Java

数据类型

int—32-bit integer

long—64-bit integer

double—64-bit floating point

float—32-bit floating point

boolean—true or false

char—Single character

byte—8-bit integer

short—16-bit integer

String—Text (object type)

var x = val—Type inference (10+)

面向对象

.length()—String length

.charAt(i)—Character at index

.substring(start, end)—Extract substring

.indexOf(str)—Find substring index

.contains(str)—Check if contains

.split(regex)—Split into array

.trim()—Remove whitespace

.replace(old, new)—Replace substring

.toUpperCase()—Uppercase conversion

.equals(str)—Compare strings (not ==)

String.format("%s", val)—Formatted string

String.valueOf(n)—Convert to string

集合

List<T> list = new ArrayList<>()—Dynamic array

list.add(item)—Add to list

list.get(i)—Get by index

list.size()—List length

list.remove(i)—Remove by index

Map<K,V> map = new HashMap<>()—Key-value map

map.put(k, v)—Add to map

map.get(k)—Get value by key

map.containsKey(k)—Check key exists

Set<T> set = new HashSet<>()—Unique collection

Collections.sort(list)—Sort list

Collections.unmodifiableList()—Immutable list

异常

if (cond) { } else { }—If-else statement

switch (val) { case: }—Switch statement

for (int i=0; i<n; i++)—For loop

for (T item : collection)—Enhanced for-each

while (cond) { }—While loop

do { } while (cond)—Do-while loop

break / continue—Exit / skip iteration

cond ? a : b—Ternary operator

输入输出

class MyClass { }—Define a class

new MyClass()—Create instance

extends BaseClass—Inheritance

implements Interface—Implement interface

interface IName { }—Define interface

abstract class A { }—Abstract class

super()—Call parent constructor

@Override—Override annotation

this.field—Current instance field

static method()—Class-level method

Access Modifiers

public—Accessible everywhere

private—Class only

protected—Class + subclasses

(default)—Package-private

final—Cannot override/reassign

static—Belongs to class

Exception Handling

try { } catch (E e) { }—Try-catch block

finally { }—Always executes

throw new Exception(msg)—Throw exception

throws Exception—Declare throwable

try (Resource r = ...) { }—Try-with-resources

catch (A | B e)—Multi-catch

NullPointerException—Null access error

IllegalArgumentException—Bad argument error

Streams & Lambda

list.stream()—Create stream

.filter(x -> cond)—Filter elements

.map(x -> transform)—Transform elements

.collect(Collectors.toList())—Collect to list

.forEach(x -> action)—Iterate stream

.reduce(init, (a,b) -> a+b)—Reduce to value

.sorted()—Sort stream

.distinct()—Remove duplicates

Optional<T>—Nullable wrapper

Optional.ofNullable(val)—Wrap nullable value

Common Operations

System.out.println()—Print to console

Arrays.sort(arr)—Sort array

Arrays.asList(arr)—Array to list

Integer.parseInt(str)—Parse string to int

Math.max(a, b)—Maximum of two

Math.random()—Random 0.0-1.0

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