Integer Class
1. Why Integer Is #1 Wrapper (beginner)
Integer wraps int (the default whole-number type!). Most-used wrapper: CLI args parsing, ArrayList<Integer>, string↔number exams. Master this one deeply!
int n = Integer.parseInt("42"); // String → int (THE method!)
Integer obj = Integer.valueOf(42); // int → Integer (cached -128..127!)
System.out.println(obj + " max=" + Integer.MAX_VALUE); // 2147483647
System.out.println(Integer.min(10, 20) + " " + Integer.max(10, 20)); // 10 20
System.out.println(Integer.sum(10, 20)); // 30
System.out.println(Integer.toBinaryString(5)); // 101
System.out.println(Integer.toHexString(255)); // ff
System.out.println(Integer.compare(5, 10)); // negative (5 < 10)
int x = obj.intValue(); // unbox
4. Traps
parseInt("abc")/parseInt(null)→NumberFormatException/ NPE.new Integer(5)deprecated (Java 9+) — usevalueOf/ autoboxing.- Unbox null → NPE:
Integer o = null; int i = o;CRASH!
5. Exam Points
- Parse + cache (
127vs128) + binary/hex — 7 marks, very frequent. - CLI sum program with
parseInt— 5 marks, frequent.