Util Package

Util Package

1. What It Is (beginner)

java.util = toolbox of everyday classes: Scanner (input), ArrayList (list), HashMap (dictionary), Random, Date, Collections. Import it to use 90% beginner programs.

2. Most-Used Classes (exam list)

Class	One-line use	Example
Scanner	keyboard input	new Scanner(System.in)
ArrayList	resizable array	list.add("hi")
HashMap	key→value	map.put("id", 101)
Random	random numbers	new Random().nextInt(100)
Arrays	array helpers	Arrays.sort(arr)
Collections	list helpers	Collections.sort(list)
Date / LocalDate	dates	LocalDate.now()

3. Code — Toolbox Demo

import java.util.*; // star imports whole toolbox (fine for beginners)
public class Tool {
    public static void main(String[] args) {
        ArrayList<String> names = new ArrayList<>();
        names.add("Asha"); names.add("Ravi");
        Collections.sort(names); // [Asha, Ravi]
        Random r = new Random();
        System.out.println(names + " lucky=" + r.nextInt(100));
        HashMap<String,Integer> marks = new HashMap<>();
        marks.put("Asha", 87);
        System.out.println(marks.get("Asha")); // 87
    }
}

4. Import Rules

• java.lang auto (no import). java.util needs import java.util.Scanner; (one) or import java.util.*; (all — beginners ok, pros prefer single).
• Wrong import → cannot find symbol ERROR.

5. Exam Points

• List 5 util classes + use — 5 marks, frequent.
• Import syntax (* vs single) — 2-3 marks, frequent.