I/O — Scanner & System.out

Output — System.out

System.out is a PrintStream — every Java program has it pre-wired to stdout.

System.out.print("a");        // a
System.out.print("b");        // ab
System.out.println();          // newline
System.out.println("hello");   // hello\n

printf and format

C-style format strings, type-checked:

System.out.printf("name=%s age=%d pi=%.3f%n", "Asha", 19, 3.14159);
// name=Asha age=19 pi=3.142
String s = String.format("x=%-5d y=%-5d", 42, 7);   // returns instead of prints
String f = "Total: %.2f".formatted(99.95);          // Java 15+ — instance method

> Use %n (platform newline) instead of \n in format strings for cross-platform code.

Width, Alignment, Padding

System.out.printf("
%10s
%n",   "hi");      //
hi
   
(right-justify, width 10)
System.out.printf("
%-10s
%n",  "hi");      //
hi
    
(left-justify)
System.out.printf("
%010d
%n",  42);        //
0000000042
   
(zero-pad)
System.out.printf("
%+d
%n",    42);        //
+42
          
(always show sign)
System.out.printf("
%,d
%n",    1_000_000); //
1,000,000
    
(locale grouping)

Input — Scanner

Scanner is the simplest input class. Construct it once, read with type-specific methods:

import java.util.Scanner;
public class InputDemo {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        System.out.print("Name? ");
        String name = sc.nextLine();
        System.out.print("Age? ");
        int age = sc.nextInt();
        System.out.printf("Hi %s, age %d%n", name, age);
        sc.close();
    }
}

The nextInt() + nextLine() Trap

nextInt() consumes only the number — the trailing newline stays in the buffer. The next nextLine() will read EMPTY.

int n = sc.nextInt();
sc.nextLine();              // ← consume the leftover newline!
String line = sc.nextLine();  // now reads the actual next line

Common Scanner Methods

Reading Until EOF

Scanner sc = new Scanner(System.in);
int sum = 0;
while (sc.hasNextInt()) {
    sum += sc.nextInt();
}
System.out.println(sum);

> On Windows the EOF key is Ctrl+Z; on macOS/Linux it's Ctrl+D.

Reading From a File

import java.io.File;
Scanner sc = new Scanner(new File("input.txt"));
while (sc.hasNextLine()) {
    System.out.println(sc.nextLine());
}
sc.close();

The constructor throws FileNotFoundException — declare throws on main or wrap in try/catch.

Custom Delimiter

Scanner sc = new Scanner("a,b,c,d");
sc.useDelimiter(",");
while (sc.hasNext()) System.out.println(sc.next());

Faster Input — BufferedReader

For 10⁵+ tokens, Scanner is the bottleneck. BufferedReader is ~5–10× faster but you parse manually:

import java.io.*;
BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
int n = Integer.parseInt(br.readLine().trim());
String[] parts = br.readLine().split(" ");
int[] arr = new int[n];
for (int i = 0; i < n; i++) arr[i] = Integer.parseInt(parts[i]);
br.close();

For huge multi-token input, combine BufferedReader with StreamTokenizer or your own StringTokenizer:

import java.util.StringTokenizer;
BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
StringTokenizer st = new StringTokenizer(br.readLine());
int a = Integer.parseInt(st.nextToken());
int b = Integer.parseInt(st.nextToken());

Output Buffering

System.out.println flushes after every call — slow when you have 10⁶ writes. Wrap in a PrintWriter:

PrintWriter pw = new PrintWriter(new BufferedWriter(new OutputStreamWriter(System.out)));
for (int i = 0; i < 1_000_000; i++) pw.println(i);
pw.flush();

Try-With-Resources (Auto-Close)

Scanner and BufferedReader both implement AutoCloseable. Use try-with-resources to ensure they close even on exceptions:

try (Scanner sc = new Scanner(System.in)) {
    int n = sc.nextInt();
    System.out.println(n * n);
}   // sc.close() automatic

Modern Java I/O Quick Hits

// Java 11+ — Files API
String all = Files.readString(Path.of("input.txt"));
List<String> lines = Files.readAllLines(Path.of("input.txt"));
// Console — for password input (no echo)
Console c = System.console();
char[] pw = c.readPassword("Password: ");

Cheat-Sheet

You can now read anything stdin throws at you, and print neatly. Next: control flow.