Java Notes

Java Cheat Sheet

Complete Java programming cheat sheet with syntax reference, common operations, built-in methods, and code snippets for quick lookup.

Data Types & Variables

java example

// Primitives
byte b = 127;                  // 1 byte (-128 to 127)
short s = 32000;               // 2 bytes
int i = 2_147_483_647;         // 4 bytes (use _ for readability)
long l = 9_223_372_036_854L;   // 8 bytes (L suffix)
float f = 3.14f;               // 4 bytes (f suffix)
double d = 3.14159265;         // 8 bytes
char c = 'A';                  // 2 bytes (Unicode)
boolean flag = true;           // 1 bit

// Type casting
int x = (int) 3.14;           // Narrowing (explicit)
double y = 5;                  // Widening (implicit)

// Constants
final double PI = 3.14159;

Array Operations

Example

// Declaration & initialization int[] arr = new int[5]; // [0, 0, 0, 0, 0] int[] arr2 = {1, 2, 3, 4, 5}; int[][] matrix = {{1,2,3}, {4,5,6}}; // Operations Arrays.sort(arr); // Sort ascending Arrays.sort(arr, Collections.reverseOrder()); // Descending (Integer[]) Arrays.fill(arr, 0); // Fill with value Arrays.copyOf(arr, newLength); // Copy Arrays.copyOfRange(arr, from, to); Arrays.equals(arr1, arr2); // Compare Arrays.binarySearch(sortedArr, key); // Search (must be sorted) Arrays.stream(arr).sum(); // Sum Arrays.stream(arr).max().getAsInt(); // Max Arrays.stream(arr).min().getAsInt(); // Min Arrays.stream(arr).average().getAsDouble(); // Average Arrays.toString(arr); // "[1, 2, 3, 4, 5]"

Collections Operations

java example

// List
List<String> list = new ArrayList<>(Arrays.asList("a", "b", "c"));
list.add("d");                 // Add to end
list.add(0, "z");              // Add at index
list.get(0);                   // Get by index
list.set(0, "x");              // Replace
list.remove(0);                // Remove by index
list.remove("a");              // Remove by value
list.contains("b");            // Check
list.indexOf("b");             // Find index
list.size();                   // Count
list.isEmpty();                // Check empty
list.clear();                  // Remove all
Collections.sort(list);        // Sort
Collections.reverse(list);     // Reverse
Collections.shuffle(list);     // Randomize
Collections.frequency(list, "a"); // Count occurrences

// Map
Map<String, Integer> map = new HashMap<>();
map.put("a", 1);
map.get("a");                  // 1
map.getOrDefault("z", 0);     // 0 (if not found)
map.containsKey("a");          // true
map.containsValue(1);          // true
map.remove("a");
map.size();
map.keySet();                  // Set of keys
map.values();                  // Collection of values
map.entrySet();                // Set of entries
map.putIfAbsent("b", 2);      // Add only if key absent
map.forEach((k, v) -> System.out.println(k + "=" + v));

// Set
Set<Integer> set = new HashSet<>(Arrays.asList(1, 2, 3, 4));
set.add(5);
set.remove(1);
set.contains(3);              // true

// Stream operations
list.stream()
    .filter(s -> s.length() > 3)
    .map(String::toUpperCase)
    .sorted()
    .distinct()
    .limit(5)
    .collect(Collectors.toList());

Control Flow

Example

// If-else if (x > 0) { } else if (x == 0) { } else { } // Ternary String result = (age >= 18) ? "Adult" : "Minor"; // Switch (Java 14+ enhanced) String day = switch (dayNum) { case 1 -> "Monday"; case 2 -> "Tuesday"; default -> "Unknown"; }; // Loops for (int i = 0; i < n; i++) { } for (String item : list) { } // Enhanced for while (condition) { } do { } while (condition); // Loop control break; // Exit loop continue; // Skip to next iteration return; // Exit method

