true.

Just to go through this, a few observations:

// It might be simpler to use a "switch" statement here
if (which.equals("Potato Chips")) {
         System.out.println("You selected Potato Chips: $"+ChipsPrice+" "+Chips+" left");
         if (Amount < ChipsPrice){
             System.out.println("Not enough money inserted");
             // Remove the semicolon - as written this won't do anything
             // Also, this condition shouldn't be here since you're not vending anyway
             // Incidentally, many people argue that you should always use curly
             // brackets, even around one-line "if" statements like this, precisely
             // to prevent errors like this
             if (Chips == 0);
             System.out.println("Sorry, out of stock");

         }
         else {
             // This can be written as Chips--;
             Chips = Chips - 1;
             // Should actually be Amount - ChipsPrice;
             // If they paid 75 cents for a 25-cent item, the change is 75 - 25 = 50 cents,
             // NOT 25 - 75 = -50 cents
             Change = ChipsPrice - Amount;
             System.out.println("Please take your chips " );
             System.out.println("Your change is "+ Change );


         }
     }
     else if (which.equals("Cookies")) {
         System.out.println("You selected Cookies: $"+CookiesPrice+" "+Cookies+" left");

         // Cookies--
         Cookies = Cookies - 1;

         if (Amount < CookiesPrice){
             System.out.println("Not enough money inserted");

             // Should be checked in the "else" statement
             if (Cookies == 0)
                 System.out.println("Sorry, out of stock");
         }
         else {
             // Cookies--
             Cookies = Cookies - 1;
             // Amount - CookiesPrice
             Change = CookiesPrice - Amount;
             System.out.println("Please take your cookies");
             System.out.println("Your change is "+ Change );

         }

     }
     else if (which.equals("Candies")) {
         System.out.println("You selected Candies: $"+CandiesPrice+" "+Candies+" left");

         if (Amount < CandiesPrice){
             System.out.println("Not enough money inserted");
             // Again, you shouldn't check this here given that you won't vend either way
             // Also, should be if (Candies == 0), NOT if (Cookies == 0)
             if (Cookies == 0)
                 System.out.println("Sorry, out of stock");
         }
         else {
             // Candies--;
             Candies = Candies - 1;
             // Should actually be Amount - CandyPrice. You use CookiesPrice instead.
             Change = CookiesPrice - Amount;
             System.out.println("Please take your candies");
             System.out.println("Your change is "+ Change );

         }
     }
     else {
         System.out.println("Please select one of the snacks below");
     }

One more thing: you're basically doing the same exact thing 3 consecutive times; in situations like that, it's usually better to try to refactor the behavior in question as a method (rather than typing it 3 separate times).

Okay, so..

// As you already stated, you know that a boolean defaults to false.
boolean turnedOn;
if(turnedOn) // Here, you are saying "if turnedOn (is true, that's implicit)
{
//then do this
}
else // if it is NOT true (it is false)
{
//do this
}

Does it make more sense now?

The if statement will evaluate whatever code you put in it that returns a boolean value, and if the evaluation returns true, you enter the first block. Else (if the value is not true, it will be false, because a boolean can either be true or false) it will enter the - yep, you guessed it - the else {} block.

A more verbose example.

If I am asked "are you hungry?", the simple answer is yes (true). or no (false).

boolean isHungry = true; // I am always hungry dammit.
if(isHungry) { // Yes, I am hungry.
   // Well, you should go grab a bite to eat then!
} else { // No, not really.
   // Ah, good for you. More food for me!

   // As if this would ever happen - bad example, sorry. ;)
}

You have used = (assignment) instead of == (comparison).

If you want to convert something like:

if(A) {
    return X;
}
else if(B) {
    return Y;
}
else {
    return Z;
}

You can write this as:

A ? X : (B ? Y : Z);

You thus write the else if as a condition in the else-part (after :) of the upper expression.

However, I would strongly advice against too much cascading. The code becomes extremely unreadable and the ? : code structure was never designed for this.