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Java Java Data Structures - Retired Exploring the Java Collection Framework Maps

Gareth Gray
Gareth Gray
3,336 Points

Spotted something unusual...

I passed the challenge with the following code:

public Map<String, Integer> getCategoryCounts() {
    Map<String, Integer> categoryCount = new HashMap<String, Integer>();
    for (BlogPost post : getPosts()) {
         String category = post.getCategory();
         Integer count = categoryCount.getOrDefault(category, 0);
         categoryCount.put(category, count + 1);
    }
         return categoryCount;
  }

Yet whenever I tried count++ instead of count + 1, it failed.

Can anyone shed light on why that would be?

Should the code above have failed?

3 Answers

Florian Tönjes
seal-mask
.a{fill-rule:evenodd;}techdegree seal-36
Florian Tönjes
Full Stack JavaScript Techdegree Graduate 50,856 Points

Hello Gareth,

with 'count++' you are using the post increment operator. It increments count by one but count still evaluates to its original value in the expression. Use the pre increment operator '++count', which increments count and evaluates to its new value, instead.

Regards, Florian

Kourosh Raeen
Kourosh Raeen
23,733 Points

It has to do with the difference between count++ and ++count. The code will pass if you use the prefix version, ++count. This is what the documentation says:

The increment/decrement operators can be applied before (prefix) or after (postfix) the operand. The code result++; and ++result; will both end in result being incremented by one. The only difference is that the prefix version (++result) evaluates to the incremented value, whereas the postfix version (result++) evaluates to the original value. If you are just performing a simple increment/decrement, it doesn't really matter which version you choose. But if you use this operator in part of a larger expression, the one that you choose may make a significant difference.

The following program, PrePostDemo, illustrates the prefix/postfix unary increment operator:

class PrePostDemo {
    public static void main(String[] args){
        int i = 3;
        i++;
        // prints 4
        System.out.println(i);
        ++i;               
        // prints 5
        System.out.println(i);
        // prints 6
        System.out.println(++i);
        // prints 6
        System.out.println(i++);
        // prints 7
        System.out.println(i);
    }
}

I suggest using count++ only if it is statement on its own and if you need to increment or decrement an argument to a method/function or if this operation is part of a larger expression use count + 1 or count - 1.

Simon Coates
Simon Coates
28,694 Points
class Main {
  public static void main(String[] args) {
    int x = 0;
    System.out.println(x++);
    System.out.println(x);
    System.out.println(++x);
    System.out.println(x);
  }
}

prints 0 1 2 2. When you run increment on it's own line, you don't usually get to see it's behaviour, but used in an expression, it bites you. using ++ after the value means the increment happens after use.