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Cheatsheet
Java
Core Java syntax and APIs at a glance.
Types & Variables
| int n = 0; | 32-bit integer primitive |
| double d = 3.14; | 64-bit float primitive |
| boolean b = true; | Boolean (lowercase in Java) |
| String s = "hi"; | String object (not a primitive) |
| var x = 42; | Local type inference (Java 10+) |
OOP & Inheritance
| class Dog extends Animal | Single inheritance |
| class Circle implements Shape | Interface implementation |
| @Override | Mark method as intentionally overriding |
| super.method() | Call parent implementation |
| abstract class / abstract method | Must be implemented by subclass |
| final class / final method | Cannot be extended or overridden |
Collections
| new ArrayList<>() | Ordered list — duplicates allowed |
| new HashSet<>() | Unordered set — no duplicates |
| new HashMap<>() | Key-value pairs — unordered |
| list.add(x) | Append element |
| map.put(k, v) | Add or replace entry |
| map.getOrDefault(k, d) | Get value or default if key absent |
Stream API
| .stream() | Create stream from collection |
| .filter(n -> n > 0) | Keep matching elements |
| .map(n -> n * 2) | Transform each element |
| .sorted() | Sort in natural order |
| .collect(Collectors.toList()) | Terminal: collect into list |
| .forEach(System.out::println) | Terminal: consume each element |
Exception Handling
| try { } catch (Ex e) { } | Catch a specific exception |
| finally { } | Always executes (cleanup) |
| throw new RuntimeException(msg) | Throw an exception |
| throws IOException | Declare checked exception in signature |