Java Cheat Sheet

Section A.1 Java Cheat Sheet

Purpose of this Cheat Sheet.

The following is intended to be useful in better understanding Java functions coming from a Python background.

Table A.1.1. Function/Method Equivalents: Python to Java

Python Function	Java Equivalent	Description
`print()`	`System.out.println()`	Prints output to the console.
`len()`	`array.length` or `list.size()`	Returns the length of an array or size of a list.
`range()`	`for (int i = 0; i < n; i++)`	Used in loops to iterate a specific number of times.
`str()`	`String.valueOf()`	Converts an object to a string.
`int()`	`Integer.parseInt()`	Converts a string to an integer.
`float()`	`Float.parseFloat()`	Converts a string to a float.
`list.append()`	`ArrayList.add()`	Adds an element to the end of a list.
`list.pop()`	`ArrayList.remove(index)`	Removes and assign the return value to use it.
`list.sort()`	`Collections.sort(list)`	Sorts a list in ascending order.
`list.reverse()`	`Collections.reverse(list)`	Reverses the order of elements in a list.
`dict.get()`	`Map.get(key)`	Retrieves the value associated with a key in a map.
`dict.keys()`	`Map.keySet()`	Returns a set of keys in a map.
`dict.values()`	`Map.values()`	Returns a collection of values in a map.
`dict.items()`	`Map.entrySet()`	Returns a set of key-value pairs in a map.
`input()`	`Scanner.nextLine()`	Reads a line of input from the console.
`open()`	`FileReader`, `BufferedReader`	Used to read from files.
`enumerate()`	`for (int i = 0; i < list.size(); i++) { ... }`	Used to iterate over a list with an index.

Table A.1.2. Operator Equivalents and Usage

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Operator Type	Operator	Description	Example
Arithmetic	`+`, `-`, `*`, `/`	Addition, Subtraction, Multiplication, Division	`5 + 2` → `7`
Arithmetic	`/`	Integer Division (truncates toward zero)	`7 / 2` → `3`
Arithmetic	`%`	Modulus (remainder)	`7 % 2` → `1`
Arithmetic	`Math.pow()`	Exponent	`Math.pow(2, 3)` → `8.0`
Assignment	`+=`, `-=`, `*=`, `/=`	Adds, subtracts, multiplies, or divides and assigns	`x += 1` → `x = x + 1`
Comparison	`==`, `!=`	Equal to, Not equal to (use `.equals()` for objects)	`x == y` → `True` or `False`
Comparison	`>`, `<`, `>=`, `<=`	Greater/Less than, or equal to	`x > 5` → `True` or `False`
Logical	`&&`, `\|\|`, `!`	Logical AND, OR, NOT	`x > 1 && y < 10` → `True` or `False`

Ternary Operator: Provides a compact, one-line if-else statement. For instance, String result = (score >= 60) ? "Pass" : "Fail"; is much shorter than a full if-else block.
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No Chained Comparisons: Java does not support chained comparisons. Range checks must use logical operators, such as if (age >= 18 && age < 65). In Python, this could be written as if 18 <= age < 65:.
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String Formatting: Java uses methods like String.format() or System.out.printf() for embedding expressions in strings, similar to Python’s F-Strings. For example, String message = String.format("Hello, %s!", name); is cleaner than traditional string concatenation.
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No Tuple or List Unpacking: Java does not have a direct equivalent to Python’s tuple and list unpacking. Assignment must be done one variable at a time, such as String name = "Alice"; int age = 30;.
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Short-Circuiting: The logical operators && (AND) and || (OR) are efficient. They stop evaluating as soon as the outcome is known. For example, in if (user != null && user.isAdmin()), the code will not attempt to call .isAdmin() if user is null, preventing an error.
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Streams API: Java’s Stream API is the idiomatic alternative to Python’s List Comprehensions. It can be used to filter, map, and reduce data in a sequence of steps. For a simpler example, to generate a basic list of numbers, instead of a multi-line loop, you can write List<Integer> numbers = IntStream.range(0, 5).boxed().toList(); This single line creates a stream of numbers from 0 to 4, prepares them for the list with the `.boxed()` method, and collects them into the final result.
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