Arrays & Strings in Java

8 cards, about a minute. Read them the night before an interview, or right after the chapter to lock it in.

1. ◆what it is

   int[] = fixed row of slots, pre-filled with 0/false/null. String = text you read but never change. StringBuilder = text you build.

   #1
2. ↯use it when

   Size known → int[]. Size unknown or a List is returned → ArrayList<Integer>. Text in a loop → StringBuilder. Only a–z → int[26].

   #2
3. { }template

   int[] count = new int[26]; for (char c : s.toCharArray()) count[c - 'a']++;

   #3
4. O( )cost

   s += c n times copies 1 + 2 + … + n chars: O(n²). sb.append(c) is O(1) amortised: O(n). substring, split, toCharArray copy: O(n).

   #4
5. !trap

   arr.length · s.length() · list.size(). And == on arrays or strings compares references — use Arrays.equals / equals.

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6. !trap

   Arrays.asList(int[]) is a List<int[]> of ONE element. split(".") splits on every char — escape it: split("\\.").

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7. ✦pro tip

   Print with Arrays.toString(a) (2D: deepToString). Plain println(a) shows junk like [I@1b6d3586.

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8. ✦pro tip

   'a' + 1 is the int 98; (char) ('a' + 1) is 'b'. Digit chars: c - '0'.

   #8

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