Java examples

Print a greeting via System.out.println.

Two println calls = two lines.

Concatenate strings with +.

println accepts int directly.

Default formatting prints up to 17 digits.

char prints as the literal symbol.

boolean prints true/false.

\t = tab, \n = newline.

Chain + with println.

A Java program need not print anything.

int holds a 32-bit whole number.

double holds a 64-bit decimal.

char is a single 16-bit Unicode character.

boolean is true or false.

String is a sequence of chars (not primitive).

var picks the type from the initializer (Java 10+).

final cannot be reassigned after init.

Declare multiple ints in one line.

(int) truncates a double.

L suffix marks a long literal.

+ on two ints.

- works as expected.

* multiplies.

int / int truncates toward zero.

Double precision when one operand is double.

% returns the remainder.

Compares ints by value.

&& short-circuits when left is false.

++x updates first; x++ returns old value first.

+= adds in place.

Run a block when a condition is true.

Two-way branch.

Multi-way branch.

Use modulus to test parity.

Cases for fixed values.

No break = fallthrough.

Compact if/else expression.

if inside another if.

&& and || in one condition.

switch as an expression with arrow syntax.

Print 0..4.

Decrement to count down.

Increment by 2.

Loop accumulator.

Run while a condition holds.

Body runs at least once.

break exits the loop immediately.

continue jumps to next iteration.

Iterate without index.

3×3 multiplication table.

void method with no return.

Returns the sum.

Same name, different parameters.

Method gets a copy of primitives.

Arrays are passed by reference.

Accept any number of arguments.

Method calls itself.

Static = on class; instance = on object.

Return as soon as you know the answer.

Method returns an int[].

Fixed-size int array.

a.length is a field, not a method.

Loop accumulator.

Track max while iterating.

Resizable list.

Sorts in place.

Collections.sort works on Lists.

List.contains.

List.remove(int) removes one element.

Nested loops over rows×cols.

.length() returns char count.

+ joins two strings.

substring(start, end) — end is exclusive.

indexOf returns position or -1.

replace(old, new).

.equals() compares contents (NOT ==).

Char-by-char with for-each.

Returns a new uppercased string.

StringBuilder.reverse.

Integer.parseInt converts a string to int.

class with public field and method.

Initialise on construction.

No-arg ctor lets you write `new T()`.

private + getter/setter.

Derived class reuses base behaviour.

@Override + virtual dispatch.

Same parent type, runtime dispatch.

Shared across all instances.

Customise printout.

Concise immutable data class.

Catch a specific exception.

finally runs whether or not an exception happened.

One block for several exception types.

Extend Exception or RuntimeException.

Auto-close any AutoCloseable.

<T> declared before the return type.

Box<T> holds a value of type T.

Accept any subtype.

throw raises; throws declares.

Wrap a cause exception.