LESSON 121 OF 671 · ARRAYS - CHALLENGES

Add If Missing in Java: contains Before add

You get a shopping list items, an ArrayList of strings, and a word item. Add item at the end only when it isn't in the list yet. Return the list.

When item is already there, the list stays as it is. The list may start empty.

Example 1:

Input: items = ["milk", "eggs"], item = "bread"
Output: ["milk", "eggs", "bread"]
Explanation: The word "bread" is not in the list yet.
It goes at the end.

Example 2:

Input: items = ["milk", "eggs"], item = "milk"
Output: ["milk", "eggs"]
Explanation: The word "milk" is already there.
Nothing is added.

Example 3:

Input: items = [], item = "apples"
Output: ["apples"]
Explanation: The list is empty, so "apples" is new.
It becomes the only item.

Constraints:

items has 0 to 9 words: it may be empty.
item and every word in items are made of lowercase English letters.

STEP 1 OF 3

Guided

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Add If Missing in Java, explained

Problem overview

The method addIfMissing receives a shopping list items (a list of strings) and a word item. It adds item at the end only when the list does not hold it yet, and returns the list. When the word is already there, the list stays as it was. The list may start empty.

For ["milk", "eggs"] and "bread" the result is ["milk", "eggs", "bread"]. For "milk" it stays ["milk", "eggs"]. For an empty list and "apples" it becomes ["apples"].

Approach

The rule has a condition ("only when it is missing"), so an if is needed. items.contains(item) is true when the word is in the list, and ! turns that around, so !items.contains(item) is true exactly when the word is missing. Inside the if, a plain add puts the word at the end. After the if, the list is returned in both cases.

An empty list needs no special case: contains on an empty list is false, so the word is added.

Step by step

Write the condition if (!items.contains(item)) and close its braces right away.
Inside it, call items.add(item). This runs only for a new word, so the list never gets the same word twice.
After the if, return items. If nothing was added, the list comes back as it was.

The solution in Java

class Solution {
    public List<String> addIfMissing(List<String> items, String item) {
        if (!items.contains(item)) {
            items.add(item);
        }
        return items;
    }
}

Details worth knowing:

contains compares with equals, so it checks the text, not the identity of the string object. That is the right behaviour for words.
The match is exact, so "Milk" and "milk" are different words. The lesson's words are all lowercase.
The method changes the list it receives and returns that same list.

Edge cases and common mistakes

Adding without the check. A bare items.add(item) puts "milk" in twice.
Forgetting the !. if (items.contains(item)) adds only words that are already there, which is the opposite of the task.
Returning inside the if only. If the return sits inside the braces, nothing is returned when the word was already there, and the method does not compile.
Empty list. It works with no special code.

Why not just add and clean up?

You could add the word every time and remove duplicates afterwards, but the check is simpler and keeps the list correct at every moment. A list that never holds the same word twice is the property the task is about, and the if guarantees it. For long lists, a Set is built for exactly this "have I seen it" question, but the lesson uses a list, and contains is the right tool for it.

Tracing the examples

Adding "bread" to ["milk", "eggs"]: contains is false, ! makes the condition true, so the word is added at the end.
Adding "milk" to the same list: contains is true, the condition is false, and the body is skipped.
Adding "apples" to an empty list: nothing matches, so the word goes in and is the only item.

Testing

Inputs and the output of the solution:

["milk", "eggs"], "bread" gives milk eggs bread.
["milk", "eggs"], "milk" gives milk eggs.
[], "apples" gives apples.

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