Python → Java Glossary

Quick reference for the most common syntax differences.

Null / None

Python	Java	Notes
`None`	`null`	Null value
`if not x`	`if (x == null)`	Null check
`x or y`	`x != null ? x : y`	Null fallback

Data Structures

Python	Java	Notes
`dict`	`HashMap<K,V>`	Key-value map
`set`	`HashSet<T>`	Unique elements
`list`	`ArrayList<T>`	Dynamic array
`deque`	`ArrayDeque<T>`	Double-ended queue
`[]` as stack	`Stack<T>`	LIFO stack
`deque`	`LinkedList<T>`	Queue (FIFO)
`[0]*n`	`new int[n]`	Fixed array
`[0]*n` (list)	`Arrays.fill(arr, 0)`	Initialize

String Operations

Python	Java	Notes
`s[::-1]`	`new StringBuilder(s).reverse().toString()`	Reverse string
`s == t`	`s.equals(t)`	String equality — never use == in Java
`for c in s`	`for (char c : s.toCharArray())`	Iterate chars
`s[i]`	`s.charAt(i)`	Access char
`s[i:j]`	`s.substring(i, j)`	Slice string
`'x' * n`	`"x".repeat(n)`	Repeat string (Java 11+)
`int(c)`	`c - '0'`	Char to int digit
`ord(c)`	`(int) c`	Char to ASCII
`str(n)`	`String.valueOf(n)`	Int to string
`''.join(lst)`	`String.join("", list)`	Join list

Math & Numbers

Python	Java	Notes
`float('inf')`	`Integer.MAX_VALUE`	Positive infinity / max
`float('-inf')`	`Integer.MIN_VALUE`	Negative infinity / min
`a // b`	`a / b`	Integer division (both ints)
`a ** b`	`Math.pow(a, b)`	Power
`abs(x)`	`Math.abs(x)`	Absolute value
`max(a, b)`	`Math.max(a, b)`	Max of two
`min(a, b)`	`Math.min(a, b)`	Min of two
`divmod(a, b)`	`a/b, a%b`	Quotient and remainder

Control Flow

Python	Java	Notes
`and`	`&&`	Logical AND
`or`	`\|\|`	Logical OR
`not`	`!`	Logical NOT
`True / False`	`true / false`	Booleans (case sensitive)
`elif`	`else if`	Else-if branch
`while True: ... break`	`do { ... } while (cond)`	Do-while loop
`x if c else y`	`c ? x : y`	Ternary expression
`a <= x < b`	`a <= x && x < b`	Chained comparison

Sorting

Python	Java	Notes
`lst.sort()`	`Arrays.sort(arr)`	In-place sort
`sorted(lst)`	`Arrays.sort(arr.clone())`	Returns new sorted
`lst.sort(key=fn)`	`Arrays.sort(arr, (a,b) -> ...)`	Custom sort
`lst.sort(reverse=True)`	`Arrays.sort(arr, Collections.reverseOrder())`	Descending sort

Class Definitions

Python	Java	Notes
`def __init__(self)`	`ClassName() { }`	Constructor
`self.x = y`	`this.x = y`	Instance variable
`def method(self)`	`public void method()`	Instance method
`isinstance(x, T)`	`x instanceof T`	Type check