Common Conversions

Example

// String ↔ Number int n = Integer.parseInt("123"); double d = Double.parseDouble("3.14"); String s = String.valueOf(123); String s2 = Integer.toString(123); // String ↔ Char Array char[] chars = str.toCharArray(); String str = new String(chars); // Array ↔ List List<Integer> list = Arrays.asList(1, 2, 3); // Fixed-size List<Integer> list = new ArrayList<>(Arrays.asList(1, 2, 3)); // Mutable Integer[] arr = list.toArray(new Integer[0]); // Collections ↔ Stream List<String> list = stream.collect(Collectors.toList()); Set<String> set = stream.collect(Collectors.toSet());

Exception Handling

java example

try {
    int result = 10 / 0;
} catch (ArithmeticException e) {
    System.out.println("Error: " + e.getMessage());
} catch (Exception e) {
    e.printStackTrace();
} finally {
    System.out.println("Always runs");
}

// Try-with-resources (auto-close)
try (BufferedReader br = new BufferedReader(new FileReader("file.txt"))) {
    String line = br.readLine();
}

// Custom exception
public class MyException extends RuntimeException {
    public MyException(String message) { super(message); }
}

File I/O

java example

// Read file
String content = Files.readString(Path.of("file.txt"));
List<String> lines = Files.readAllLines(Path.of("file.txt"));

// Write file
Files.writeString(Path.of("output.txt"), "Hello");
Files.write(Path.of("output.txt"), lines);

// BufferedReader
try (BufferedReader br = Files.newBufferedReader(Path.of("file.txt"))) {
    br.lines().forEach(System.out::println);
}

Lambda & Streams

java example

// Lambda syntax
(parameters) -> expression
(parameters) -> { statements; }

// Functional interfaces
Predicate<Integer> isEven = n -> n % 2 == 0;
Function<String, Integer> toLength = String::length;
Consumer<String> printer = System.out::println;
Supplier<Double> random = Math::random;
Comparator<String> byLength = Comparator.comparingInt(String::length);

// Stream pipeline
List<String> result = names.stream()
    .filter(n -> n.startsWith("A"))    // Filter
    .map(String::toUpperCase)           // Transform
    .sorted()                           // Sort
    .limit(5)                           // Take first 5
    .collect(Collectors.toList());      // Collect

// Collectors
Collectors.toList()
Collectors.toSet()
Collectors.joining(", ")
Collectors.groupingBy(Student::getDepartment)
Collectors.counting()
Collectors.summarizingInt(Student::getMarks)

Continue the lesson path

Study next in this module

4 connected lessons

01NotesQuick Revision Notes — Java ProgrammingConcise Java quick revision notes covering all major topics - OOP, Collections, Multithreading, JDBC, Spring Boot in table format for fast review before exams or interviews.02NotesImportant Java ProgramsCollection of most important Java programs frequently asked in exams and interviews - patterns, number programs, string programs, and array programs.03NotesImportant Formulas & ConceptsEssential Java formulas, time complexity reference, bit manipulation tricks, and mathematical concepts used in programming and DSA.04Collections FrameworkJava Collections Framework OverviewUnderstand the Java Collections Framework from scratch — what it solves, the four main types (List, Set, Map, Queue), how each works, when to use which, with full explanations and examples.

PreviousCoding Interview Questions NextQuick Revision Notes — Java Programming

Java Notes

Java Cheat Sheet

Complete Java programming cheat sheet with syntax reference, common operations, built-in methods, and code snippets for quick lookup.

