Introduction to Java · Introduction to Java Programming

Important Classes

1. Which Classes Matter First

Beginners use 5 classes for 90% programs: String, System, Scanner, Math, ArrayList. All come free with JDK — just import where needed (java.lang auto-imported).

Class	Package	One-line use
`String`	`java.lang`	text: `"hello".length()`
`System`	`java.lang`	`System.out.println()`, `System.in`
`Scanner`	`java.util`	keyboard input
`Math`	`java.lang`	`Math.sqrt(16)`, `Math.max(a,b)`
`ArrayList`	`java.util`	resizable list

2. Code — All Five Together

import java.util.Scanner;
import java.util.ArrayList;

public class Demo {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in); // System.in = keyboard
        System.out.print("Enter name: ");
        String name = sc.nextLine();          // String class
        System.out.println("Hi " + name + ", len=" + name.length());
        System.out.println("sqrt(16)=" + Math.sqrt(16)); // Math class
        ArrayList<String> list = new ArrayList<>();      // resizable array
        list.add(name);
        System.out.println(list);
        sc.close();
    }
}

3. Beginner Traps

java.lang needs no import (String, System, Math). java.util needs import (Scanner, ArrayList).
System.out is print; System.in is input; System.err is error print.
Close Scanner on System.in once at end.

4. Exam Points

List 5 important classes + package + one method — 5 marks, frequent.
Which package needs import vs auto — 3 marks, frequent.

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