Java · Theory

PushbackInputStream

Theory

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PushbackInputStream

PushbackInputStream is modern Java (5/8 toolkit). Lambdas are short functions: (n) -> print. They implement a functional interface (one abstract method). Streams let you filter/map/collect a list without a manual loop.

For PushbackInputStream: generics keep types safe: List<String>. Autoboxing turns int into Integer when needed. varargs is String... args. Optional avoids some null checks — don’t overuse it.

Keep PushbackInputStream small. Don’t dump business logic inside forEach. Know one tiny example you can write from memory.

Let's take this on the board with one tiny Main class — no extra files. PushbackInputStream — output: [4, 16]. 1..5 → keep evens 2,4 → square → 4 and 16. Start from main, go line by line, and stop at each print. That output is the proof for PushbackInputStream.

Exam tip

Explain map/filter/collect as a 3-step stream story.

Example

import java.util.*;
import java.util.stream.*;
public class Main {
  public static void main(String[] args) {
    List<Integer> nums = Arrays.asList(1, 2, 3, 4, 5);
    List<Integer> evenSq = nums.stream()
      .filter(n -> n % 2 == 0)
      .map(n -> n * n)
      .collect(Collectors.toList());
    System.out.println(evenSq); // [4, 16]
  }
}

PushbackInputStream — output: [4, 16]. 1..5 → keep evens 2,4 → square → 4 and 16.

Short notes

DefPushbackInputStream is part of modern Java (generics / Java 8 style).
RulePushbackInputStream — lambda = (args) -> body on a functional interface.
UsePushbackInputStream — streams filter/map/collect. Generics = List<String>.
TrapPushbackInputStream — huge logic inside lambdas, or forgetting generics.

import java.util.*; import java.util.stream.*; public class Main { public static void main(String[] args) { List<Integer> nums = Arrays.asList(1, 2, 3, 4, 5); List<Integer> evenSq = nums.stream() .filter(n -> n % 2 == 0) .map(n -> n * n) .collect(Collectors.toList()); System.out.println(evenSq); // [4, 16] } }

PushbackInputStream

Example

Short notes

Questions

PushbackInputStream

Example

Short notes

Questions