Data Types & Variables

java example

// Primitives
byte b = 127;                  // 1 byte (-128 to 127)
short s = 32000;               // 2 bytes
int i = 2_147_483_647;         // 4 bytes (use _ for readability)
long l = 9_223_372_036_854L;   // 8 bytes (L suffix)
float f = 3.14f;               // 4 bytes (f suffix)
double d = 3.14159265;         // 8 bytes
char c = 'A';                  // 2 bytes (Unicode)
boolean flag = true;           // 1 bit

// Type casting
int x = (int) 3.14;           // Narrowing (explicit)
double y = 5;                  // Widening (implicit)

// Constants
final double PI = 3.14159;

Array Operations

Example

Collections Operations

java example

// List
List<String> list = new ArrayList<>(Arrays.asList("a", "b", "c"));
list.add("d");                 // Add to end
list.add(0, "z");              // Add at index
list.get(0);                   // Get by index
list.set(0, "x");              // Replace
list.remove(0);                // Remove by index
list.remove("a");              // Remove by value
list.contains("b");            // Check
list.indexOf("b");             // Find index
list.size();                   // Count
list.isEmpty();                // Check empty
list.clear();                  // Remove all
Collections.sort(list);        // Sort
Collections.reverse(list);     // Reverse
Collections.shuffle(list);     // Randomize
Collections.frequency(list, "a"); // Count occurrences

// Map
Map<String, Integer> map = new HashMap<>();
map.put("a", 1);
map.get("a");                  // 1
map.getOrDefault("z", 0);     // 0 (if not found)
map.containsKey("a");          // true
map.containsValue(1);          // true
map.remove("a");
map.size();
map.keySet();                  // Set of keys
map.values();                  // Collection of values
map.entrySet();                // Set of entries
map.putIfAbsent("b", 2);      // Add only if key absent
map.forEach((k, v) -> System.out.println(k + "=" + v));

// Set
Set<Integer> set = new HashSet<>(Arrays.asList(1, 2, 3, 4));
set.add(5);
set.remove(1);
set.contains(3);              // true

// Stream operations
list.stream()
    .filter(s -> s.length() > 3)
    .map(String::toUpperCase)
    .sorted()
    .distinct()
    .limit(5)
    .collect(Collectors.toList());

Control Flow

Example

Common Conversions

Example

Exception Handling

java example

try {
    int result = 10 / 0;
} catch (ArithmeticException e) {
    System.out.println("Error: " + e.getMessage());
} catch (Exception e) {
    e.printStackTrace();
} finally {
    System.out.println("Always runs");
}

// Try-with-resources (auto-close)
try (BufferedReader br = new BufferedReader(new FileReader("file.txt"))) {
    String line = br.readLine();
}

// Custom exception
public class MyException extends RuntimeException {
    public MyException(String message) { super(message); }
}

File I/O

java example

// Read file
String content = Files.readString(Path.of("file.txt"));
List<String> lines = Files.readAllLines(Path.of("file.txt"));

// Write file
Files.writeString(Path.of("output.txt"), "Hello");
Files.write(Path.of("output.txt"), lines);

// BufferedReader
try (BufferedReader br = Files.newBufferedReader(Path.of("file.txt"))) {
    br.lines().forEach(System.out::println);
}

Lambda & Streams

java example

// Lambda syntax
(parameters) -> expression
(parameters) -> { statements; }

// Functional interfaces
Predicate<Integer> isEven = n -> n % 2 == 0;
Function<String, Integer> toLength = String::length;
Consumer<String> printer = System.out::println;
Supplier<Double> random = Math::random;
Comparator<String> byLength = Comparator.comparingInt(String::length);

// Stream pipeline
List<String> result = names.stream()
    .filter(n -> n.startsWith("A"))    // Filter
    .map(String::toUpperCase)           // Transform
    .sorted()                           // Sort
    .limit(5)                           // Take first 5
    .collect(Collectors.toList());      // Collect

// Collectors
Collectors.toList()
Collectors.toSet()
Collectors.joining(", ")
Collectors.groupingBy(Student::getDepartment)
Collectors.counting()
Collectors.summarizingInt(Student::getMarks)

Continue the lesson path

Study next in this module

4 connected lessons

PreviousCoding Interview Questions NextQuick Revision Notes — Java Programming