An error with a ternary operator

Question

In the following code:

public Map<Integer, Integer> leavesCount = new HashMap<Integer, Integer>();

public void addLeaf(int leaf, int count){
    leavesCount.containsKey(leaf) ? leavesCount.put(leaf, leavesCount.get(leaf) + count) : leavesCount.put(leaf, count);
}

I get the following error with leaf inside the containsKey :

Type mismatch: cannot convert from int to boolean

Does anyone know how to solve the issue?

Answer 1

重写为

leavesCount.put(leaf, leavesCount.containsKey(leaf) ? (leavesCount.get(leaf) + count) : count)

Answer 2

That's not how ternary operations work. To use a ternary for this you would want to change the function to

public void addLeaf(int leaf, int count){
    leavesCount.put( leaf, leavesCount.containsKey(leaf) ? leavesCount.get(leaf) + count : count)
}

Which isn't really best practice. You are better off using an if statement.

public void addLeaf(int leaf, int count){
    if(leavesCount.containsKey(leaf)){
        leavesCount.put(leaf, leavesCount.get(leaf) + count);
    }else{
        leavesCount.put(leaf, count);
    }
}

The reason for this is readability. Putting a ternary inside of a function calls can start to get messy.

You could also move it to a var.

public void addLeaf(int leaf, int count){
    count = leavesCount.containsKey(leaf) ? leavesCount.get(leaf) + count : count;
    leavesCount.put( leaf, count)
}

Answer 3

In Java 8, there's an elegant built-in method to do what you want:

public Map<Integer, Integer> leavesCount = new HashMap<>();

public void addLeaf(int leaf, int count) {
    leavesCount.merge(leaf, count, Integer::sum);
}

This uses Map.merge method, which expects the key and the value, along with a merge function that merges the old value with the new value if the key was already present in the map.

For the merge function, I'm using Integer::sum , which is a method reference to the Integer.sum method. This method reference acts like a BiFunction<Integer, Integer, Integer> , ie it expects two values and returns their sum.

Answer 4

You should replace leavesCount.containsKey(leaf) ? leavesCount.put(leaf, leavesCount.get(leaf) + count) : leavesCount.put(leaf, count); leavesCount.containsKey(leaf) ? leavesCount.put(leaf, leavesCount.get(leaf) + count) : leavesCount.put(leaf, count); with

    if (leavesCount.containsKey(leaf)) {
        leavesCount.put(leaf, leavesCount.get(leaf) + count);
    } else {
        leavesCount.put(leaf, count);
    }